Effect of exogenous bovine somatotropin on pubertal mammary development in heifers

Is bovine somatotropin an alternative strategy to overcome the detrimental effects of high-gain diets on prepubertal Holstein × Gyr heifers?

Feeding high-gain diets and an inadequate energy and protein ratio during pre-puberty may lead to impaired growth and mammary gland development of heifers. Thus, frequent application of bovine somatotropin (bST) may prevent future losses in productivity, improve mammary development and animal performance. We aimed to evaluate the effects of bST on digestibility, performance, blood metabolites, mammary gland development, and carcass composition of high-performance prepubertal Holstein × Gyr heifers. Thirty-four Holstein × Gyr heifers with an average initial body weight of 218 ± 49 kg and 14 ± 4 months of age were submitted to an 84-day trial evaluating the effects of no bST or bST injections. Treatments were randomly assigned to each animal within one of the tree blocks. The bST did not influence digestibility or performance parameters. Regarding blood results, IGF1 concentration presented an interaction between treatment and day, where bST heifers had the highest IGF1 concentration. Heifers receiving bST also showed increased ribeye area; however, only an experimental day effect for backfat thickness was observed, with greater accumulation of carcass fat on day 84. Heifers receiving bST had lower pixels/mm² on parenchyma, characteristic of greater parenchymal tissue. Moreover, heifers on bST treatment also had reduced pixels/mm2, characteristic of reduced fat pad tissue. Lastly, bST injections did not influence liver and muscle gene expression, nor most genes evaluated in mammary gland tissue, except for IGFBP3 expression, which was greater for bST heifers. In summary, we confirm the efficacy of bST injections to overcome the detrimental effects of high-gain diets on mammary gland growth and to improve lean carcass gain of prepubertal Holstein × Gyr heifers.

Modern Imaging Techniques in the Study and Disease Diagnosis of the Mammary Glands of Animals

The study of the structure and function of the animals' mammary glands is of key importance, as it reveals pathological processes at their onset, thus contributing to their immediate treatment. The most frequently studied mammary diseases are mastitis in cows and ewes and mammary tumours in dogs and cats. Various imaging techniques such as computed tomography, positron emission tomography, magnetic resonance imaging, and ultrasonographic techniques (Doppler, contrast-enchanced, three-dimensional and elastography) are available and can be applied in research or clinical practice in order to evaluate possible abnormalities in mammary glands, as well as to assist in the differential diagnosis. In this review, the above imaging technologies are described, and the perspectives of each method are highlighted. It is inferred that ultrasonographic modalities are the most frequently used imaging techniques for the diagnosis of clinical or subclinical mastitis and treatment guidance on a farm. In companion animals, a combination of imaging techniques should be applied for a more accurate diagnosis of mammary tumours. In any case, the confirmation of the diagnosis is provided by laboratory techniques.

Whole-genome resequencing reveals genetic diversity and selection characteristics of dairy goat

The dairy goat is one of the earliest dairy livestock species, which plays an important role in the economic development, especially for developing countries. With the development of agricultural civilization, dairy goats have been widely distributed across the world. However, few studies have been conducted on the specific characteristics of dairy goat. In this study, we collected the whole-genome data of 89 goat individuals by sequencing 48 goats and employing 41 publicly available goats, including five dairy goat breeds (Saanen, Nubian, Alpine, Toggenburg, and Guanzhong dairy goat; n = 24, 15, 11, 6, 6), and three goat breeds (Guishan goat, Longlin goat, Yunshang Black goat; n = 6, 15, 6). Through compared the genomes of dairy goat and non-dairy goat to analyze genetic diversity and selection characteristics of dairy goat. The results show that the eight goats could be divided into three subgroups of European, African, and Chinese indigenous goat populations, and we also found that Australian Nubian, Toggenburg, and Australian Alpine had the highest linkage disequilibrium, the lowest level of nucleotide diversity, and a higher inbreeding coefficient, indicating that they were strongly artificially selected. In addition, we identified several candidate genes related to the specificity of dairy goat, particularly genes associated with milk production traits (GHR, DGAT2, ELF5, GLYCAM1, ACSBG2, ACSS2), reproduction traits (TSHR, TSHB, PTGS2, ESR2), immunity traits (JAK1, POU2F2, LRRC66). Our results provide not only insights into the evolutionary history and breed characteristics of dairy goat, but also valuable information for the implementation and improvement of dairy goat cross breeding program.

The role of the IGF system in mammary physiology of ruminants

The IGF system plays a central role in all stages of mammary development, lactation and involution. IGFs exert their effects on the mammary gland through both endocrine and paracrine/autocrine mechanisms and the importance of circulating versus local IGF action remains an open question, especially in ruminants. At the whole organ level, a critical role for IGFs in ductal morphogenesis and lobuloalveolar development has been established, while at the cellular level the ability of IGFs to stimulate cell proliferation and control cell survival contributes to the number of milk-secreting cells in the gland. Much of this work has been conducted in rodents which provide an affordable research model and allow for genetic manipulation of specific components of the IGF system. Research into the role of the IGF system in dairy cows has generally supported information obtained with rodents though large gaps in our knowledge remain and species differences are not well defined. Examples include whether exogenous somatotropin exerts its effects on the mammary gland through local IGF-1 synthesis which is accepted dogma in rodents, what the role of IGF-1 versus IGF-2 is in the mammary gland, and how the IGFBPs regulate IGF bioactivity. This last area is particularly under-investigated in ruminants both at the whole animal and the cellular and molecular levels. Given that the IGF system may underlie many management practices that could contribute to enhancing productive efficiency of lactation, more research into the basic biology of this important system is warranted.

