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.

Pre- and post-weaning nutrition status affects sheep mammary gland morphology and promotes differential gene expression

Little is known about how varying the plane of nutrition before and after weaning can influence gene expression that drives mammary gland development (MGD). Therefore, the aim of this study was to investigate this paradigm in a seasonal sheep breed. Forty pre-weaning 30-day old Kurdish female lambs were fed either a low-nutrient and protein (L; 2.0 Mcal ME/kg DM; 8.70% crude protein [CP]) or high-nutrient and protein diet (H; 2.5 Mcal ME/kg DM; 14.80% CP). At d 120 (weaning), lambs were allocated into a 2 × 2 arrangement based on pre- and post-weaning diets, resulting in four groups (LL, LH, HL, HH). On d 210, mammary biopsy samples were taken for histomorphological and gene expression studies. Alveoli were larger in HH compared with all other groups (p < 0.05). Whilst estrogen receptor-alpha, progesterone receptor, growth hormone receptor and insulin-like growth factor binding protein-1 gene expression was modulated depending on pre- or post-weaning nutritional levels (p < 0.01). Overall, a consistent high plane of nutrition promoted MGD, with more a complex relationship evident between the expression of genes critical for mammary gland function and development. These findings provide scope for future tailored nutritional strategies to optimize both liveweight gain and MGD.

Udder Ultrasonography of Dairy Cows: Investigating the Relationship between Echotexture, Blood Flow, Somatic Cell Count and Milk Yield during Dry Period and Lactation

Udder health of dairy cows is related to their productivity and welfare. The period from dry-off to calving and early lactation is crucial. Ultrasonography is a useful and practical tool for the examination of the mammary parenchyma and blood flow. This observational study investigated the relationship between udder echotexture features, blood flow volume (BFVol) in the milk vein, milk somatic cell count (SCC) and daily milk yield (DMY) from late lactation, throughout the dry period and consecutive early lactation. Seventeen repeated measurements were performed on twenty-one Holstein cows. The udder parenchyma was examined with B-mode ultrasonography. Udder echotexture was studied using 15 features: Numerical Pixel Value (NPV), Pixel Standard Deviation (PSD), Skewness, Excess, Contrast, Homogeneity, Correlation, Entropy, Run Percentage, Long-Run Emphasis, Grey Value Distribution, Runlength Distribution, Gradient Mean Value, Gradient Variance and Percentage of Non-zero Gradients. Blood flow in the milk vein was examined with spectral Doppler. Linear mixed-effects models were employed to investigate relationships between BFVol, udder echotexture features, SCC and DMY throughout the study period. Our models showed that a 1 kg increase in DMY was associated with a significant increase of 0.25 L/min in the expected BFVol and that a 1,000,000-cells/mL increase in SCC was associated with a significant BFVol decrease of 0.49 L/min, keeping all other variables constant. Multivariable models showed significant associations between DMY and NPV, between PSD and Long-Run Emphasis, and between SCC and NPV, PSD, Gradient Mean Value, Homogeneity, Gradient Variance and Entropy. In conclusion, udder echotexture and BFVol in the milk vein are related to SCC and milk yield. Ultrasonography can be used for the comprehensive assessment of udder health in support of precision dairy farming.

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.

In utero hyperthermia in late gestation derails dairy calf early-life mammary development

