Changes in tissue composition associated with mammary gland growth during lactation in sows.

Twenty-four primiparous sows were used to determine the extent of mammary gland growth during lactation. Litter size was set to nine or 10 pigs immediately after birth. Sows were slaughtered in groups representing d 0 (within 12 h after farrowing), 5, 10, 14, 21, and 28 of lactation. Sows were provided 17.5 Mcal ME and 65 g of lysine per day during lactation. Mammary glands were collected at slaughter and trimmed of skin and extraneous fat pad. Each gland was weighed, cut in half to measure cross-sectional area, and ground for chemical analysis. Dry matter content, dry fat-free tissue (DFFT) content, protein content, amino acids composition, ash content, and DNA content were measured. Only glands known to have been suckled were included in these data. Wet and dry tissue weight; cross-sectional area; and the amount of DFFT, tissue protein, and amino acids in each suckled mammary gland increased (P < .05) during lactation to a peak on d 21. Fat percentage of each suckled gland declined (P < .05) and the percentage of protein and DFFT increased (P < .05) as lactation progressed. These results suggest that hypertrophy occurred in the tissue during lactation. There was a linear increase in the amount and percentage of DNA during lactation (P < .05), suggesting hyperplasia of the mammary tissue. Mammary tissue growth continues in suckled glands during lactation in sows, with gland wet weight increased by 55% and total gland DNA increased by 100% between d 5 and 21 of lactation.     

Effect of nutrient intake on mammary gland growth in lactating sows

Sixty-one primiparous sows were used to determine the response of mammary gland growth to different energy and protein intakes during lactation. After birth, litter size was set to 9 or 10 pigs. Sows were slaughtered at selected times up to 30 d of lactation. Individual sows were fed one of four diets that were combinations of different amounts of energy and protein (3.0 Mcal ME and 8.0 g lysine/kg diet; 3.0 Mcal ME and 16.2 g lysine/kg diet; 3.5 Mcal ME and 6.4 g lysine/kg diet; or 3.5 Mcal ME and 13.0 g lysine/kg diet). Mammary glands were collected at slaughter and trimmed of skin and the extraneous fat pad. Each gland was weighed, cut in half to measure cross-sectional area, ground, and stored at -20 degrees C for chemical analysis. Frozen, ground tissue was used to determine dry matter, dry fat-free tissue (DFFT), total tissue protein, ash, and DNA content. Only glands known to have been suckled were included in this data set. Response surface regression was used for statistical analysis. The percentage of protein, fat, ash, and DNA in each suckled mammary gland was affected only by total energy intake (P<.05). The percentage of dry tissue and fat decreased as the total energy consumed during lactation increased, whereas the percentage of protein and DFFT increased as total energy intake increased. There were quadratic effects (P<.05) of both total energy and protein intake on wet weight, dry weight, protein amount, DFFT amount, and DNA amount of each suckled mammary gland during lactation. This study shows that mammary gland growth is affected by nutrient intake during lactation. The weight of suckled mammary glands and the amount of mammary tissue protein, DFFT, and total DNA were maximal on d 27.5 of lactation when sows had consumed an average of 16.9 Mcal of ME and 55 g of lysine per day during lactation. Provision of adequate amounts of nutrients to sows during lactation is important for achieving maximal growth of mammary glands and maximal milk production.     

Quantification of mammary gland tissue size and composition changes after weaning in sows

