Production responses to bovine somatotropin in northeast dairy herds

The Enhanced Milk Yield Effect of Early Lactation Increased Milking Frequency and Bovine Somatotropin Is Additive and Not Synergistic

Dairy farm profitability depends on milk yield, so the dairy industry manages cows to improve their productivity. Both bovine somatotropin (bST) and early lactation increased milking frequency (IMF) and milk yield (MY) in dairy cows. The objective of this study was to evaluate the effects of mid-lactation bST administration on milk production in established lactation when combined with the milk yield carry-over effect from early lactation IMF. Thirteen multiparous Holstein cows were milked unilaterally for 20 days in early lactation. The left udder halves were milked twice daily (2X) and the right udder halves were milked four times daily (4X). Udder halves milked 4X produced 8.60 ± 1.40 kg more than 2X on the final day of IMF treatment. Cows were then returned to 2X milking for the remainder of lactation and sampled on alternate days from 74-94 days in milk (DIM). Bovine somatotropin was administered to all cows at 80 DIM. The 4X halves continued to make 2.66 ± 0.12 kg/d more milk than 2X through 94 DIM. Fat, protein, and lactose yields were significantly greater in the 4X halves compared to the 2X from 74-94 DIM. Overall milk yield increased by 2.71 kg/d with bST administration. However, there was no significant interaction between MF and bST administration. We can infer from these data that the mechanisms by which bST and IMF in early lactation increase milk yield are complementary due to their non-synergistic nature of enhancing MY.

Effects of administering exogenous bovine somatotropin to beef heifers during the first trimester on conceptus development as well as steroid- and eicosanoid-metabolizing enzymes

The aim of this study was to evaluate the effect of bovine somatotropin (bST) on fetal and placental development during the first third of gestation in beef heifers. Angus heifers (n = 97) were randomly assigned to either receive a 500-mg injection of bST (BST) biweekly on days 0, 15, 29, 43, and 57 of gestation or not receive bST (CTL) throughout the experiment. Body weight (BW) was assessed on days -9, -3, 0, 15, 22, 29, 43, 50, 57, 64, and 77, while blood samples were collected on days 0, 22, 50, and 64. Pregnancy status was determined via transrectal ultrasonography on days 29 and 64. A subset of pregnant heifers (BST, n = 7; CTL, n = 5) were harvested on day 84, and complete gravid reproductive tracts and liver tissue were collected for analysis. Cytochrome P450 1A (CYP1A), 2C (CYP2C), 3A (CYP3A), and uridine 5'-diphospho-glucuronosyltransferase (UGT) activities were determined. Mean change in BW and average daily gain of heifers between fixed-time artificial insemination (day 0) and day 77 did not differ between treatments (P ≥ 0.05). Mean concentrations of insulin-like growth factor 1 (IGF-1) were greater (P < 0.001) in BST (347 ± 27.7 ng/mL) compared with CTL (135 ± 32.8 ng/mL) heifers. Mean placental weight, fetal membrane weight, uterine weight, and ovarian and corpus luteum (CL) weights, as well as fetal morphometric data, did not differ (P ≥ 0.05) between treatments. However, BST heifers had greater (P = 0.03) quantities of combined fetal fluid compared with CTL (521.6 ± 22.9 vs. 429.6 ± 27.14 g, respectively). Tendencies were observed for BST heifers to have reproductive tracts with fewer placentomes (P = 0.08) and fetuses with greater umbilical diameters (P = 0.09) compared with CTL. The activity of CYP1A did not differ (P ≥ 0.05) within the maternal and fetal liver, caruncle, cotyledon, or CL tissue samples between treatments. Furthermore, CYP3A activity was only observed in maternal liver samples and was not different between treatments (P ≥ 0.05). Interestingly, CYP2C activity was greater (P = 0.01) in the liver of BST vs. CTL heifers, and UGT activity was greater (P = 0.02) in the CL from BST heifers compared with CTL. In conclusion, the administration of bST during the first third of gestation increased plasma concentrations of IGF-1, which resulted in an increase in fetal fluid, decrease in placentome number, and greater umbilical diameter, but failed to alter fetal development.

Detection of methionine- and alanine-recombinant bovine somatotropins and their induced antibodies in serum and milk of cows suggests blood-milk barrier specificity for these compounds

The elimination of recombinant bovine somatotropin (rbST) and its induced antibodies through milk of 2 formulations is studied to propose a control strategy for its use or abuse. Two dairy cows were treated with alanine-rbST (Ala-rbST), which is identical to endogenous bovine somatotropin, and ten dairy cows were treated with methionine-rbST (Met-rbST), which differs by 1 amino acid from endogenous bovine somatotropin. We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method able to measure rbST at a decision limit (CCα) of 0.8 ng/mL and 2.3 ng/mL for serum and milk, respectively. The results show that the administered Ala-rbST is transferred from blood to milk but that this is not the case for Met-rbST. This suggests a blood-milk barrier-related specificity for these compounds. In addition, rbST-induced antibodies were formed in animals treated with Ala-rbST and those treated with Met-rbST. In both treatments, the rbST-induced antibodies were transferred from blood to milk, showing no blood-milk barrier specificity for these antibodies. These elimination patterns show that, for enforcement purposes, the detection of rbST-induced antibodies in tank milk can serve to screen for rbST administration, and subsequent confirmatory serum analysis by LC-MS/MS is needed to identify whether Ala-rbST or Met-rbST has been used.