Prenatal hyperthermia has immediate and long-term consequences on dairy cattle growth, immunity, and productivity. While changes in the molecular architecture are reported in the mature mammary gland (MG), any influence on early-life mammary development is unknown. Herein, we characterize the impact of late-gestation in utero heat stress on heifer mammary gross and cellular morphology at early-life developmental stages (i.e., birth and weaning). During summer, pregnant dams were exposed to environmental heat stress (shade of a free-stall barn) or offered active cooling (shade, fans, and water soakers) for 54 ± 5 d before parturition (avg. temperature-humidity index = 79). Heifer calves born to these dams were either in utero heat-stressed (IU-HT; n = 36) or in utero cooled (IU-CL; n = 37) and were managed as a single cohort thereafter. A subset of heifers was euthanized at birth (d0; n = 8/treatment; 4.6 ± 2.3 h after birth) and after weaning (d63; n = 8/treatment; 63.0 ± 1.5 d) to harvest the whole MG. An ultrasound of rear mammary parenchyma (MPAR) was taken prior to d63 and correlated to harvested MPAR cross-sectional area and weight. Portions of mammary fat pad (MFP) and MPAR were preserved for compositional and histological analysis, including ductal structure number and cross-sectional area, connective tissue area, and adipocyte number and cross-sectional area. Cellular proliferation in MPAR was assessed via Ki-67 immunohistochemistry. Relative to IU-CL heifers, the MGs of IU-HT heifers were shorter in length at d0 and d63 (P ≤ 0.02). There were moderate correlations between d63 ultrasound and harvest measures. The IU-HT heifers had reduced MG and MFP mass at d0 and d63 (P ≤ 0.05), whereas MPAR mass was reduced only at d0 (P = 0.01). IU-HT heifers had greater MPAR protein and DNA content at d63 (P ≤ 0.04), but there were no MFP compositional differences (P ≥ 0.12). At d0, IU-HT heifers had fewer MPAR ductal structures (P ≤ 0.06), but there were no differences at d63. Yet, MPAR luminal and total ductal structure cross-sectional areas of IU-HT heifers were reduced at both d0 and d63 (P ≤ 0.01). The MFP adipocytes of IU-HT heifers were smaller at d0 (P ≤ 0.01), but differences were not detected at d63. The IU-HT heifers had diminished MPAR total, stromal, and epithelial cellular proliferation at both d0 and d63 (P < 0.01). Prenatal hyperthermia derails dairy calf early-life mammary development with potential carry-over consequences on future synthetic capacity.

Outcome prediction from the first examination in clinical mastitis using ultrasonography in dairy cows

The aim of this study was to examine whether ultrasonography can be used to predict the outcome of clinical mastitis in dairy cows. Forty-seven mastitic quarters of Holstein-Friesian cows were examined using ultrasonography at the time of the first examination. In mastitic mammary tissue, three sonographic signs indicating tissue abnormality were found: a hyperechoic spot in the parenchyma area, structural changes to the milk duct, and non-homogeneous parenchyma. Logistic regression was used to evaluate whether the abnormal findings in the sonographic images can be used to predict the outcome of clinical mastitis. The outcomes of clinical mastitis were defined by the return, or failure to return, to marketable milk production. The sonogram finding of non-homogeneous parenchyma in the first examination did predict the outcome of clinical mastitis, whereas the type of systemic symptoms (severe or moderate) was not a predictor in this regression model. Therefore, ultrasound examinations of mammary glands in the first examination could be a useful method for predicting outcome of clinical mastitis. There is an economic benefit if ultrasound examination in first examination helps in the decision of whether or not to treat the mastitic cows.

Effect of whole-milk allowance on liveweight gain and growth of parenchyma and fat pads in the mammary glands of dairy heifers at weaning

The rates of development of 2 tissues in mammary glands, parenchyma (PAR) and the mammary fat pad (MFP), in response to nutrition in early life might have a major bearing on lifetime milk production. Historical studies reported that feeding greater amounts of dietary nutrients from postweaning to puberty increased growth rates of heifers and stimulated the growth of MFP at the expense of PAR, which might suggest compromised mammary development and future milk production. The current study sought to determine if a higher volume of whole milk (8 vs. 4 L/d) offered to calves would increase rates of growth and development of PAR in mammary glands at weaning (1 to 12 wk). To measure these tissues, we developed 2 simple methods to assess the size of PAR and MFP at the time of screening using ultrasound. We report that calves offered 8 L/d of whole milk had greater rates of growth until weaning (0.86 ± 0.06 vs. 0.81 ± 0.09 kg/d), compared with calves offered 4 L/d. Ultrasonography showed that despite the faster rates of growth in calves offered 8 L/d of milk/d, the ratio of PAR:MFP depth was 40% less at weaning in the front glands (34%) compared with calves offered 4 L of milk/d. Rear glands were less impaired. The ultrasound methods developed here might be useful to monitor the development of mammary glands in response to different nutritional regimens during the preweaning period.