The objectives of this study were to characterize the tissue compositional changes in porcine mammary glands after weaning and to determine whether administration of estradiol alters the profile of these tissue changes. Forty-five primiparous sows were assigned randomly to one of two treatment groups after weaning, control or estrogen treated. Estrogen-treated sows received twice-daily injections of estradiol-17beta (0.125 mg/kg of BW); control sows received vehicle injections. Sows were weaned at d 21 of lactation and killed on either d 0 (d of weaning; n = 5) or on d 2, 3, 4, 5, or 7 after weaning (n = 4 per treatment on each day). Teat order relative to suckling behavior was observed on the day before weaning to determine which mammary glands the piglets suckled. Suckled and non-suckled glands were identified from the teat order observation, and individual mammary glands were collected at slaughter. Mammary glands were trimmed of skin and extraneous fat pad, individually weighed, and bisected to measure cross-sectional area. The remaining half of each gland was ground and stored at -20 degrees C for chemical analyses. Frozen tissue was used for measuring tissue DNA, DM, protein, fat, and ash contents. Suckled mammary glands of sows undergo significant and dramatic changes during the initial 7 d after weaning, with significant changes occurring even by d 2 after weaning. Mean cross-sectional area of parenchymal tissue in suckled mammary glands decreased from 59.7 +/- 2.1 cm2 on the day of weaning to 26.8 +/- 2.3 cm2 by d 7 after weaning (P < 0.0001). Mammary gland wet weight decreased from 485.9 +/- 22.0 g on the day of weaning to 151.5 +/- 24.8 g by d 7 after weaning (P < 0.0001), whereas DNA decreased from 838.8 +/- 46.2 g on the day of weaning to 278.4 +/- 52.5 g by d 7 after weaning (P < 0.0001). The changes in gland wet weight and DNA during the period of mammary gland involution in the sow represent loses of over two-thirds of the parenchymal mass and nearly two-thirds of the cells that were present on the day of weaning. Estrogen treatment did not affect overall mammary involution during the first 7 d after weaning. Mammary glands that were not suckled during lactation had no further loss of parenchymal tissue during the first 7 d after weaning. Mammary gland involution in the sow is a rapid process and is probably irreversible within 2 or 3 d after weaning.     

Ultrasonographic assessment of the feline mammary gland

The aim of this study is to characterise the feline mammary echotexture using B-mode ultrasonography, which is not routinely used to examine the feline mammary gland. Using a 5-9 MHz linear transducer the ultrasonographic appearance of non-stimulated and stimulated mammary glands was determined in 35 mature intact non-pregnant, pregnant and lactating queens aged from 16 months to 8 years. In intact non-pregnant queens, mammary glands are fairly underdeveloped and on the ultrasonograms they appear with a regular hypoechoic texture and generally show a thickness of less than 2.0mm. The stimulated mammary tissue typically presents a more hyperechoic appearance compared to the non-stimulated gland and a fine granular echotexture. Maximum echogenicity of the mammary gland is reached during lactation. In late pregnancy, the mammary glands reach 6-9 mm in thickness. During lactation, the size of the glands depends on the existence of a suckling stimulus, with the suckled glands reaching about 11 mm in thickness. Ductal structures can only be imaged during late pregnancy and lactation. Ultrasonographic evaluation of the feline mammary gland can become a valuable diagnostic tool to characterise physiological changes and may further contribute to a better characterisation of diseased mammary tissue.     

The regression of unsuckled mammary glands during lactation in sows: the influence of lactation stage, dietary nutrients, and litter size.

During lactation in the sow, mammary glands that are not regularly suckled undergo regression. This study characterizes the regression of unsuckled mammary glands and how that regression is affected by dietary nutrients and litter size. Sixty-nine primiparous sows were fed one of four diets containing combinations of two protein levels (32 or 65 g lysine/d) and two energy levels (12 or 17.5 Mcal ME/d) during lactation. Litter size was adjusted to 10. Sows were killed on d 0, 5, 10, 14, 21, or 28 of lactation. In another experiment, twenty-eight primiparous sows were allotted to have different litter sizes and were killed on d 21 of lactation. The day before slaughter, teat order of each litter was observed. After death, mammary glands were removed and dissected. Skin and extraneous fat pads were removed from the mammary glands and individual glands were separated. Each gland was weighed, cut in half to measure cross-sectional area, and ground for chemical analysis. The amounts of dry tissue, protein, fat, ash, and DNA were measured. Only glands observed to be unsuckled were included in the results. Regression of unsuckled mammary glands occurred rapidly during the first 7 to 10 d of lactation, as indicated by a decline in wet weight, dry weight, protein, fat, DNA, and cross-sectional area. The rate of regression was slowed after the early lactation period. The rate of regression of unsuckled glands was affected by dietary nutrient levels. Dietary energy level affected (P < 0.05) the decline in wet and dry weights, protein, fat and DNA content, and cross-sectional area, whereas dietary protein level affected (P < 0.05) the decline in dry weight and fat content. At d 5 of lactation, the wet weight of unsuckled mammary glands in sows fed the high-energy high-protein diet was 91% greater (P < 0.05) than in sows fed the low-energy low-protein diet. Effects of litter size on size and composition of unsuckled glands were not significant by d 21 of lactation. Unsuckled mammary glands regress rapidly during early lactation, and the rate of regression is affected by dietary nutrient intake.     