Bovine Somatotropin Alters Myosin Heavy Chains and Beta Receptors in Skeletal Muscle of Feedlot Heifers with Little Impact on Live or Carcass Performance

The objective was to determine whether recombinant bovine somatotropin (rbST) enhanced live performance,skeletal muscle biological activity, and beta-adrenergic receptor expression of feedlot heifers during the finishing phase. Heifers (n = 16; initial body weight = 457 ± 3 kg) were randomly assigned to pens (4 pens/treatment; 2 heads/pen) and treatment: (1) no rbST (Control); (2) 500 mg/hd of sometribove zinc at day 0 and 14 (rbST; Posilac®; Elanco AnimalHealth, Greenfield, IN). Longissimus muscle biopsies for muscle chemistry were collected on day 0, 14, 28, 42, and 56. The rbST heifers had increased expression of AMP-activated protein kinase alpha and beta 3 adrenergic receptor (P < 0.05). Day of the study affected the expression of myosin heavy chain-IIA (MHC-IIA), MHC-IIX, beta 2 adrenergic receptor, peroxisome proliferator-activated receptor gamma, and stearoyl-CoA desaturase (P < 0.05). Day had a significant effect on muscle fiber cross-sectional area and proportion (P < 0.05). As days on feed increased, the area of MHC-I fibers decreased whereas MHC-IIA and IIX area increased (P < 0.05). The rbST heifers had decreased proportions of MHC-I fibers and increased proportions of MHC-IIX fibers (P < 0.05). The greatest density of Paired Box 7-positive cells was on day 0, 28, and 42 (P < 0.05), and the greatest density of Myogenic factor 5-positive cells was on day 42 and 56 (P < 0.05). Also, the greatest density of cells positive for Paired Box 7:Myogenic factor 5 was measured on day 28 (P < 0.05). These data indicate that, as days on feed increase, the effects of skeletal muscle biological activity are not dependent on rbST administration but may be more due to physiological changes occurring as the animal reaches physio-logical maturity.

Exogenous porcine somatotropin stimulates mammary development in late-pregnant gilts

The goal of this project was to determine if increasing insulin-like growth factor-1 (IGF-1) concentrations in late pregnancy can stimulate mammogenesis in gilts. Yorkshire × Landrace gilts of a similar body weight (BW; 196.2 ± 6.2 kg) on day 89 of gestation were separated in 2 groups, namely, controls (CTL, n = 17) that were injected with sterile water, and porcine somatotropin-treated (pST, n = 20) that received injections of 5 mg of pST (Reporcin). Injections were given daily from days 90 to 109 of gestation and gilts were slaughtered on day 110 to collect mammary glands for compositional analyses. Blood samples were obtained on days 89, 96, 103, and 109 of gestation to measure IGF-1, free fatty acids (FFA), urea, glucose, and insulin concentrations. Treated gilts gained more BW (22.7 vs. 18.2 kg, P < 0.05) and lost more backfat (P < 0.05) than CTL gilts during the treatment period. There was a treatment × day effect (P < 0.01) on IGF-1, glucose, and urea concentrations. Concentrations of IGF-1 increased 4-fold (P < 0.01) in pST compared with CTL gilts on days 96, 103, and 109 of gestation. Insulin values were also greater on days 96 (P < 0.01) and 103 (P = 0.01), and tended to be greater (P < 0.10) on day 109 of gestation in pST gilts. Glucose was greater in pST than CTL gilts on days 96 (P < 0.01), 103 (P < 0.01), and 109 (P = 0.01). Concentrations of urea were lower (P < 0.01) on days 96, 103, and 109 of gestation in gilts receiving pST injections, and FFA was not altered by treatment on any sampling day (P > 0.10). Injections of pST did not affect mammary extraparenchymal tissue weight (P > 0.10) but increased mammary parenchymal mass (1922 vs. 1576 ± 124 g, P < 0.05). The composition of parenchymal tissue was also altered by treatment. Mammary parenchyma from pST gilts contained more (P < 0.05) protein, DNA and RNA and less fat (P < 0.05) and dry matter (P < 0.01) than that from CTL gilts. These findings provide a clear demonstration that increasing circulating IGF-1 in late-pregnant gilts can stimulate mammary development both in terms of total parenchymal mass and of parenchymal tissue composition.

Effects of rumen-undegradable protein on intake, performance, and mammary gland development in prepubertal and pubertal dairy heifers

The objective of this study was to evaluate the influence of different amounts of rumen-undegradable protein (RUP) on intake, N balance, performance, mammary gland development, carcass traits, and hormonal status of Holstein heifers at different physiological stages (PS). Sixteen prepubertal (PRE) heifers (initial BW = 106 ± 7.6 kg; age = 4.3 ± 0.46 mo) and 16 pubertal (PUB) heifers (initial BW = 224 ± 7.9 kg; age = 12.6 ± 0.45 mo) were used in an experiment over a period of 84 d. Four diets with increasing RUP contents (38, 44, 51, and 57% of dietary crude protein) and heifers at 2 PS (PRE or PUB) were used in a 4 × 2 factorial arrangement of treatments in a completely randomized design. Throughout the experiment, 2 digestibility trials were performed over 5 consecutive days (starting at d 36 and 78) involving feed and ort sampling and spot collections of feces and urine. At d 0 and 83, body ultrasound images were obtained for real-time carcass trait evaluation. The mammary gland was ultrasonically scanned at d 0 and every 3 wk during the experiment. Blood samples were taken at d 0 and 84 to determine serum concentrations of progesterone, estrogen, insulin-like growth factor I (IGF-I), and insulin. No interaction between PS and the level of RUP was found for any trait. Apparent digestibility of dry matter, organic matter, and neutral detergent fiber corrected for ash and protein was not affected by RUP level but was lower for PRE compared with PUB heifers. Sorting against neutral detergent fiber corrected for ash and protein (tendency only) and for crude protein was greater for PUB than PRE heifers. Pubertal heifers had greater average daily gain (905 vs. 505 g/d) and N retention (25.9 vs. 12.5 g/d) than PRE heifers. In addition, average daily gain and N retention were greatest at 51% RUP of dietary protein. Mammary ultrasonography indicated no effects of RUP amounts on mammary gland composition, whereas PRE heifers had greater pixel values than PUB, indicating higher contents of fat rather than protein in the mammary glands of PRE heifers. Serum progesterone and IGF-I concentration was affected only by PS, and PRE heifers had greater values of progesterone and IGF-I concentrations than PUB heifers. Serum insulin concentration was unaffected by PS but tended to be higher at 51% of RUP. In conclusion, an RUP level of 51% increases body weight, average daily gain, feed efficiency, and N retention in heifers regardless of the PS. In addition, PRE heifers have a lower sorting ability and reduced intake, total-tract digestibility, and N retention. They also have higher amounts of fat in their mammary glands, even at moderate growth rates.