Preweaning nutrient supply alters mammary gland transcriptome expression relating to morphology, lipid accumulation, DNA synthesis, and RNA expression in Holstein heifer calves

The objective of this study was to analyze the mammary gland transcriptome to determine how preweaning nutrient supply alters the molecular mechanisms that regulate preweaning mammary development. Holstein heifers were fed via milk replacer (MR) either an elevated level of nutrient intake (ELE; on average, 5.9 ± 0.2 Mcal of ME in 8.4 L of MR/d, n = 6) or a restricted amount of nutrients (RES; 2.8 ± 0.2 Mcal of ME in 4 L of MR/d, n = 5) for 54 d after birth, at which point they were slaughtered and samples of mammary parenchyma tissue were obtained. Parenchymal mRNA was analyzed, and the fold change (FC) of 18,111 genes (ELE relative to RES) was uploaded to Ingenuity Pathway Analysis (IPA) software (Qiagen Bioinformatics, Redwood City, CA) for transcriptomic analysis. Using a threshold of P < 0.05, IPA identified that the FC of 1,931 of 18,811 differentially expressed genes (DEG) could be used for the analysis. A total of 18 molecular and cellular functions were relevant to DEG arising from the treatments; the 5 functions most associated with DEG were cell death and survival, cellular movement, cellular development, cellular growth and proliferation, and lipid metabolism. Based on the directional FC of DEG, the mammary gland of ELE heifers was predicted to have increased epithelial-mesenchymal transition (Z = 2.685) and accumulation of lipid (Z = 2.322), whereas the synthesis of DNA (Z = -2.137), transactivation of RNA (Z = -2.254), expression of RNA (Z = -2.405), transcription (Z = -2.482), and transactivation (Z = -2.611) were all predicted to be decreased. Additionally, IPA predicted the activation status of 13 upstream regulators with direct influence on DEG as affected by ELE feeding that were ligand-dependent nuclear receptors (n = 2), enzymes (n = 1), or transcription regulators (n = 10). Of these, 6 were activated (Z > 2) and 7 were inhibited (Z < -2). In summary, feeding ELE preweaning altered the mammary transcriptome of Holstein heifers, affecting cell functions involved in the morphological and physiological development of the mammary gland.

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.

In vivo measures of mammary development in gestating gilts

Potential links between measures of udder morphology obtained in live pregnant gilts and mammary gland development and composition measured in mammary tissue collected at slaughter were studied. Thirty-three gilts were used. In vivo measures of gland morphology using a tape or ultrasound imaging (parenchymal area measured by ultrasound [AREA]) were obtained on d 108 ± 1 of gestation. Gilts were then slaughtered on d 110 ± 1 of gestation to collect mammary glands for dissection and compositional analyses. The various tape measures were the distance between each teat on one side of the udder (DIST-TEAT), the distance between each teat pair (DIST-PAIR), the length of the udder (sum of all DIST-TEAT), the distance between the base of the teat and the ventral midline section of the udder (MID), and the distance between the base of the teat and the exterior junction of the udder with the abdomen (EXT). The variables MID, DIST-TEAT, DIST-PAIR, and length had very poor correlations with parenchymal weight, extraparenchymal weight, or any of the measured compositional variables. On the other hand, both AREA and EXT were correlated ( < 0.01) with the weight of parenchymal tissue, total parenchymal protein, total DNA, and total RNA. The ultrasound measure AREA and the tape measure EXT were also correlated with each other ( < 0.05). These measures could, therefore, be helpful to estimate mammary development in studies where animals cannot be slaughtered. The tape measure EXT seemed to better reflect the volume of the gland than MID, and it provided as reliable an estimate of parenchymal weight as the measure of parenchymal area using ultrasound while being much easier and cheaper to obtain.