Apoptosis and oxidative stress of infiltrated neutrophils obtained from mammary glands of goats during various stages of lactation

OBJECTIVE: To examine apoptosis in infiltrated neutrophils during involution of mammary glands and compare them with those obtained during late and peak lactation, and to measure oxidative stress and activities of antioxidant enzymes and determine involvement of free radicals in apoptosis of infiltrated neutrophils. SAMPLE POPULATION: Neutrophils from mammary gland secretions of 8 goats at 4 stages (late and peak lactation and 1 and 2 weeks after end of lactation). PROCEDURE: DNA fragmentation was evaluated to characterize apoptosis. Concentration of thiobarbituric acid reactive substances (TBARS) was used to evaluate oxidative stress. Activities of superoxide dismutase and glutathione peroxidase were determined. RESULTS: Neutrophils from secretions obtained after end of lactation of all goats and from late-lactation milk of some goats underwent prominent apoptosis, whereas neutrophils from peak lactation secretions did not. Higher lipid peroxidation and lower antioxidant enzyme activities in neutrophils during involution were observed, compared with those during late and peak lactation. A significant negative correlation existed between TBARS concentrations and antioxidant enzyme activities during the nonlactating period. CONCLUSIONS AND CLINICAL RELEVANCE: Apoptosis is a feature of infiltrated neutrophils during involution of mammary glands in goats. This feature may allow prompt resorption and clearance of infiltrated neutrophils without damaging surrounding tissues. Increased oxidative stress in infiltrated neutrophils from secretions obtained after end of lactation is probably related to a deficiency in antioxidant enzyme activities. Understanding the relationship between apoptosis and oxidative stress will lead to new strategies for manipulating involution and reducing tissue damage.     

The influence of suckling interval on milk production of sows

The objective of this study was to determine whether sow milk yield per gland could be increased by reducing the interval between suckles (suckling interval). Eighteen sows were allocated at their first farrowing to three treatments comprising litter sizes of 6 or 12 piglets or a cross-suckling treatment that was imposed to increase suckling frequency. The cross-suckled treatment comprised two groups of six piglets each. Each suckling group was allowed to suckle the sow during 30-min intervals each day between d 6 and d 28 of lactation. The suckling interval was shorter (P < .05) for cross-suckled sows than for sows suckling single litters of 6 or 12 piglets during early lactation (d 10 to 14) and late lactation (d 24 to 28). Average piglet growth rate between birth and 28 d of age was greatest (P < .05) for piglets in the single litters of six and lowest for piglets in the cross-suckled treatments. Single litters of 12 piglets had the highest (P < .05) litter growth rates, followed by the cross-suckled litters and then the single litters of six piglets. The concentration of lactose and fat in milk from sows remained relatively stable, although milk from the cross-suckled sows contained more protein in early lactation (P < .05). Milk yield of sows was not significantly increased (P > .05) by the cross-suckle treatment, although during early lactation, milk yield tended to be greater from sows in the cross-suckle treatment than from sows suckling single litters of six (8,920 g/d vs 7,819 g/d, P < .1). The concentration of DNA and total RNA and the RNA:DNA ratio in mammary glands was unaffected by treatment (P > .05). Sows with single litters of 12 piglets had the greatest total DNA in their udders (P < .05). However, individual gland weights were heavier (P < .05) in cross-suckled sows than in sows with single litters of 6 or 12 piglets. Increased suckling frequency seemed to play a role in increased mammary gland weight and milk production during lactation.     