TRIENNIAL LACTATION SYMPOSIUM/BOLFA:Historical perspectives of lactation biology in the late 20th and early 21st centuries

The latter half of the 20th century and the early portion of the 21st century will be recognized as the "Golden Age" of lactation biology. This period corresponded with the rise of systemic, metabolomic, molecular, and genomic biology. It includes the discovery of the structure of DNA and ends with the sequencing of the complete genomes of humans and all major domestic animal species including the dairy cow. This included the ability to identify polymorphisms in the nucleic acid sequence, which can be tied to specific differences in cellular, tissue, and animal performance. Before this period, classical work using endocrine ablation and replacement studies identified the mammary gland as an endocrine-dependent organ. In the early 1960s, the development of RIA and radioreceptor assays permitted the study of the relationship between endocrine patterns and mammary function. The ability to measure nucleic acid content of tissues opened the door to study of the factors regulating mammary growth. The development of high-speed centrifugation in the 1960s allowed separation of specific cell organelles and their membranes. The development of transmission and scanning electron microscopy permitted the study of the relationship between structure and function in the mammary secretory cell. The availability of radiolabeled metabolites provided the opportunity to investigate the metabolic pathways and their regulation. The development of concepts regarding the coordination of metabolism to support lactation integrated our understanding of nutrient partitioning and homeostasis. The ability to produce recombinant molecules and organisms permitted enhancement of lactation in farm animal species and the production of milk containing proteins of value to human medicine. These discoveries and others contributed to vastly increased dairy farm productivity in the United States and worldwide. This review will include the discussion of the centers of excellence and scientists who labored in these fields to produce the harvest of knowledge we enjoy today.

Effect of exogenous somatotropin in Holstein calves on mammary gland composition and proliferation

Pubertal mammary gland growth and development are hormonally regulated, but the details are poorly understood in calves. Our purpose was to evaluate the effects of exogenous growth hormone (GH) on the biochemical composition of the prepubertal mammary gland, mRNA expression of selected genes, and histological characteristics of the developing parenchyma (PAR). In this experiment, 19 calves (7 ± 4 d of age) were randomly assigned to 1 of 2 treatments: bovine somatotropin (bST, 500 mg; n = 10) or placebo (Sal; 0.9% saline; n = 9). Animals were treated every 3 wk beginning on d 23. Calves were assigned to an early (65 d; tissue harvested after 2 treatment injections) or late collection time (107 d; tissue harvested after 4 treatment injections). Calves were fed milk replacer and calf starter for 8 wk and starter and hay thereafter. Parenchyma and mammary fat pad (MFP) from one udder half were harvested for analysis of protein, lipid, and DNA. Additional tissues were preserved for histological analysis or snap-frozen for quantitative real-time PCR. Somatotropin treatment did not significantly alter the mass of PAR or MFP or the general pattern of development of epithelial structures. Significant increases were observed in protein/100 kg of body weight (BW), total protein, DNA concentration, DNA/100 kg of BW, and total DNA in 107-d calves, and a significant treatment by day interaction was observed for DNA and lipid concentrations in PAR. In MFP, a significant decrease was observed in protein/100 kg of BW in bST-treated calves and in total MFP protein in 65-d calves. A treatment by day interaction was found for total protein, DNA, and protein/100 kg of BW. In PAR, relative expression of ATPase-binding cassette 3 and growth hormone receptor were reduced by bST and both were lower in 107-d-harvest calves. Epithelial cell retention of bromodeoxyuridine (BrdU; possible indicator of stem-like cells) was greatest in 65-d bST-treated calves, and a significant time of sampling response and treatment × time interaction were observed. Expression of the proliferation marker protein Ki67 was numerically higher in bST-treated calves but the difference was nonsignificant. Retention of the BrdU label was reduced in 107-d calves. Exogenous growth hormone given to calves may affect mammary tissue composition and epithelial cell gene expression in subtle ways but exogenous supplementation with bST alone is not likely to alter overall development patterns or affect the mass of mammary parenchymal tissue. Whether such subtle changes have an effect on subsequent development or function is unknown.

Mammogenesis and first lactation milk yields of identical-twin heifers following pre-pubertal administration of bovine growth hormone

The effects of long-term administration of bovine growth hormone (GH) on pre-pubertal mammogenesis and subsequent milk production were examined in pasture-fed dairy heifers. Purified bovine GH (specific activity 0·78 i.u. per mg) was administered daily for 21 weeks (0·6 mg GH per kg M 0·75) to one member of each of 12 sets of twins from 3·5 (range 2·5 to 4-5) months of age. GH administration resulted in a significantly smaller mammary gland compared with control heifers at slaughter at the end of treatment (717 v. 839 g; P < 005 (no. = 4)) and increased the proportion of mammary fat pad containing parenchyma (461 v. 383 g/kg; P < 0·01). Mammary parenchymal dry, fat-free tissue was increased in the GH-treated group (25·8 v. 22·5 g; P < 0·05), and fat-pad adipose tissue was reduced (323 v. 435 g; P < 005), compared with the control group. Morphometric analysis of the mammary gland revealed an increase in the volume fraction of connective tissue (P < 0·1) and a tendency toward less fat in GH-treated heifers compared with control heifers. Estimates of the composition of the whole gland confirmed that GH treatment reduced total mammary fat (372 v. 496 g; P < 0·05). At calving, udder volume was greater in the heifers treated with GH compared with the untreated group (14·8 v. 11·2 1; P < 0·05 (no. = 4)). However, there were no differences in the mean daily yields of milk or milk components between groups.

Measurement of bovine somatotropin (bST) and insulin-like growth factor-1 (IGF-1) in bovine milk using an electrochemiluminescent assay

Bovine somatotropin (bST) and insulin-like growth factor-1 (IGF-1) are peptide hormones that are involved in the regulation of milk production in dairy cows. Because these hormones are present at extremely low concentration in fresh and processed bovine milk, a highly sensitive and specific electrochemiluminescent immunoassay (ECLIA) has been developed to better estimate the concentration of these hormones in milk. The assay employs an imager, a capture antibody bound to a carbon electrode, and a detection antibody coupled to a ruthenium label. In the presence of tripropylamine and an electric pulse, ruthenium generates light proportional to the amount of antigen bound, and the light is captured as signal by a charge-coupled device (CCD) camera. Using bovine milk as the starting matrix, 99.69% of bST and 104.79% of IGF-1 were recoverable. The limit of detection (LOD) was <5 pg/mL for bST and <1 pg/mL for IGF-1. The limit of quantification (LOQ) was <14 pg/mL for bST in milk and <2 pg/mL of IGF-1. The assay is highly specific and shows <0.2% cross-reactivity with other peptide hormones found in bovine milk such as insulin and IGF-2. These data indicate this new, ECLIA is highly sensitive and specific for estimating the concentration of bST or IGF-1 in milk.