TRIENNIAL LACTATION SYMPOSIUM/BOLFA: Mammary growth during pregnancy and lactation and its relationship with milk yield

The number of secretory cells in the mammary gland is often cited as a major determinant of milk production. However, literature data for proxy measures of secretory cell number do not fully support such a claim. In particular, measurements of total mammary DNA in livestock explain only <25% of variation in milk yield, probably because of tissue heterogeneity for DNA concentration. Relative to BW, measurements of udder size in dairy cattle, as total DNA or udder weight, are approximately double those seen in most other livestock classes. Therefore, selection for dairy production, not surprisingly, has resulted in cows with greater secretory capacity. There is limited evidence that genetic selection is still increasing udder size in some cattle populations, but more recent data are needed. It is contended that the most important period of mammary growth for determination of milk yield is that occurring in pregnancy and early lactation. Mammary development is largely complete, at term, in sheep, goats, and cattle, but in pigs, the udder continues to grow during the first 3 wk of lactation, depending, in part, on litter size. Increased litter size in sheep and goats will enhance the extent of mammary development at the end of gestation (and milk yield) by 20 to 25%. However, twinning in dairy cattle does not affect milk production and, by inference only, is not likely to affect numbers of secretory cells at term. Milking frequency and suckling intensity in very early lactation will increase milk yield in cows and increase mammogenesis and milk yield in sheep, indicating that even at term, the ruminant gland retains some capacity for further development, if demand requires it. There is limited understanding of the hormonal signals in pregnancy that regulate mammary development relative to the number of young carried. Furthermore, the genetic differences between dairy and beef cattle that lead to substantially greater udder size in the dairy breeds have not been identified. During lactation, the drivers for secretory cell loss in relation to milking frequency and nutritional status are still not known. Measurement of mammary development and using this phenotype in genomewide association studies to identify key genetic variants for mammogenesis will provide knowledge that is fundamental to understanding the quantitative regulation of milk production.

A Point Mutation in Suppressor of Cytokine Signalling 2 (Socs2) Increases the Susceptibility to Inflammation of the Mammary Gland while Associated with Higher Body Weight and Size and Higher Milk Production in a Sheep Model

Mastitis is an infectious disease mainly caused by bacteria invading the mammary gland. Genetic control of susceptibility to mastitis has been widely evidenced in dairy ruminants, but the genetic basis and underlying mechanisms are still largely unknown. We describe the discovery, fine mapping and functional characterization of a genetic variant associated with elevated milk leukocytes count, or SCC, as a proxy for mastitis. After implementing genome-wide association studies, we identified a major QTL associated with SCC on ovine chromosome 3. Fine mapping of the region, using full sequencing with 12X coverage in three animals, provided one strong candidate SNP that mapped to the coding sequence of a highly conserved gene, suppressor of cytokine signalling 2 (Socs2). The frequency of the SNP associated with increased SCC was 21.7% and the Socs2 genotype explained 12% of the variance of the trait. The point mutation induces the p.R96C substitution in the SH2 functional domain of SOCS2 i.e. the binding site of the protein to various ligands, as well-established for the growth hormone receptor GHR. Using surface plasmon resonance we showed that the p.R96C point mutation completely abrogates SOCS2 binding affinity for the phosphopeptide of GHR. Additionally, the size, weight and milk production in p.R96C homozygote sheep, were significantly increased by 24%, 18%, and 4.4%, respectively, when compared to wild type sheep, supporting the view that the point mutation causes a loss of SOCS2 functional activity. Altogether these results provide strong evidence for a causal mutation controlling SCC in sheep and highlight the major role of SOCS2 as a tradeoff between the host’s inflammatory response to mammary infections, and body growth and milk production, which are all mediated by the JAK/STAT signaling pathway.

Mastitis is an inflammation of the mammary gland mainly caused by invading bacteria. Ruminants show natural variability in their predisposition to mastitis, and therefore provide unique models for study of the genetics and physiology of host response to bacterial infection. A genome-wide association study was conducted in a dairy sheep population for milk somatic cell counts as a proxy for mastitis. Fine mapping, using whole genome sequencing, led to the identification of a mutation in the Suppressor of Cytokine Signaling 2 gene (socs2). This mutation was shown to cause a loss of functional activity of the SOCS2 protein, which suggested impairment of feedback control of the JAK/STAT signaling pathways in susceptible animals. Additionally, size, weight and milk production were increased in animals carrying the susceptible variant suggesting a pleiotropic effect of the gene on production versus health traits. Results gave strong evidence of the role of SOCS2 in the host’s inflammation of the udder and provided new insights into the key mechanisms underlying the genetic control of mastitis.

Meta-analysis of the effects of sometribove zinc suspension on the production and health of lactating dairy cows

OBJECTIVE

To provide an updated evaluation of the efficacy and safety of sometribove zinc suspension (rbST-Zn), a form of recombinant bovine somatotropin, in lactating dairy cows.

DESIGN

Meta-analysis.

SAMPLE

26 studies published in peer-reviewed journals or reviewed by a regulatory agency.

PROCEDURES

To be included, a study had to involve the use of the rbST-Zn formulation available to US producers in accordance with the label instructions for treatment initiation (57 to 70 days postpartum), dose (500 mg, q 14 d), and route (SC).

RESULTS

For cows treated with rbST-Zn, mean milk, 3.5% fat-corrected milk, fat, and protein yields were increased by 4.00, 4.04, 0.144, and 0.137 kg/d (8.8, 8.89, 0.32, and 0.30 lb/d), respectively; however, the concentration of milk components did not change. Pregnancy proportion for the first 2 breeding cycles was increased by 5.4%, and pregnancy proportion for the duration of the trial was reduced by 5.5% for rbST-Zn-treated cows, compared with proportions for untreated cows. Mean body condition score (1 to 5 scale) was reduced by 0.06 points during the period of rbST-Zn use for treated cows. Administration of rbST-Zn had no effect on milk somatic cell count, the number of days to pregnancy, or inseminations per pregnancy; rates of fetal loss, twins, cystic ovaries, clinical lameness, lameness lesions, or traumatic lesions of the integumentary system; and odds of clinical mastitis or culling.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that rbST-Zn administration to dairy cows effectively increases milk production with no adverse effects on cow health and well-being.