Mammary gland growth as influenced by litter size in lactating sows: impact on lysine requirement

Twenty-eight primiparous sows were used to determine the effect of litter size on the growth of mammary glands and nursing pigs during lactation. Litter size was set to 6, 7, 8, 9, 10, 11, or 12 pigs by cross-fostering immediately after birth. Four sows were allotted to each litter-size group. Sows were allowed to consume a daily maximum of 13.6 Mcal ME and 46.3 g of lysine during lactation. Sows were slaughtered on d 21 (20.6+/-1.1) of lactation. Mammary glands were collected at slaughter and trimmed of skin and the extraneous fat pad. Each gland was separated, weighed, and ground for chemical analysis. Dry matter, dry fat-free tissue (DFFT), crude protein, ash, and DNA contents were measured. Only glands known to have been nursed were included in the data set. Wet and dry weights and the amounts of DFFT, protein, DNA, ash, and fat in individual nursed mammary glands linearly decreased (P<.05) as litter size increased. Percentages of DFFT, protein, and DNA were quadratically affected (P<.05) by litter size on d 21 of lactation. Total mammary wet and dry weights and total DFFT, protein, DNA, fat, and ash amount of all nursed mammary glands of each sow were increased as litter size increased (P<.05). Changing litter size from 6 to 12 pigs resulted in 2,098, 432, 253, 227, 4.4, 178, and 20 g increases in the amounts of total mammary wet weight, dry weight, DFFT, protein, DNA, fat, and ash, respectively, on d 21 of lactation. Litter weight gain was 18.1 kg greater in sows with 12 pigs than in sows with 6 pigs. Sows with a larger litter size had a greater increase in total mass of mammary gland tissue and litter weight but had lower growth of individual nursed mammary glands and individual pigs than sows with the smaller litter size. The need for nutrients to support additional mammary gland and litter growth as litter size increases should be considered when estimating nutrient requirements for lactating sows. Sows need an additional .96 g lysine per day to account for mammary gland growth for each pig added to a litter.     

Adrenomedullin (AM) and adrenomedullin binding protein (AM-BP) in the bovine mammary gland and milk: Effects of stage of lactation and experimental intramammary E. coli infection

Adrenomedullin (AM) has been characterized as an endogenous tissue survival factor and modulator of many inflammatory processes. Because of the increased susceptibility of the mammary gland to infection during the time surrounding parturition in the cow, we investigated how milk and tissue content of AM and its binding protein (AM-BP) might be affected by the stage of lactation and the udder health status. Milk and mammary biopsy samples were obtained from Holstein cows 21 days prior to and at various times after calving to represent the dry period and early and mid-stages of lactation. Additional cows received an intramammary challenge with Escherichia coli for immunohistochemical characterization of AM and AM-BP. Milk AM concentrations were relatively constant across the stages of lactation while AM-BP increased two-fold (P<0.04) between early and mid-lactation. Milk AM (P<0.04) and AM-BP (P<0.03) increased as somatic cell counts (SCCs) increased within a given stage of lactation. Tissue content of both (AM and AM-BP) were significantly affected by stage of lactation, lowest in the dry period and progressively increasing to peak at mid-lactation as well as increasing in association with higher levels of SCCs. Following E. coli challenge, AM increased in epithelial cells surrounding mammary alveoli presenting high levels of SCCs. The data suggest that AM and AM-BP are cooperatively regulated in the mammary gland during lactation; changes in localized tissue AM and AM-BP content reflect a dynamic regulation of these tissue factors in the bovine mammary gland consistent with their protective effects within inflamed tissue.     