Derivation of a growth hormone gene cassette for goat by mutagenesis of the corresponding bovine construct and its expression in Pichia pastoris

Recombinant bovine growth hormone (rbGH), a 191-aa polypeptide that affects animal growth and lactation, has been used for several years to increase milk production in dairy cattle. It has also been used in goats (Capra hircus) instead of their own hormone (chGH), which is still not available in the market. Since both hormones differ in only one amino acid residue, a strategy based on PCR mediated site-directed mutagenesis, was used to convert the bGH expression cassette harbored by an integration plasmid for Pichia pastoris into a chGH. Transformation by homologous recombination of Pichia pastoris GS115 strain with the linearized new plasmid resulted in transformants that, upon fermentation and induction with methanol, secreted a band with the expected size and immunoreactivity for GH. Production of total proteins secreted into culture medium (50 ml) was 20 microg/ml, of which 60% was chGH as judged by densitometry in SDS-PAGE. Its biological activity was confirmed in vitro when 3T3 pre-adipocytes exposed to the induced culture medium differentiated into adipocytes in cell culture.

Growth hormone-releasing hormone (GHRH) polymorphisms associated with carcass traits of meat in Korean cattle

BACKGROUND

Cold carcass weight (CW) and longissimus muscle area (EMA) are the major quantitative traits in beef cattle. In this study, we found several polymorphisms of growth hormone-releasing hormone (GHRH) gene and examined the association of polymorphisms with carcass traits (CW and EMA) in Korean native cattle (Hanwoo).

RESULTS

By direct DNA sequencing in 24 unrelated Korean cattle, we identified 12 single nucleotide polymorphisms within the 9 kb full gene region, including the 1.5 kb promoter region. Among them, six polymorphic sites were selected for genotyping in our beef cattle (n = 428) and five marker haplotypes (frequency > 0.1) were identified. Statistical analysis revealed that -4241A>T showed significant associations with CW and EMA.

CONCLUSION

Our findings suggest that polymorphisms in GHRH might be one of the important genetic factors that influence carcass yield in beef cattle. Sequence variation/haplotype information identified in this study would provide valuable information for the production of a commercial line of beef cattle.

Leptin Does Not Act Directly on Mammary Epithelial Cells in Prepubertal Dairy Heifers

The mammary gland of prepubertal dairy heifers consists of parenchyma expanding into the stroma, a matrix of connective and adipose tissue. High planes of nutrition increase stromal mass, but inhibit growth of parenchyma. The parenchyma consists of epithelial cells proliferating in response to growth factors such as insulin like growth factor-I (IGF-I). These observations have led to the hypothesis that elevated planes of nutrition increase leptin production, which in turn inhibits IGF-I-mediated epithelial cell proliferation. To assess this possibility, heifers were offered planes of nutrition sustaining average daily gains of 715 g/d (normal; NP) or 1,202 g/d (high; HP) from 42 d of age until slaughter at 240 kg. At slaughter, HP heifers had 2-fold greater plasma leptin concentration and 3-fold greater leptin mRNA abundance in mammary stroma and parenchyma. To assess the causal nature between leptin and parenchymal development, the induction of signaling events and functional responses in the MAC-T cell line and in primary mammary epithelial cells by leptin was examined. Leptin did not induce phosphorylation of signal transducers and activators of transcription (STAT)3, STAT5, extracellular signal-regulated kinase (ERK1/2), or AKT/Protein kinase B. Consistent with its inability to signal, leptin did not alter basal- or IGF-I-stimulated thymidine incorporation or increase suppressors of cytokine signaling 3 (SOCS3) expression in these cells. Transcripts corresponding to the short leptin receptor form were present in mammary tissue, but those corresponding to the long signaling form were not detected in either mammary tissue or cells. In conclusion, elevated planes of nutrition increase leptin synthesis in mammary stroma, but leptin does not act directly on bovine mammary epithelial cells.

Effect of Feeding Level Pre- and Post-Puberty and Body Weight at First Calving on Growth, Milk Production, and Fertility in Grazing Dairy Cows

The effect of feeding to achieve differential growth rates in Holstein-Friesian (HF; n = 259) and Jersey (n = 430) heifers on time to puberty and first lactation milk production was investigated in a 3 x 2 factorial design. Holstein-Friesian and Jersey calves were reared to achieve a BW of 100 and 80 kg, respectively, at 100 d. At target weight, all calves were randomly allocated to one of 3 feeding treatments to achieve different growth rates. Holstein-Friesian and Jersey calves were fed fresh pasture to achieve average daily growth rates of 0.77, 0.53, or 0.37 kg of BW/d (HF) and 0.61, 0.48, or 0.30 kg of BW/d (Jersey), respectively. Period 1 (prepubertal) was imposed until HF and Jersey treatment groups averaged 200 and 165 kg of BW, respectively. Following period 1, HF and Jersey calves from each treatment group were randomly allocated to one of 2 feeding treatments to achieve average daily growth rates of 0.69 or 0.49 kg of BW/d (HF) and 0.58 and 0.43 kg of BW/d (Jersey), respectively. Period 2 (postpubertal) was imposed until 22 mo, when heifers were returned to their farms of origin. Body weight, body condition score, height, heart girth circumference (HGC), milk production, and fertility-related data were collected until the end of the third lactation. Time to reach puberty was negatively associated with level of feeding, and heifers attained puberty at the same BW (251 +/- 25.4 and 180 +/- 24.0 kg for HF and Jersey heifers, respectively). Heifers on high feed allowances during periods 1 and 2 were heavier, taller, and had greater HGC than their slower grown counterparts until 39 mo of age when height and HGC measurements stopped. Body weight differences remained until 51 mo, when measurements ceased. High feed allowance during period 1 (prepubertal) did not affect milk production during the first 2 lactations, but did reduce milk production in lactation 3. It is possible that the expected negative effect of accelerated pre-pubertal growth was masked by greater calving BW, as BW-corrected milk yield declined in both breeds with increasing prepubertal feed allowance. Growth rate during period 2 was positively correlated with first lactation milk production. Milk yield increased 7% in first lactation heifers on the high feed allowance, which resulted in higher growth rate during period 2. Milk production during subsequent lactations was not affected. Results suggest that accelerated prepubertal growth may reduce mammary development in grazing dairy cows, but this does not affect milk production in early lactations because of superior size. Body weight at calving and postpubertal growth rate management are important in first lactation milk production, but do not affect milk production in subsequent lactations.