Preweaning milk replacer intake and effects on long-term productivity of dairy calves

The preweaning management of dairy calves over the last 30 yr has focused on mortality, early weaning, and rumen development. Recent studies suggest that nutrient intake from milk or milk replacer during the preweaning period alters the phenotypic expression for milk yield. The objective of this study was to investigate the relationship between nutrient intake from milk replacer and pre- and postweaning growth rate with lactation performance in the Cornell dairy herd and a commercial dairy farm. The analysis was conducted using traditional 305-d first-lactation milk yield and residual lactation yield estimates from a test-day model (TDM) to analyze the lactation records over multiple lactations. The overall objective of the calf nutrition program in both herds was to double the birth weight of calves by weaning through increased milk replacer and starter intake. First-lactation 305-d milk yield and residuals from the TDM were generated from 1,244 and 624 heifers from the Cornell herd and from the commercial farm, respectively. The TDM was used to generate lactation residuals after accounting for the effects of test day, calving season, days in milk, days pregnant, lactation number, and year. In addition, lactation residuals were generated for cattle with multiple lactations to determine if the effect of preweaning nutrition could be associated with lifetime milk yield. Factors such as preweaning average daily gain (ADG), energy intake from milk replacer as a multiple of maintenance, and other growth outcomes and management variables were regressed on TDM milk yield data. In the Cornell herd, preweaning ADG, ranged from 0.10 to 1.58 kg, and was significantly correlated with first-lactation yield; for every 1 kg of preweaning ADG, heifers, on average, produced 850 kg more milk during their first lactation and 235 kg more milk for every Mcal of metabolizable energy intake above maintenance. In the commercial herd, for every 1 kg of preweaning ADG, milk yield increased by 1,113 kg in the first lactation and further, every 1 kg of prepubertal ADG was associated with a 3,281 kg increase in first-lactation milk yield. Among the 2 herds, preweaning ADG accounted for 22% of the variation in first-lactation milk yield as analyzed with the TDM. These results indicate that increased growth rate before weaning results in some form of epigenetic programming that is yet to be understood, but has positive effects on lactation milk yield. This analysis identifies nutrition and management of the preweaned calf as major environmental factors influencing the expression of the genetic capacity of the animal for milk yield.

The environmental impact of recombinant bovine somatotropin (rbST) use in dairy production

The environmental impact of using recombinant bovine somatotropin (rbST) in dairy production was examined on an individual cow, industry-scale adoption, and overall production system basis. An average 2006 U.S. milk yield of 28.9 kg per day was used, with a daily response to rbST supplementation of 4.5 kg per cow. Rations were formulated and both resource inputs (feedstuffs, fertilizers, and fuels) and waste outputs (nutrient excretion and greenhouse gas emissions) calculated. The wider environmental impact of production systems was assessed via acidification (AP), eutrophication (EP), and global warming (GWP) potentials. From a producer perspective, rbST supplementation improved individual cow production, with reductions in nutrient input and waste output per unit of milk produced. From an industry perspective, supplementing one million cows with rbST reduced feedstuff and water use, cropland area, N and P excretion, greenhouse gas emissions, and fossil fuel use compared with an equivalent milk production from unsupplemented cows. Meeting future U.S. milk requirements from cows supplemented with rbST conferred the lowest AP, EP, and GWP, with intermediate values for conventional management and the highest environmental impact resulting from organic production. Overall, rbST appears to represent a valuable management tool for use in dairy production to improve productive efficiency and to have less negative effects on the environment than conventional dairying.

The declining phase of lactation: peripheral or central, programmed or pathological?

In most species the functional activity of the mammary gland during lactation follows a biphasic developmental pattern. This pattern starts with a rapid increase in milk output that occurs with secretory activation and continues with a more gradual increase until the point of peak lactation is reached. Following this gain-of-function phase, the ability of the gland to produce milk decreases. This decrease occurs even if the lactation is prolonged by the presence of continued suckling stimulus and complete milk removal. This review describes the current state of our knowledge concerning the factors that regulate milk synthesis capacity by the mammary gland during the lactation cycle. The review describes four potential alternatives as mechanisms governing the process, which we refer to as secretory diminution. These alternatives are not presented as mutually exclusive of each other or other possible mechanisms, but are proposed as potential contributing mechanisms.

Relationship of Thermal Status to Productivity in Heat-Stressed Dairy Cows Given Recombinant Bovine Somatotropin

The responses of lactating Holstein cows to daily administration of bovine somatotropin (bST) were measured at thermoneutrality (Tn) and under both constant and cycled heat-stress conditions to determine the relationship between thermal status and bST-induced shifts in milk production. All tests included a 5-d acclimation period at Tn (18 degrees C), followed by a 2-d increase in ambient temperature to 28.5 degrees C. After d 3, ambient temperature was cycled between 28.5 (day) and 25.5 degrees C (night) for 4 d. Daily injections with either 31 mg of bST or saline began on d 1 of the experiment. Milk production, feed intake, and respiratory rate (RR) were measured daily. Intraperitoneal, telemetric temperature transmitters were used for a continuous measure of core body temperature (T(core)). Blood samples were collected during each phase to evaluate the changes in serum chemistry in response to bST and heat stress. Following a 15-d recovery, cows were switched across injection treatments and the study was repeated. Milk production decreased by approximately 18.4% below the initial yield at Tn by the end of 7 d of heat challenge. Although a reduction in milk production occurred during heat stress in both groups, milk production was higher in bST-treated cows compared with control cows during periods of constant and cyclic heat. Likewise, bST treatment during the entire period increased the milk-to-feed ratio over the control level by approximately 11.3%. Plasma insulin-like growth factor 1 and serum nonesterified fatty acids accompanied the increased growth hormone level with bST treatment (approximately 122.0 and 88.8%, respectively), whereas plasma urea nitrogen was reduced by approximately 13.3% to reflect the shift to lipid metabolism. There was no difference in T(core) of the treatment and control groups at Tn. Both bST and control cows increased RR and T(core) above the Tn level by approximately 94.8 and 2.9%, respectively, during constant heat, with a greater increase in T(core) of bST-treated compared with control cows (approximately 0.6%). The increase in RR during heat stress preceded T(core) by 1 d for both groups. During cyclic heat, T(core) decreased by approximately 0.4% compared with constant heat in both the control and bST-treated groups. Bovine somatotropin treatment increased milk production similarly during the Tn and heat-stress periods, approximately 8.3% over the control; however, the bST-induced increase in milk-to-feed ratio was greatest during the continuous and cyclic heat-stress phases, approximately 16.2%. This increase occurred together with the elevation in T(core).