Performance strategies affect mammary gland development in prepubertal heifers

In Brazil, the majority of dairy cattle are Holstein × Gyr (H×G). It is unknown whether excessive energy intake negatively affects their mammary development to the same extent as in purebred Holsteins. We hypothesized that mammary development of H×G heifers can be affected by dietary energy supply. We evaluated the effect of different average daily gains (ADG) achieved by feeding different amounts of a standard diet during the growing period on biometric measurements, development of mammary parenchyma (PAR) and mammary fat pad (MFP), and blood hormones. At the outset of this 84-d experiment, H×G heifers (n = 18) weighed 102.2 ± 3.4 kg and were 3 to 4 mo of age. Heifers were randomly assigned to 1 of 3 ADG programs using a completely randomized design. Treatments were high gain (HG; n = 6), where heifers were fed to gain 1 kg/d; low gain (LG; n = 6), where heifers were fed to gain 0.5 kg/d; and maintenance (MA; n = 6), where heifers were fed to gain a minimal amount of weight per day. Heifers were fed varying amounts of a single TMR to support desired BW gains. Over the 84 d, periodic biometric and blood hormone measurements were obtained. On d 84, all heifers were slaughtered and carcass and mammary samples were collected. At the end, HG heifers weighed the most (181 ± 7.5 kg), followed by LG (146 ± 7.5 kg) and MA (107 ± 7.5 kg) heifers. The ADG were near expected values and averaged 0.907, 0.500, and 0.105 ± 0.03 kg/d for HG, LG, and MA, respectively. In addition, body lengths, heart girths, and withers heights were affected by dietary treatment, with MA heifers generally being the smallest and HG heifers generally being the largest. Body condition scores differed by treatment and were highest in HG and lowest in MA heifers; in vivo subcutaneous fat thickness measurement and direct analysis of carcass composition supported this. The HG heifers had the heaviest MFP, followed by LG and then MA heifers. Amount of PAR was highest in LG heifers and was the same for HG and MA heifers. The percentage of udder mass occupied by PAR was lowest in HG heifers, differing from LG and MA heifers. Composition of MFP was not evaluated. Regarding PAR composition, no differences in ash or DM were found. On the other hand, CP concentration of PAR for HG heifers was lower than that for LG heifers, which was lower than that for MA heifers. Regarding the fat content, HG treatment was higher than LG and MA treatment, which did not differ from each other. In PAR, differences in relative abundance of genes related to both stimulation and inhibition of mammary growth were observed to depend on dietary treatment, sampling day, or both. The same can be said for most of the blood hormones that were measured in this experiment. In this experiment, high ADG achieved by feeding different amounts of a standard diet during the growing period negatively affected mammary development.

Technical note: Mammary gland ultrasonography to evaluate mammary parenchymal composition in prepubertal heifers

Bovine mammary gland development studies are often terminal or involve invasive biopsy procedures. Therefore, noninvasive means of assessing mammary development should be considered as alternative methods in live animals. The objective was to test if mammary ultrasonography can be used as a noninvasive way to estimate mammary parenchyma (PAR) composition in prepubertal dairy heifers with different average daily body weight gains. In the 84 d preceding, the ultrasound exam heifers were maintained in 1 of 3 treatment groups. Individual heifers were fed a high gain (1 kg/d; n = 6), low gain (0.5 kg/d, n = 6), or maintenance (n = 6) treatment diet. To achieve desired body weight gains, heifers were fed differing amounts of the same silage-based diet. Mammary glands of 18 crossbred heifers Holstein:Gyr underwent a single mammary ultrasound exam immediately before heifer slaughter, which took place when heifers weighed 142.0 ± 8.0 kg and were 200 d old. The 4 mammary glands of each heifer were evaluated using a real-time B-mode ultrasound machine equipped with a 6.5-MHz micro-convex transducer. Digital images (8-bit) of glands were obtained and PAR was identified within gland. Average pixel values per unit of PAR area were determined for each gland and analyzed at the level of heifer. Pixel results were interpreted on the basis that lower average pixel values reflect PAR with relatively high amounts of protein as opposed to fat. To help validate that the pixel value within PAR is associated with composition of PAR, pixel findings were compared with histological [number of adipocytes in PAR (Nad) and epithelial area in PAR (Ep)] and biochemical [percent crude protein in PAR (%CP), percent ether extract in PAR (%EE), PAR weight (WPAR), and mammary fat pad weight (WFAT)] composition of PAR in these same heifers. Within PAR, %EE and WFAT were positively correlated with pixel values, whereas %CP, Ep, and Nad were negatively correlated. Parenchyma weight did not correlate with pixel values. Regression analyses (fixed effect log-pixel value; random effect treatment) were used to estimate Nad, Ep, %CP, %EE, WPAR, and WFAT. Sensitivity analysis of regression equations revealed that accuracy of tested equations ranged from 0.77 to 0.93 and precision ranged from 0.56 to 0.82. Concordance correlation coefficients of the equations ranged from 0.41 to 0.76. In conclusion, ultrasonography of PAR can accurately measure and predict PAR composition in prepubertal dairy heifers growing at various rates of gain.