Ovarian and IGF-I axis control of mammary development in prepubertal heifers

Although much is known about the endocrine control of bovine mammary development, most heifer work has focused on periods near the time of puberty or during gestation. However, we have found that ovariectomy in the prepubertal period also markedly impacts mammary development well before the onset of estrus would have normally occurred. Interactions between the pituitary and ovary to control udder development are mediated at least in part via alteration in concentrations of local IGF-I axis molecules within the developing mammary gland. For example, in heifers treated with growth hormone or estrogen, expression of IGF-I binding proteins (IGFBP-3) protein was reduced, thus effecting an increase in free IGF-I. Ovariectomized heifers had reduced rates of epithelial cell proliferation, fewer IGF-I receptors, and less local IGF-I. Mammary tissue expression of fibronectin was increased in ovariectomized heifers, but laminin expression was higher in controls. Thus, alterations in specific extracellular matrix proteins likely impact heifer mammary development. As a result, we have initiated calfhood studies. At 30 days of age, it is difficult to detect parenchymal tissue in the udder. Only a thin cord of parenchymal tissue (150 mg per gland) is discernible. By 75 days of age, a rounded, walnut-like mass of mammary parenchymal tissue becomes very evident and at 90 days of age, this mass of tissue has grown to approximately 10 g, a approximately 60-fold increase. At 2 months of age, most proliferating epithelial cells (>92%) are confined to a population of light and intermediate-staining parenchymal cells. Between 2 and 5 months of age, a dark-staining cell population markedly emerges, but these dark cells were rarely labeled with bromodeoxyuridine (BrdU) and are likely to represent a more differentiated or committed cell lineage. The coordinated change in the proportions of each cell type suggests a progression from light-, to intermediate-, to dark-staining cell phenotypes. We are currently focusing on the importance of the ovary and mammary tissue synthesis of estrogens on emergence of specific populations of putative mammary stem cells.

Prospects for zero days dry

Recent studies in high-producing dairy cows have demonstrated the dry period requirement is greatly influenced by parity and management practice. Multiparous cows that were continuously milked and treated with bST demonstrated negligible production losses in the next lactation. First-lactation heifers, however, demonstrated large reductions in milk yield. These reductions were not overcome by using bST or IMF the next lactation. No studies have been performed examining use of LDPP or SDPP in combination with bST and IMF on the dry-period requirement. Cows that are continuously milked demonstrated higher feed intakes during the peripartum period, which may greatly improve metabolic health. Future studies must examine potential benefits of continuous milking on health in the next lactation.

A local increase in the mammary IGF-1: IGFBP-3 ratio mediates the mammogenic effects of estrogen and growth hormone

A single epithelium-free mammary fat pad was surgically prepared in each of twenty-five one-month-old, Friesian heifers. At 18 mo of age, heifers were randomly assigned to one of four treatment groups. Treatments were: control (C), growth hormone (GH), estrogen (E) or growth hormone + estrogen (GE). Hormones were administered for 40 hr before the animals were sacrificed to provide mammary samples of parenchyma (PAR), intact fat pad (MFP), and epithelium-free or "cleared" fat pad (CFP). IGF-1 and IGF binding protein-3 (IGFBP-3) mRNA was highest in CFP and MFP whereas the protein products were highest in PAR. IGFBP-2, a 28-kDa IGFBP and a 24-kDa IGFBP were more abundant in CFP and MFP. E and GH increased incorporation of [(3)H]thymidine into DNA of PAR. Incorporation of [(3)H]thymidine into the DNA of MFP or CFP was minimal. Coincident with the changes observed in mammary epithelial proliferation, E increased IGF-1 protein in MFP and PAR, and to a lesser extent in CFP. E tended to increase IGF-1 mRNA levels in MFP, but not CFP implying that the regulation of IGF-1 expression is modulated by adjacent epithelium. GH and E reduced IGFBP-3 protein in PAR and increased the 24-kDa IGFBP in CFP and MFP. Increased proliferation of mammary parenchymal cells was associated with increased IGF-1 and reduced IGFBP-3 protein in mammary tissue. An increase in the ratio of mammary IGF-1: IGFBP-3 likely increases the proportion of the mammary IGF-1 available to stimulate proliferation. These data also indicate that stromal: epithelial interactions regulate the IGF-1 axis in mammary tissue.

High body weight gain and reduced bovine mammary growth: physiological basis and implications for milk yield potential

Available evidence concerning the relationship between growth rate, mammary growth and milk yield in heifers leads to these conclusions: 1) Increased growth rate due to high feeding level before puberty onset can lead to reduced pubertal mammary growth and reduced milk yield potential. 2) Increased growth rate due to high feeding level after puberty and during pregnancy have no effect on mammary growth and milk yield. 3) Higher body weight gain due to higher genetic potential for growth is positively related to milk yield. The negative effect of high feeding level before puberty occurs in all breeds, but the level of feeding causing reduced yield varies. Variation in responses between experiments suggests that feeding regimes that support high growth rates without negative effect on yield can be developed. A breakthrough most likely will originate from increased knowledge of the physiological relationship between nutrition and mammary development. Our investigations suggest that blood growth hormone (GH) is important for mammary development, and that the negative effect of high feeding level on mammary development may be due to reduced blood GH. GH, however, does not bind to mammary tissue. Experiments with exogenous GH suggest that GH acts on mammary tissue via IGF-I, but IGF-I is increased by high feeding level - not decreased as GH. This paradoxical relationship cannot be explained by changes in circulating IGF binding proteins. However, the sensitivity of mammary tissue to IGF-I is reduced by high feeding level, probably due to the action of locally produced binding proteins and/or growth factors.

Selection for milk production from a lactation biology viewpoint

The success of selection for increased milk production in dairy cows is apparent. Certainly, many herds now have average production levels that would have only been associated with the best producers in the herd 30 yr ago. There are, of course, many reasons for this success. Among these are improvements in genetic selection methods and associated use of artificial insemination, better fulfillment of nutritional needs and diet formulation, and careful attention to mastitis control and milking management. Development of new management tools (i.e., bovine somatotropin, improved crops, estrus detection devices, estrus synchronization, monitoring of individual animal performance, and disease prevention) should not be forgotten. Although many aspects of a dairy operation determine overall performance and profitability, the focus of this paper is the udder. Information indicates that both the structure and function of the bovine mammary gland have been directly impacted by long-term selection for increased milk production but improved functionality may have been more important. This review also considered studies that attempt to develop techniques and measurements for possible selection of genetically superior animals including measurement of circulating hormones and direct assay of mammary tissue function.