A rapid surface plasmon resonance (SPR) biosensor immunoassay for screening of somatotropins in injection preparations

The use of growth hormones (recombinant somatotropins (rSTs)) is approved in several countries, e.g. the USA, Brazil and Australia to enhance growth or lactating performances of livestock. Their use in the EU is banned, however, due to the widespread application, the illegal use within the EU cannot be excluded. To screen for rSTs in injection preparations, a biosensor immunoassay (BIA) using surface plasmon resonance (SPR) technology was developed. Compared to existing analysis methods for rSTs, like radio immunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA), this technique provides a rapid (7 min) alternative. A direct BIA was compared to an indirect (inhibition) BIA and the performances of several antibodies against (r)STs were compared in the indirect BIA. In the final inhibition assay, using rabbit anti-bovine rST, extracts from several injection preparations were shown to contain bovine rST (rbST). The limit of detection for rbST in the assay is 0.008 microg mL(-1) which is far below the expected concentrations in injection preparations. Although the cross-reactivities for STs of other species were low, screening of injection preparations for porcine, equine and human ST was feasible through the analysis of less diluted extracts. Tryptic digestion followed by nano-electrospray liquid chromatography-ion trap tandem mass spectrometry (nano-LC-MS/MS) was used to identify STs.

Fertility of Lactating Dairy Cows Administered Recombinant Bovine Somatotropin During Heat Stress

Administration of recombinant bovine somatotropin (bST) to lactating dairy cows during heat stress increases milk yield, but it also can increase body temperature and may therefore compromise fertility. However, it is possible that bST treatment could increase fertility during heat stress because it has been reported to increase fertility in lactating cows. In addition, bST increases secretion of insulin-like growth factor-I (IGF-I) that promotes embryo survival. The purpose of this study was to determine effects of bST on reproductive function in lactating dairy cows during heat stress. The experiment was conducted in southern Georgia from July to November 2005 using lactating Holstein cows (n = 276 for reproductive traits). For first service timed artificial insemination (TAI), cows were presynchronized with 2 injections of PGF(2alpha) given 14 d apart followed by a modified Ovsynch protocol (GnRH and insemination at 72 h following PGF(2alpha) ). Pregnancy was diagnosed by using ultrasonography on d 29 and reconfirmed by palpation between d 45 and 80 post-TAI. Nonpregnant cows were resynchronized with the modified Ovsynch protocol and received a second TAI. Treatment with bST started 1 wk before the start of Ovsynch and continued at 2-wk intervals. Blood samples were collected from a subset of cows to determine IGF-I profiles immediately before the first bST injection, 1 wk later, and at d 35 of bST treatment. Rectal temperatures were assessed on d 29 of bST treatment. Pregnancy rates (d 45 to 80 post-TAI) did not differ between bST and control cows for first- (16.7 vs. 15.2%) or second-service TAI (14.8 vs. 17.2%). Plasma concentrations of IGF-I and milk yield were greater for bST-treated cows following the initiation of bST treatment and bST increased rectal and vaginal temperatures. Body condition score was less for bST-treated cows. In conclusion, treatment with bST during heat stress increased IGF-I concentrations, milk yield over time, and rectal and vaginal temperatures without affecting first- or second-service pregnancy rates. Thus, at least under certain housing conditions, bST can be used to improve milk yield during heat stress without compromising fertility.

Genetic parameters for yield traits of cows treated or not treated with bovine somatotropin

The objective of this study was to estimate genetic correlations between yield traits of cows treated with bovine somatotropin (bST) and the same yield traits of untreated cows. Lactation records from registered Holstein cows were divided by parity into 3 data sets: 1, 2, and 3 through 5. Approximately 10% of the records in each data set were from cows treated with bST. The numbers of records of treated and untreated cows in the data sets were 4,337 and 48,765; 3,730 and 37,796; and 3,645 and 33,957. Two-trait animal models (records for cows treated or not treated) were used to estimate genetic parameters for milk production traits and somatic cell score (SCS). Estimates of heritability for milk yield for records of treated and untreated cows for the 3 data sets were 0.13, 0.16, and 0.09, and 0.18, 0.18, and 0.14, respectively, with estimates of repeatability of 0.50 and 0.41 for data set 3. Estimates of heritability for fat yield for records of treated and untreated cows were 0.31, 0.16, and 0.12, and 0.27, 0.21, and 0.16. Estimates of repeatability were 0.50 and 0.43 for data set 3. Heritability estimates for protein yield for records of treated and untreated cows were 0.13, 0.17, and 0.12, and 0.20, 0.23, and 0.16, with estimates of repeatability of 0.52 and 0.47. Estimates of heritability for SCS for treated and untreated cows were 0.08, 0.15, and 0.13, and 0.11, 0.13, and 0.13 with repeatability estimates of 0.52 and 0.45. Estimates of genetic correlations between milk yields with and without bST treatment in lactations 1, 2, and 3 to 5 were all 0.99. Estimates of genetic correlations for fat and protein yields were 0.96 for all data sets. Estimates for SCS were 0.99. Estimates of genetic correlations between records of treated and untreated cows were large enough to conclude that records of treated and untreated cows could be considered to be one trait, with treatment as a fixed effect to account for differences in means.