Growth, intake, and health of Holstein heifer calves fed an enhanced preweaning diet with or without postweaning exogenous estrogen

Research has shown that changes in nutrition both before and after weaning can affect mammary development. Additionally, estrogen is known to be a potent mammogenic stimulant. Our objectives were to determine effects of altered preweaning feeding and exogenous estradiol postweaning on growth, intake, and health. Thirty-six Holstein heifer calves were reared on (1) a restricted milk replacer (MR) diet fed at 0.44kg powder dry matter (DM)/day [R; 20.9% crude protein (CP), 19.8% fat, DM basis], or (2) an enhanced MR fed at 1.08kg powder DM/d (EH; 28.9% CP, 26.2% fat, DM basis). The MR feeding was reduced 50% during wk 8 to prepare for weaning. Starter was offered after wk 4 but balanced between treatments. Body weight and frame were measured weekly with intakes and health monitored daily. At weaning, a subset of calves were slaughtered (n=6/diet). Enhanced-fed calves had greater carcass, thymus, liver, spleen, and mammary gland (parenchyma and mammary fat pad) weights. The EH calves also had greater average daily gain (ADG) starting during wk 1 (0.36 vs. -0.06kg/d) and lasting through wk 7 (1.00 vs. 0.41kg/d). Remaining calves received estrogen implants or placebo and were slaughtered at the end of wk 10, creating 4 treatments: (1) R, (2) R + estrogen (R-E2), (3) EH, and (4) EH + estrogen (EH-E2). Postweaning ADG was similar between R, EH, and EH-E2 calves, but greater in R-E2 calves than E calves. The EH-E2 calves had the heaviest mammary glands, and R-E2 calves had heavier mammary glands than R calves. The EH calves consumed more MR DM, CP, and fat preweaning. The R-fed calves consumed more starter DM preweaning. Fecal score was greater for EH calves (1.74 vs. 1.50) preweaning, but days medicated did not differ. Fecal scores were lower for R-E2 calves postweaning. Improved preweaning feeding of calves increased body weights and frame measures. Differences in body weights remained postweaning. Enhanced-fed calves showed greater ADG during the preweaning period but not postweaning. Exogenous estrogen may elicit diet-dependent growth responses. Analysis of collected samples will allow determination of cellular and molecular processes responsible for the marked differences in mammary development observed.

Tamoxifen impairs prepubertal mammary development and alters expression of estrogen receptor α (ESR1) and progesterone receptors (PGR)

Research has shown that prepubertal heifers experience allometric mammary growth that is influenced by the ovaries. Our purpose was to determine the role of estrogen in prepubertal mammary gland development. Sixteen Holstein calves were randomly assigned to 1 of 2 treatment groups: tamoxifen-injected (TAM) or control (CON). Calves were administered the antiestrogen tamoxifen (0.3 mg kg(1) d(1)) or placebo from 28 to 120 d of age. At 120 d, calves were euthanized and udders removed. Weight and DNA content of trimmed parenchymal tissue were halved (P ≤ 0.0001) in TAM compared with CON calves. Parenchymal samples from 3 zones of the left rear mammary gland (lower, middle, and outer regions) were processed for immunohistochemical staining for estrogen receptor α (ESR1) and progesterone receptor (PGR), Ki67-positive cells, and 5-bromo-2'-deoxyuridine label retaining cells (LRCs). Overall, neither the percentage nor location within the epithelial tissue layer of either ESR1- or PGR-positive cells was impacted by TAM treatment. However, image analysis indicated a 6.2-fold lower (P = 0.0001) level of ESR1 protein expression in TAM calves. Similarly, messenger RNA expression of ESR1 was also reduced (P = 0.0001) in TAM heifers. In contrast, expression of PGR protein was greater by 43% (P = 0.03) in TAM calves, but messenger RNA expression did not differ between treatments. Overall, TAM calves had a higher (P ≤ 0.03) percentage and density (cells per tissue area) of Ki67-positive cells. Irrespective of treatment, there were also more Ki67-labeled cells in the outer zones of the mammary gland (P ≤ 0.001). We were able to effectively use multispectral imaging to identify positive cells and quantify the expression of ESR1 and PGR protein. We also identified and counted the proportion of label retaining cells (LCR) (putative epithelial stem cells). We noted an overall 2.9-fold greater number of LRCs in TAM heifers and more LRCs in the outer sampling zones. This suggests that a cohort of LCR cells in TAM remained inactivated in comparison with CON heifers, which exhibited markedly increased growth of the mammary parenchymal tissue over the treatment period. These results suggest that the impacts of ovariectomy are partially explained by loss of ESR1 expression and/or estrogen receptor signaling in the prepubertal bovine mammary gland. The significance of mammary expression of PGR in control of prepubertal bovine mammary development remains unresolved.