Hormones, mammary growth, and lactation: a 41-year perspective

When I was a beginning graduate student 41 yr ago it had been established that estrogen caused mammary duct growth; a combination of estrogen and progesterone was required for lobule-alveolar development of the mammary glands; and prolactin and growth hormone were essential for mammary growth. In laboratory species exogenous prolactin, glucocorticoids, and estrogen would initiate secretion of milk provided the mammary glands had a well-developed lobule-alveolar system. It was not known with certainty that progesterone inhibited the process. For some species, prolactin and thyroxine had been shown to stimulate lactation, while glucocorticoids suppressed lactation. Definitive roles for growth hormone and insulin during lactation had not been established. Studies of hormonal control of mammary growth and function in cattle were few. In vitro methods to study hormonal regulation of the mammary glands were in their infancy. Quantitative measures of changes in mammary cell numbers and specific components of milk in response to hormones were rare. The concepts for quantification of hormone concentrations, hormone receptors, growth factors, and binding proteins in blood; hormonal regulation of nutrient partitioning; and hormonally induced mechanisms of action within mammary cells were waiting to be discovered. And eventually they were. However, lest we become too enamored with our current understanding of the hormones that control mammary growth and lactation, it remains a fact that the greatest physiological stimulus for milk yield is pregnancy, not some cocktail of exogenous hormones, growth factors, receptor agonists/antagonists, or gene therapies. Viva la mom!

Local IGF-I axis in peripubertal ruminant mammary development

The regulation of mammary growth and development in heifers is accomplished by complex interactions of hormones, growth factors, and extracellular matrix molecules. Many of these growth stimulators are believed to be locally produced in the mammary gland and to be affected by developmental and nutritional status. Although estrogen and growth hormone are considered critical to pubertal mammogenesis, results summarized in this review suggest that IGF-I and IGF binding proteins are especially important locally-produced growth regulators in peripubertal ruminants. This assertion is supported by studies of ovariectomized heifers, in which increased stromal IGFBP-3 and reduced IGF-I correspond with a failure of udder development. Similarly, reduced mammary development with overfeeding coincides with reduced mitogenic activity of mammary tissue extracts and altered concentrations of IGF-I and IGFBPs. In vitro studies convincingly demonstrate that much of the mitogenic activity of mammary extracts or serum can be attributed to IGF-I and that alterations in IGFBP-3 modulate its effectiveness. Thus by analogy to second messenger mechanisms of action for protein hormones, local mammary-derived growth factors likely explain many of the effects attributed to the classic mammogenic hormones.

Regulation of local synthesis of insulin-like growth factor-I and binding proteins in mammary tissue

Our objective was to investigate the mammary expression of insulin-like growth factor-I (IGF-I) and IGF-binding proteins in prepubertal heifers and regulation of IGF-I by bovine somatotropin (bST) and feeding level. Twenty-four prepubertal Friesian heifers were divided into six blocks according to genotype and starting date for the experiments. Within blocks, heifers were assigned to daily bST treatment (0 or 15 mg/d) at low or high feeding level (0.55 kg/d or 1.1 kg/d average daily gain, respectively) for 5 wk so that the mean body weight and standard error was approximately equal for all four treatment groups. At high feeding level, content of IGF-I protein in mammary tissue extracts was increased 46% by somatotropin compared with placebo. Somatotropin tended to increase abundance of IGF-binding protein-3 (40 to 43 kD) in mammary extracts. High feeding level increased abundance of a 24-kD binding protein and reduced abundance of IGF-binding protein-2 (32 kD) in mammary extracts. High feeding level reduced abundance of IGF-binding protein-1 mRNA in mammary tissue, but there was no significant effect of feeding level or somatotropin on mRNA levels of other IGF-binding proteins. These results suggest that effects of somatotropin treatment and feeding level on the prepubertal mammary gland are mediated in part by alterations in local synthesis of IGF-I and IGF-binding proteins.

Growth hormone and mammary development

Classic studies in rodents conducted in the 1950s showed that growth hormone (GH) is essential for mammary development both in the pubertal phase and during pregnancy. Since then, a considerable number of experiments have been carried out in ruminants to investigate the role of GH for regulation of normal mammary development and to examine the possibility of enhancing mammary growth by administration of GH. The available evidence demonstrates that GH treatment stimulates mammary growth before puberty, but the data do not convincingly support the idea that the effect is translated into increased milk yield. GH treatment during late pregnancy seems to stimulate both mammary growth and milk yield during lactation. The limited data concerning the effect of GH on mammary growth during lactation indicate that mammary growth is unaffected by GH treatment in early lactation, whereas GH seems to increase the amount of mammary parenchyma in mid-lactation. The mechanism of action of GH remains a puzzle, but the effect of exogenous GH most likely involves insulin-like growth factor-I (IGF-I). Full understanding of the role of endogenous GH for regulation of normal mammary development requires more knowledge about the interaction between GH and IGF-I and the interplay between the GH-IGF-I axis and locally produced factors, including receptors, binding proteins, and growth factors.

Diet energy requirements of growing Holsteins

The conceptual bases of recent NRC publications of nutrient requirements to meet dietary energy needs of growing Holsteins, emphasizing heifers gaining 0.75 kg/d, are described and compared with several world systems and with body composition data. Requirements of Holsteins for net energy for maintenance are generally thought to be higher (from 7 to 20%) than those of British beef breeds. Total estimates of metabolizable energy requirements are quite similar for most energy systems for cattle at 200 to 400 kg of body weight. For heavier cattle, the energy density estimates based on beef breed equations project higher metabolizable energy for gain and total metabolizable energy requirements. Extrapolations from the chemical composition of growing Holstein heifers indicate that, at the slower growth that is typical of replacements, energy densities are lower than for more rapidly growing heifers and that net energy requirements are consistent with current US standards.

Effects of diet and bovine somatotropin on heifer growth and mammary development

Forty Holstein heifers [body weight (BW) = 126 kg] were blocked by BW into groups of 4, and, within each block, heifers were randomly assigned to one of four treatments. Twenty heifers had ad libitum access to a diet formulated to produce a BW gain of 0.8 kg/d (control diet), and 20 heifers had ad libitum access to a diet formulated to produce a BW gain of 1.2 kg/d. (high diet). Half of the heifers fed each diet were injected daily with bovine somatotropin (bST; 25 micrograms/ kg of BW). The high diet increased daily BW gain as well as body condition score. Injection of bST also increased daily BW gain, but did not affect body condition score. The high diet reduced age at puberty by 58 d, but did not affect BW, withers height at puberty, or pelvic area at slaughter. Injection of bST had no effect on age at puberty, but increased BW, withers height at puberty, and pelvic area at slaughter. The high diet did not affect mammary parenchymal DNA, RNA, or the ratio of RNA to DNA. The injection of bST increased mammary parenchymal DNA, RNA, and the ratio of RNA to DNA. The high diet was more cost effective for rearing dairy heifers from 120 d of age to potential breeding size (> or = 363 kg of BW and postpubertal) than was the control diet. In conclusion, the high protein, high energy diet increased growth rate without detrimental effects on mammary development. Injection of bST increased BW, skeletal size, and mammary development.