Pregnancy, Bovine Somatotropin, and Dietary n-3 Fatty Acids in Lactating Dairy Cows: I. Ovarian, Conceptus, and Growth Hormone–Insulin-Like Growth Factor System Responses

The objective was to examine effects of bovine somatotropin (bST), pregnancy, and dietary fatty acids on reproductive responses in lactating dairy cows. Beginning at approximately 17 d in milk (DIM), a comparison was made of isoenergetic diets comprising supplementary lipids of whole cottonseed vs. calcium salts of fish oil enriched lipid (FO). Ovulation was synchronized in cows with a presynchronization plus Ovsynch protocol, and cows were inseminated artificially by appointment or not inseminated (d 0 = time of synchronized ovulation; 77 +/- 12 DIM). On d 0 and 11, cows received bST (500 mg) or no bST. All cows were slaughtered on d 17. Number of cows in each group was as follows: control diet had 5 bST-treated cyclic (bST-C), 5 non-bST-treated cyclic (no bST-C), 4 bST-treated pregnant (bST-P), and 5 non-bST-treated pregnant (no bST-P) cows; and cyclic cows fed FO diet had 4 bST-treated (bST-FO) and 5 non-bST-treated cyclic (no bST-FO-C) cows. Feeding FO increased milk production, number of class 1 follicles (2 to 5 mm), and decreased insulin during the period before d 0 compared with control-fed cows. The bST increased milk production, pregnancy rate [83% (5/6) vs. 40% (4/10)], conceptus length (45 vs. 34 cm), and interferon-tau in the uterine luminal flushings (9.4 vs. 5.3 microg) with no effect on interferon-tau mRNA concentration in the conceptus. Treatment with bST increased plasma growth hormone (GH) and insulin-like growth factor (IGF)-I. Among control-fed cows (cyclic and pregnant), bST decreased progesterone concentration in plasma. Cows fed FO had less plasma insulin than control-fed cyclic cows, and FO altered the plasma GH (bST-FO > bST-C) and IGF-I (bST-C > bST-FO-C) responses to bST injections. Endometrial IGF-I mRNA was reduced in pregnant cows and tended to decrease in those fed FO. The IGF-II mRNA was increased in the endometrium of pregnant and bST-treated cows fed the control diet. Cows fed FO had increased concentrations of IGF-II mRNA, when bST was not injected. The insulin-like growth factor binding protein-2 (IGFBP-2) mRNA was increased in bST-P cows, whereas bST decreased the IGFBP-2 mRNA in all cyclic cows. In summary, bST and FO seemed to modulate reproductive responses that may be beneficial to the developing conceptus and pregnancy rate.

Overexpression of des(1-3) insulin-like growth factor 1 in the mammary glands of transgenic mice delays the loss of milk production with prolonged lactation

During prolonged lactation, the mammary gland gradually loses the capacity to produce milk. In agricultural species, this decline can be slowed by administration of exogenous growth hormone (GH), which is believed to act through insulin-like growth factor 1 (IGF1). Our previous work demonstrated delayed natural mammary gland involution in des(1-3)IGF1-overexpressing transgenic mice (Tg[Wap-des{1-3}IGF1]8266 Jmr), hereafter referred to as WAP-DES mice. The present study tested the hypothesis that overexpressed des(1-3)IGF1 would delay the loss of milk production during prolonged lactation. Accordingly, we examined lactational performance in WAP-DES mice by artificially prolonging lactation with continual litter cross-fostering. Over time, lactational capacity and mammary development declined in both WAP-DES and control mice. However, the rate of decline was 40% slower in WAP-DES mice. Mammary cell apoptosis increased by 3-fold in both groups during prolonged lactation but was not different between genotypes. Plasma concentrations of murine IGF1 were decreased in WAP-DES mice, while those of the transgenic human IGF1 were elevated during prolonged lactation. Phosphorylation of the mammary IGF1 receptor was increased in the WAP-DES mice, but only during prolonged lactation. Plasma prolactin decreased with prolonged lactation in nontransgenic mice but remained high in WAP-DES mice. The WAP-DES mice maintained a higher body mass and a greater lean body mass during prolonged lactation. These data support the conclusion that overexpressed des(1-3)IGF1 enhanced milk synthesis and mammary development during prolonged lactation through localized and direct activation of the mammary gland IGF1 receptor and through systemic effects on prolactin secretion and possibly nutrient balance.

Bovine mammary progenitor cells: current concepts and future directions

Although cell number is positively correlated with milk production, much remains to be learned about the bovine mammary stem cell and progenitor cells. Bovine mammary development is driven by many of the same classic mammogenic hormones studied in murine models, yet histologic features of bovine mammary development differ from that of rodent models. Most notably, terminal end buds, as they have been described for murine models, do not exist in the bovine mammary gland. However, among the most important common features of mammary development in disparate species is the involvement of histologically distinct, lightly staining epithelial cells as putative stem and progenitor cells. Although stem cell research has often focused on mammary development, mammary stem cells seemingly provide the basis for mammary growth and cell turnover in the mature animal. These cells provide an obvious focus for research aimed at increasing the efficiency of milk production. This review addresses recent findings concerning the histology and molecular physiology of putative bovine mammary stem and progenitor cell populations, areas where more study is critically needed, and areas where studies of bovine mammary physiology may present a unique opportunity to better understand mammary physiology in many species.