Effects of melatonin and plane of nutrition on mammary development in prepubertal boutsiko mountain breed ewe lambs

The effects of melatonin (implants, M or no implants, C) and plane of nutrition (high, H or low, L) on mammary development and growth hormone (GH) concentrations were investigated in prepubertal Boutsiko mountain breed ewe lambs. Eighty female lambs were assigned to each of 4 treatments: ad libitum feeding control (HC), HM, LC and LM. The rearing treatments started and ended at mean ages of 63 and 160 d, respectively. Feed restriction resulted in a mean daily gain of 70.6% of the ad libitum-fed lambs during the experimental period. Melatonin (18 mg Regulin) was administered at 68 d of age (January 10) and replaced on March 1. Blood samples were collected from 10 lambs in each treatment group at the end of the experiment for GH measurements. At a mean age of 160 d, seven lambs from each treatment group were slaughtered and the udder was removed. One udder half was trimmed and the parenchyma and fat pad portions were kept for determination of deoxyribonucleic acid (DNA) content. Melatonin did not influence mammary development parameters, while the mass of parenchyma tended to be greater in lambs on low than high nutrition planes (P<0.10). Mean mammary parenchymal weight and DNA content were 25.1 and 29.2 g and 52.5 and 58.2 mg in high and low nutrition lambs, respectively. Mean plasma GH concentrations were not affected by melatonin treatment and were higher in low than high nutrition lambs (P<0.01). There were no correlations between mean plasma GH concentrations and parenchymal DNA content, or between mean daily weight gain and parenchyma (g), in contrast to those found in a previous experiment with lambs of the same breed but greater age at slaughter. The results suggest that a period of accelerated mammary development occurs later than 140 d of age in Boutsiko mountain breed ewe lambs.

Influence of prepubertal dietary regimen on mammary growth of Holstein heifers

One hundred-sixteen Holstein heifers (mean BW, 175 kg) were randomly assigned to diets of alfalfa silage or corn silage and were fed to gain approximately 725 or 950 g/d in order to study the influence of prepubertal diet and rate of gain on mammary growth and milk production. Blood was collected before puberty for hormone determination, and 8 heifers per group were killed at puberty for evaluation of tissue variables. Serum growth hormone was reduced, and IGF-I was increased, in the group of heifers reared at a high rate of gain on the corn silage diet. Accompanying the decline in growth hormone, total mammary parenchymal DNA and RNA was reduced in heifers reared at a high rate of gain on the corn silage diet. Mammary parenchyma in heifers of the latter group contained a greater volume of adipocytes and a lower volume of epithelial cells than did mammary parenchyma in heifers of other groups. Data are consistent with previous investigations that showed a deleterious effect of prepubertal rapid weight gain on mammogenesis when accompanied by excess body fat deposition. However, this effect did not cause a decline in subsequent milk production.

Effects of prepubertal growth rate and diet on lipid metabolism in lactating Holstein cows

The objectives were to determine the effects of rate of BW gain and type of silage fed before puberty on the partitioning of excess dietary energy between synthesis of milk and BW gain in second or third lactation. Accordingly, 41 Holstein heifers weighing 175 kg were fed diets containing either alfalfa silage or corn silage to gain either 725 or 950 g/d until BW was 325 kg and two estrous cycles were observed. Puberty occurred near 281 kg of BW. During second (n = 36) or third (n = 5) lactation, the cows were fed a control diet (60% forage and 40% concentrate) and a high energy diet (20% forage and 80% concentrate) in a double-reversal experimental design with three 6-wk periods. The rate of BW gain before puberty did not affect the magnitude of changes in DMI, milk yield, milk composition, or concentrations of thyroid hormones, insulin, bST, glucose, or lipids in serum when cows were switched from a control to a high energy diet during second or third lactation. However, compared with cows fed a corn silage diet, cows fed alfalfa silage between 175 and 325 kg of BW had more depressed yields of fat, total solids, and FCM when fed the high energy diet than when fed the control diet during second or third lactation. Increased deposition of fat in adipose and mammary tissues of cows with mean BW gain in excess of 950 g/d or fed a corn silage diet between 175 and 325 kg of BW did not result in more pronounced depression of milk fat percentage when cows were switched from a control to a high energy diet during second or third lactation. Overall, neither rate of BW gain nor type of silage fed between 175 and 325 kg of BW had a major influence on partitioning of excess dietary energy between synthesis of milk and BW gain during second or third lactation.

Relationships between nutrition, puberty and mammary development in cattle

The available studies concerning the relationships between nutrition, puberty and mammary development demonstrate the importance of pubertal mammary growth for the future development and ultimate milk-producing capacity of the mammary gland. A relationship between reproductive development and mammary development is also evident and mammary development at puberty is clearly influenced by the feeding level at that time. The role of specific nutrients has not been thoroughly investigated, but results suggest that specific fatty acids may be involved in the regulation of mammary growth. Mammary growth during puberty is affected by oestrogen and GH, but their respective roles and mechanisms of action have not yet been clarified.

Raising dairy replacements to meet the needs of the 21st century

Producing high quality replacement heifers at minimum cost will be one of the many challenges facing the dairy farm of the 21st century. Because replacement heifers represent a large portion of the total cost of milk production, dairy farmers will have to meet the replacement needs of their lactating herds at minimum cost to maintain the farm's profitability. To keep the US dairy industry competitive into the 21st century, researchers, extension, industry, and producers must examine the database of research in order to determine how to apply the known information to current production and research efforts. Important research is needed to improve the dairy heifers of the future. This review summarizes research of the past 12 yr related to dairy replacements and incorporates those findings into possible scenarios for future dairy calf and heifer management systems. Additional research is needed to determine how dairy replacement raising systems affect the lifetime productivity and profitability of the dairy cow.

Effect of prepartum bovine somatotropin in primigravid ewes on mammogenesis, milk production, and hormone concentrations

Twenty-five primigravid ewes were used to investigate the effect of bST, between 97 and 124 d of gestation, on mammogenesis and subsequent milk production. Five ewes (reference group) were slaughtered at 96 d of gestation, and the remaining ewes were injected daily with saline (control group: n = 10) or .1 mg/kg of BW of bST (bST group: n = 10). Following bST treatment, 5 control and 5 bST group ewes were slaughtered (slaughter group). The remaining ewes were slaughtered after lambing and being milked for 8 wk (production group). Weekly blood samples were obtained from both slaughter and production group ewes. Slaughter group ewes were also subjected to 8-h serial blood sampling at 98 d (period 1) and 123 d (period 2) of gestation. Milk production was 42% higher in ewes treated prepartum with bST than in those treated with saline. Results suggest that the increase in milk was due to an increase in mammary parenchymal cell number rather than to an increase in cellular activity. The high rate of [3H]thymidine incorporation into parenchymal tissue in reference group ewes suggests that the increase in parenchyma during the second trimester of gestation is due to cellular hyperplasia but that cellular hypertrophy may be more important during the last trimester. Plasma IGF-I concentrations were significantly higher during bST treatment and remained elevated between daily injections; the increase was greatest in period 2.

The effect of manipulation in energy allowance during the rearing period of heifers on hormone concentrations and milk production in first lactation cows

Fifteen Holstein heifers that were 175 +/- 4.0 d old and at BW of 175 +/- 4.9 kg were used to determine the effect of three feeding regimens from 6 to 12 mo of age on growth, blood concentration of several hormones, and milk production during first lactation. The feeding regimens consisted of two periods, the first lasting for 4 mo and the other for the subsequent 2 mo. For group A (restricted) heifers, the diet during period 1 was restricted to 85% of NRC (1988) recommendations (a daily BW gain of .7 kg); during period 2, a high energy, high protein diet was provided for ad libitum intake. Group B (control) heifers received a diet that corresponded to 100 and 90% of the NRC (1988) recommendations in periods 1 and 2, respectively. Group C (ad libitum) intake heifers received a high energy, high protein diet throughout both periods. Daily BW gains of heifers of groups A, B, and C were, respectively, .625, .768, and 1.100 kg for period 1 and 1.162, .705, and .797 kg for period 2. The different feeding regimens influenced the age at which the heifers achieved puberty but did not affect BW at puberty. Milk production during 250 d of lactation was 7056, 6070, and 5975 kg for groups A, B, and C, respectively. A statistical model that included serum derived mitogenic activity and serum prolactin of period 2 accounted for 63% of the difference in milk production at first lactation.

Bovine somatotropin

Bovine somatotropin has the potential to alter profoundly the way that dairy herds are managed and to change the ways in which veterinarians provide services to those herds. This article describes the structure, function, and action of somatotropin. The metabolic effects of somatotropin on dairy cattle are discussed.

Growth and subsequent lactation in primigravid Holstein heifers after prepartum bovine somatotropin treatment

Thirty-seven primigravid Holstein heifers (600.7 +/- 9.4 kg) were used to determine the effect of recombinant bST, administered during the last trimester of gestation, on growth and subsequent 270-d milk yield. Treatments consisted of daily subcutaneous injections of saline or of 20 or 40 mg of recombinant bST. Four weeks before expected date of parturition, 8 heifers (3 receiving saline, 3 receiving 20 mg of bST, and 2 receiving 40 mg of bST) were fitted with jugular catheters, and blood plasma bST and insulin-like growth factor-I concentrations were determined in samples collected during a 9-h period. Feed efficiency (DMI/gain) was improved in heifers treated with 20 mg of bST, but DMI was not affected by treatment. Average daily gain was increased in heifers treated with 20 mg of bST, but not in those treated with 40 mg of bST. Fat-uncorrected milk yield following prepartum bST treatment was 19% higher in heifers treated with 20 mg of bST, but the 40-mg dose of bST did not increase milk yield. Percentage of milk fat was lower in heifers treated with 20 mg of bST prepartum, but daily milk fat yields did not differ among groups. Solids-not-fat yield was highest in the group treated previously with 20 mg of bST. The higher milk yield of these heifers became significant (P less than .05) only after 90 d of lactation. Plasma bST and insulin-like growth factor-I concentrations were both elevated in bST-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bovine somatotropin on production and reproduction in prepubertal Friesian heifers

This experiment was conducted to evaluate the effect of bST on average daily gain, onset of puberty, first lactation milk yield, and reproductive efficiency in Friesian heifers. Heifers (n = 16 per treatment) were allocated to either: 1) control (1.5 ml of vehicle) or 2) bST (15 mg of bST in 1.5 ml of vehicle) using a randomized complete block design. Subcutaneous injections of bST were administered once daily from 7 mo of age for 120 d or until heifers reached puberty. Heifers were weighed every 2 wk, and blood samples were collected twice weekly after heifers reached 200 kg. Progesterone concentrations were used to determine onset of puberty. Heifers were bred between 16 and 18 mo of age and, following parturition, milk yield and composition were recorded twice weekly and once every 2 wk, respectively. Heifers assigned to bST treatment had an average daily gain (kg) of .8 compared with .7 in control heifers. Number of days from birth to onset of puberty for bST-treated heifers was 401 compared with 381 for control heifers. Treatment with bST had no effect on milk yield, milk composition, or reproductive efficiency during the first lactation. These data demonstrate that daily administration of bST to Friesian heifers from 7 to 11 mo of age does not affect average daily gain, onset of puberty, reproductive efficiency, or first lactation milk yield of heifers.

Cloning and in vivo expression of bovine growth hormone receptor mRNA

A cDNA for the bovine growth hormone (bGH) receptor has been cloned out of a cDNA library prepared from liver of a pregnant Holstein heifer. The cDNA clone hybridizes to a single 4.5 kb mRNA species and shares a high degree of sequence homology with growth hormone receptors cloned from other species. Utilizing the bGH receptor cDNA as a probe, a relatively high level of bGH-receptor mRNA was detected in bovine liver. In comparison to liver values, lower concentrations of bGH-receptor mRNA were detected in bovine kidney, anterior pituitary, and mammary gland. Because specific binding sites for bGH have not been convincingly demonstrated in isolated cell membranes from whole bovine mammary tissue, mammary tissue from two pregnant heifers (separate experiments) was separated into fractions enriched for epithelium, stroma, and blood components. These fractions were then probed for growth hormone receptor mRNA using solution hybridization-nuclease protection assays performed on isolated RNA. The assay results indicated that a low level of bGH-receptor mRNA is relatively evenly distributed throughout the mammary tissues of the two cows studied. In contrast, experiments using a probe to bovine insulin-like growth factor-I (IGF-I) indicate that the IGF-I mRNA is localized in the stromal/blood component of the mammary gland. These data suggest a possible paracrine mechanism for bGH action in the mammary gland.