Inseminations at Estrus Induced by Presynchronization Before Application of Synchronized Estrus and Ovulation

A controlled field study examined conception rates after 2 timed artificial insemination (TAI) breeding protocols conducted on 2 commercial dairy farms. Estrous cycles in postpartum lactating cows were presynchronized with 2 injections of PGF(2alpha) given 14 d apart (Pre-synch) and then, after 12 d, the standard Ovsynch protocol (injection of GnRH 7 d before and 48 h after an injection of PGF(2alpha), with one TAI at 12 to 16 h after the second GnRH injection) or Heatsynch protocol [injection of GnRH 7 d before an injection of PGF(2alpha), followed 24 h later by 1 mg of estradiol cypionate (ECP) and one TAI 48 h after ECP] was applied. Experimental design allowed artificial insemination to occur anytime after the second Presynch injection and during the designed breeding week when estrus was detected. Of the 1846 first services performed, only 1503 (rate of compliance = 81.4%) were performed according to protocol. Numbers of cows inseminated, logistic-regression adjusted conception rates, and days in milk (DIM) were for inseminations made: 1) during 14 d after first Presynch injection (n = 145; 22.6%; 54 +/- 0.4 DIM); 2) during 12 d after second Presynch injection (n = 727; 33%; 59 +/- 0.2 DIM); 3) during 7 d after the first GnRH injection of Ovsynch or Heatsynch (n = 96; 32.1%; 74 +/- 0.5 DIM); 4) after estrus as part of Heatsynch (n = 212; 44.6%; 76 +/- 0.3 DIM); 4) after TAI as part of Heatsynch (n = 154; 21.1%; 76 +/- 0.4 DIM); 5) after estrus as part of Ovsynch (n = 43; 48.7%; 77 +/- 0.7 DIM); and 6) after TAI as part of Ovsynch (n = 271; 24.4%; 77 +/- 0.3 DIM). Conception rates when AI occurred after one Presynch injection were less than when AI occurred after 2 Presynch injections. Conception rates for those inseminated after either Presynch injection did not differ from those inseminated after combined Heatsynch + Ovsynch. Cows in the Ovsynch and Heatsynch protocols inseminated after estrus during the breeding week had greater conception rates than those receiving the TAI, but overall conception rates did not differ between protocols. Among cows inseminated after detected estrus, conception was greater for cows in the Heatsynch + Ovsynch protocol (77 +/- 0.4 DIM) than for those inseminated after either Presynch injection (54 +/- 0.4 or 59 +/- 0.2 DIM). We concluded that conception rates after Heatsynch and Ovsynch were similar under these experimental conditions, and that delaying first AI improved fertility for cows inseminated after detected estrus.

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.

Induced Lactation in Nonpregnant Cows: Profitability and Response to Bovine Somatotropin

Significant culling of high-producing cows with low fertility reduces profitability of dairy farms as those cows are replaced with heifers. Induced lactation of nonpregnant cows may be a management alternative to reduce culling and increase profits. The objectives of this study were to evaluate the efficacy of bovine somatotropin (bST) to increase milk production in cows induced into lactation with estrogen plus progesterone, and to determine the profitability of inducing cows into lactation vs. using replacement heifers entering the herd as first-lactation cows. Parity 1 or greater, nonpregnant, healthy Holstein cows (n = 28) were induced into lactation by administration of estradiol-17beta (0.075 mg/kg of body weight [BW] per d) and progesterone (0.25 mg/kg of BW per d) for 7 d. Milking began on d 18. Cows were randomly assigned to control or bST treatment groups on d 37 +/- 20 of milking, and milk production was compared for 70 d. After the 70-d comparison, all cows received bST for the duration of lactation. Cows receiving bST produced more milk (28.4 kg/d) than controls (24.1 kg/d), with variable yields among cows. For the economic analysis, induced cows were compared to first-lactation cows in the same herd using fair market value for costs and multiple component pricing for milk. Net present value for an induced cow (1966 dollars) was significantly greater than that for a first-lactation cow (1446 dollars). Our data suggest that bST use in induced cows is profitable. If a reliable method were developed and approved by the FDA, inducing nonpregnant cows into lactation could be used by dairy producers to increase profitability.

Effect of bST and Reproductive Management on Reproductive Performance of Holstein Dairy Cows

The objective was to determine the effects of bovine somatotropin (bST) and two artificial insemination (AI) protocols on reproductive performance of Holstein cows. Lactating cows (n = 840) were assigned at 37 d in milk (DIM) to one of four treatments in a 2 x 2 factorial arrangement. Treatments consisted of either bST (500 mg/14 d) starting at 63 +/- 3 DIM or no bST (control), with cows either submitted for timed AI following a synchronized ovulation (Ovsynch) protocol or assigned to receive AI based on estrus detection (ED). Two injections of PGF2, at 37 +/- 3 and 51 +/- 3 DIM were used to presynchronize estrous cycles. Cows then received an injection of GnRH at 63 +/- 3 DIM, followed 7.5 d later by PGF2. Cows assigned to ED treatments were inseminated after observed estrus during a 7-d period. Cows in Ovsynch treatments received a second GnRH injection 48 h after the last PGF2alpha and received timed AI 16 to 18 h later. Pregnancy was diagnosed by ultrasound at 31 d after AI and confirmed 14 d later. Frequency of anovulation (18.5%) at 63 DIM was similar across treatments, but proportions of anovulatory cows decreased quadratically as body condition at 70 DIM increased from 2.25 to 3.75. Estrus detection rate after PGF2alpha tended to be lower in multiparous cows receiving bST, and bST reduced returns to estrus in nonpregnant cows. Conception rates were higher in cows receiving AI after ED and bST improved conception rates to first AI in cyclic cows by reducing embryonic mortality. Pregnancy loss was similar for cows inseminated following ED or the Ovsynch protocol. There was a positive impact of bST on fertility of cyclic cows inseminated at fixed time or at detected estrus, but effective resynchronization protocols are needed to optimize reinsemination of non-pregnant bST-treated cows.

Within-Herd Effects of Age at Test Day and Lactation Stage on Test-Day Yields

Variance ratios were estimated for random within-herd effects of age at test day and lactation stage, on test-day yield and somatic cell score to determine whether including these effects would improve the accuracy of estimation. Test-day data starting with 1990 calvings for the entire US Jersey population and Holsteins from California, Pennsylvania, Wisconsin, and Texas were analyzed. Test-day yields were adjusted for across-herd effects using solutions from a regional analysis. Estimates of the relative variance (fraction of total variance) due to within-herd age effects were small, indicating that regional adjustments for age were adequate. The relative variances for within-herd lactation stage were large enough to indicate that accuracy of genetic evaluations could be improved by including herd stage effects in the model for milk, fat, and protein, but not for somatic cell score. Because the within-herd lactation stage effect is assumed to be random, the effect is regressed toward the regional effects for small herds, but in large herds, lactation curves become herd specific. Model comparisons demonstrated the greater explanatory power of the model with a within-herd-stage effect as prediction error standard deviations were greater for the model without this effect. The benefit of the within-herd-stage effects was confirmed in a random regression model by comparing variance components from models with and without random within-herd regressions and through log-likelihood ratio tests.

Lactation persistency: Insights from mammary cell proliferation studies

A persistent lactation is dependent on maintaining the number and activity of milk secreting cells with advancing lactation. When dairy cows are milked twice daily, the increase in milk yield from parturition to peak lactation is due to increased secretory activity per cell rather than to accretion of additional epithelial cells. After peak lactation, declining milk yield is due to loss of mammary epithelial cells by apoptosis. During lactation, only 0.3% of mammary cells proliferate in a 24-h period. Yet this proliferative rate is sufficient to replace most mammary epithelial cells by the end of lactation. Management practices can influence lactation persistency. Administration of bovine somatotropin may enhance persistency by increasing cell proliferation and turnover, or by reducing the rate of apoptosis. Increased photoperiod may also increase persistency of lactation by mechanisms that are as yet undefined. Increased milking frequency during the first weeks of lactation increases milk yield, even after return to less frequent milking, with increases of approximately 8% over the entire lactation. A mammary cell proliferation response to frequent milking during early lactation appears to be involved. Conversely, advanced pregnancy, infrequent milking, and mastitis increase death of epithelial cells by apoptosis. Regulation of mammary cell renewal provides a key to increasing persistency. Investigations to characterize epithelial cells that serve as the proliferative population in the bovine mammary gland have been initiated. Epithelial cells that stain lightly in histological sections are evident through all phases of mammary development and secretion and account for nearly all proliferation in the prepubertal gland. Characterization of these cells may provide a means to regulate mammary cell proliferation and thus to enhance persistency, reduce the effects of mastitis, and decrease the necessity for a dry period.

Molecular dissection of a quantitative trait locus: a phenylalanine-to-tyrosine substitution in the transmembrane domain of the bovine growth hormone receptor is associated with a major effect on milk yield and composition

We herein report on our efforts to improve the mapping resolution of a QTL with major effect on milk yield and composition that was previously mapped to bovine chromosome 20. By using a denser chromosome 20 marker map and by exploiting linkage disequilibrium using two distinct approaches, we provide strong evidence that a chromosome segment including the gene coding for the growth hormone receptor accounts for at least part of the chromosome 20 QTL effect. By sequencing individuals with known QTL genotype, we identify an F to Y substitution in the transmembrane domain of the growth hormone receptor gene that is associated with a strong effect on milk yield and composition in the general population.

The composition of bovine milk lipids: January 1995 to December 2000

Data from recent publications on bovine milk lipids are presented and discussed. This includes extraction of lipids, triacylglycerols, phospholipids, other complex lipids, sterols, isoflavones, and fatty acids. Improved gas-liquid and high performance liquid chromatography were used. Data on the trans and cis isomers of fatty acid and of conjugated linoleic acids are given, and the analyses are described. Papers about the lipids in milks and dairy products from the United States are few; where with the exception of trans-fatty acid isomers and conjugated linoleic acids, almost no research has been reported.

Timing of onset of somatotropin supplementation on reproductive performance in dairy cows

The objective of this field trial was to determine whether delaying the start of recombinant bovine somatotropin (rbST) supplementation from 9 to 10 wk postpartum to 17 to 18 wk postpartum would improve reproductive performance in lactating dairy cows. Cows from nine herds (n = 798 cows; 766 Holsteins, 32 Jerseys) were assigned at random to receive rbST supplementation at 14-d intervals beginning during wk 9 to 10 (n = 399) or wk 17 to 18 (n = 399) after calving. Effects of herd, season of calving, parity, and onset of rbST supplementation (9 to 10 wk vs. 17 to 18 wk) on days to first service and days open were determined. In primiparous but not multiparous cows, there tended to be fewer days to first service and fewer days open when onset of rbST supplementation was delayed. Percentages of cows pregnant at 150, 200, and 250 d postpartum were also examined. Time of onset of rbST did not affect percentages of multiparous cows pregnant at 150, 200, and 250 d postpartum. However, there appeared to be a slight tendency for percentages of pregnant primiparous cows to be greater at 200 and 250 d postpartum for those receiving rbST supplementation beginning at 17 to 18 wk compared to those receiving rbST starting at 9 to 10 wk. In conclusion, delaying the start of rbST supplementation to wk 17 to 18 postpartum had no beneficial effect on reproductive performance of multiparous cows but tended to improve some measures of reproductive performance in primiparous cows.

Lactation and gestation in dairy cows: flexibility avoids nutritional extremes

The modern dairy cow has been selectively bred to produce large amounts of milk. Partly as a result, food consumption is considerably less than milk energy output in early lactation. It is only at 2 months or more postpartum that intake increases to the point where positive energy balance is regained, the initial production being achieved by a substantial mobilisation of body reserves. These reserves are laid down before parturition, but it is certainly not the case that the pregnant cow will accumulate adipose tissue recklessly; in the last third of pregnancy well-fed cows in good body condition exhibit reduced, not increased, appetite. There is a fine balancing act to perform. Excessive body condition at parturition quickly leads to metabolic problems such as ketosis, but cows who subsequently become too thin have increased risk of metabolic diseases such as mastitis and lameness. The biological mechanisms regulating output of milk are reasonably well understood, those controlling appetite less well so, and there has been little attempt at systematic integration of the two. The transition from pregnancy to lactation represents a major challenge to homeostasis, made more complicated in multiparous cows by the fact that much of gestation is concurrent with lactation. Herein lies the potential for nutritionally-entrained flexibility. In the wild, concurrent pregnancy and lactation only occur when nutritional conditions are favourable. If conditions are poor, rebreeding will be delayed and lactation will continue, at an energetically-sustainable level, for much longer than its 'normal' duration. In this way the twin energetic burdens of pregnancy and lactation are separated, and extremes are avoided. Given the increasing public concern about stresses suffered by intensively-managed dairy cows, this case may be one where commercial dairying could learn useful lessons from nature.

Mammary cell number, proliferation, and apoptosis during a bovine lactation: relation to milk production and effect of bST

This investigation evaluated mammary cell loss and replacement during lactation and the impact of administration of bST on these processes. During lactation, a gradual decrease in number of mammary epithelial cells within the mammary glands occurs and largely accounts for the decline in milk production with advancing lactation. This decrease is not appreciably impacted by the loss of viable epithelial cells in milk. Rather, the net decline in cell number (approximately 50% during the entire lactation) results from continual death by apoptosis. Accompanying the decline in mammary cell number by apoptosis is a degree of cell renewal. Approximately 0.3% of mammary cells in lactating, nonpregnant cows were labeled by a 24-h in vivo treatment with the thymidine analog, bromodeoxyuridine. During the entire lactation, the number of new cells amounts to approximately 50% of the number of cells initially present. By the end of lactation, most cells present in the mammary gland were formed after calving. Increasing cell replacement or decreasing apoptosis during lactation may provide a means to increase persistency of lactation. Indeed, administration of bST to Holstein cows during midlactation increased the proportion of mammary epithelial cells expressing the nuclear proliferation antigen, Ki-67, from 0.5 to 1.6%. Bovine somatotropin appears to increase the rate of cell renewal in the lactating mammary gland. Knowledge of molecular regulation of apoptosis and cell proliferation should provide a means to modulate cell turnover in the mammary gland. A change in the ratio of epithelial proliferation to cell death during lactation will affect the persistency of lactation.

Reproductive loss in high-producing dairy cattle: where will it end?

The dairy industry in the United States has changed dramatically in the last decade. Milk production per cow has increased steadily because of a combination of improved management, better nutrition, and intense genetic selection. Dairy farms are larger, and nearly 30% of the dairy cows in the United States are on farms with 500 or more cows. The shift toward more productive cows and larger herds is associated with a decrease in reproductive efficiency. Cows with the greatest milk production have the highest incidence of infertility, but epidemiological studies suggest that, in addition to milk production, other factors are probably decreasing reproductive efficiency in our dairy herds. The reproductive physiology of dairy cows has changed over the past 50 yr, and physiological adaptations to high milk production may explain part of the reproductive decline. Critical areas for new research include control of the estrous cycle, metabolic effects of lactation on reproduction, mechanisms linking disease to reproduction, and early embryonic mortality. Solving reproductive loss in dairy cows will not be easy because only a small number of research groups study reproduction in postpartum dairy cows. Therefore, the present research base will need to be expanded. For this to occur, research funding must be increased above its current level and a renewed emphasis must be placed on solving the emerging crisis of infertility in dairy cows.

Effects of sustained release bovine somatotropin (sometribove) on animal health in commercial dairy herds

The health of dairy cows given bovine somatotropin (bST) for one lactation was evaluated in 28 commercial herds located in four regions of the United States. At least six herds were in a region and at least one herd/region contained fewer than 60 cows. Cows (n = 1213) were assigned randomly to control or bST groups and were treated beginning in wk 9 to 10 of lactation and every 14 d until dry-off or d 400 of lactation. Management was according to site practices. Cows were observed for health-related signs by farm personnel daily and by the herd veterinarian biweekly. Average 305-d test-day milk yields were 932 kg greater for bST-treated cows. Pregnancy rates, days open, twinning, cystic ovaries, or abortions were unaffected by treatments. Supplementation of cows with bST had no effect on total mastitis cases, total days of mastitis, duration of mastitis, or the odds ratio of a cow to develop mastitis. Cows supplemented with bST used more medications for health events other than mastitis. This usage was associated primarily with treatments for disorders of the foot and hock. Supplemented cows had a slight increase in foot disorders. There was no effect of supplementation with bST on culling from the herd or removal from study. Overall, the results confirm that label directions for bST are adequate for safe use under field conditions. All clinical signs observed in this study occur normally in dairy herds and were managed in cows supplemented with bST.

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.

Bovine somatotropin and lactation: from basic science to commercial application

Bovine somatotropin (bST) results in increased milk yield and an unprecedented improvement in efficiency. Beginning in the 1930s to present day, investigations have examined animal-related factors such as nutrition, bioenergetics, metabolism, health and well being and consumer-related factors such as milk quality, manufacturing characteristics, and product safety. Overall, bST is a homeorhetic control involved in orchestrating many physiological processes. Direct effects involve adaptations in many tissues and the metabolism of all nutrient classes--carbohydrates, lipids, protein, and minerals. Mechanisms include alterations in key enzymes, intracellular signal transduction systems, and tissue response to homeostatic signals. Indirect effects involve the mammary gland and are thought to be mediated by the insulin-like growth factor (IGF) system. Specific changes include increased cellular rates of milk synthesis and enhanced maintenance of secretory cells. Indirect effects are modulated by environment and management factors, especially nutritional status. This modulation is a central component in allowing ST to play a key role in regulating nutrient utilization across a range of physiological situations. U.S. commercial use began in 1994, and adoption has been extensive. From a consumer perspective, bST was unique, and special interest groups loudly predicted dire consequences. However, introduction of bST had no impact on milk consumption, and milk labeled as recombinant bST-free occupies a minor niche market. From a producer perspective, commercial use verified scientific studies and enhanced net farm income. Overall, ST is a key homeorhetic control regulating nutrient partitioning, and the ST/IGF system plays a key role in animal performance and well being across a range of physiological situations.