Effect of suckling intensity on human growth hormone binding, biochemical composition and histological characteristics of ovine mammary glands

Suckling both, or only one contralateral mammary gland during 15 days postpartum was utilized to study lactogenic hormone binding to mammary microsomal membranes and quantitative mammary morphology in ewes. Binding of radiolabeled human growth hormone was specific for lactogenic hormones. Non-radiolabeled human growth hormone, ovine and bovine prolactin and human placental lactogen effectively competed with radiolabeled human growth hormone for binding sites but ovine and bovine growth hormone were completely ineffective. Specific binding of radiolabeled human growth hormone to 600 μg of membrane protein averaged 23 ± 3% in all lactating glands. Neither days postpartum nor treatment of contralateral mammary glands substantially altered hormone binding in lactating glands. Specific human growth hormone binding (6 ± 0.5%) in non-suckled glands (15 days postpartum both udder halves) was significantly lower (P<0.01) than in lactating tissue but only a moderate and variable reduction in specific binding was measured in membranes from glands non-suckled for 15 days but contralateral to a suckled gland (14 ± 4%). Specific binding was approximately doubled in assays with 600 compared with 300 μg of membrane protein and the pattern of binding among variously suckled glands was not changed by treatment of membranes with 4 M MgCl₂ prior to assay. Most secretory cells from all lactating glands had rounded, basally displaced nuclei, apical fat globules, secretory vesicles and abundant densely stained basal cytoplasm (ergastoplasm). Alveolar lumenal area was maximal (50% of tissue area) and stromal tissue area was minimal. After 15 days of non-suckling (both udder halves) mammary cells were engorged with lipid, ergastoplasm was reduced and nuclei were irregularly shaped and randomly displaced compared with lactating tissue. In addition, lumenal area was reduced and stromal tissue more evident. Lack of suckling for 5 days had little apparent effect on mammary cytology. Like lactogenic hormone binding, mammary tissue morphology was only moderately altered by 15 days of non-suckling when the remaining gland was suckled. RNA concentration was lowest (2.1 ± 0.3 mg/g) in mammary tissue from ewes in which neither gland was suckled for 15 days postpartum but non-suckling interval had no significant effect when contralateral glands were suckled. DNA concentration was not significantly influenced by suckling treatments. Relative lactogenic hormone binding closely corresponded to changes in cytological and biochemical indices of secretory cell function.     

Aberrant development of mammary glands, but precocious expression of beta-casein in transgenic females ubiquitously expressing whey acidic protein transgene

It has been suggested that whey acidic protein (WAP) may function as a protease inhibitor. However, the actual function of WAP remains obscure. We investigated the histological development of the mammary glands of transgenic mice ubiquitously expressing WAP and CAG/WAP transgene. Ubiquitous expression of WAP induced aberrant development of the lobular alveoli of the mammary glands: mammary alveoli that were either aberrantly large or small in size increased in number in the developing mammary glands of these transgenic females during pregnancy and lactation. The expression of beta-casein was precociously induced in the mammary glands of the transgenic females during early pregnancy and accompanying this was a histological observation that abnormally developed lobular alveoli filled with milk proteins appeared in the mammary glands of transgenic females during early pregnancy. However, during lactation, the development of mammary glands was impaired in transgenic females. To investigate the possible paracrine action of WAP associated with mammary gland aberration, we transplanted the mammary tissue of CAG/EGFP transgenic females into the fat pad of virgin CAG/WAP transgenic females and initiated pregnancy by mating. The development of mammary tissue transplanted to the recipient was histologically examined on day 3 of lactation. The results revealed that the development of grafted mammary tissues was impaired in a manner similar to that of the mammary glands of CAG/WAP transgenic females, indicating that the inhibitory effect of WAP acts via a paracrine mechanism. In vitro experiments using HC11 cells with forced expression of exogenous WAP demonstrated the inhibitory function of WAP on proliferation of mammary epithelial cells.     

BCL-2 family of proteins and mammary cellular fate

This review focuses on the pro-apoptotic and anti-apoptotic Bcl-2 family members involved in apoptosis, which is the predominant process controlling cell remodelling during post-lactational mammary gland involution. The members of the Bcl-2 protein family, whose expression levels are under the control of lactogenic hormones, internally control this mechanism also during lactation. They can physically interact with each other, sometimes in an antagonistic manner. Mammary glands undergo repeated cycles of structural development, functional differentiation and regression, therefore provide a unique model for investigating this family of proteins that regulate the fate of the secretory cells and consequently milk yield. The involvement of Bcl-2 family members is reviewed in mammary tissue during morphogenesis, at different stages of lactation cycle and in comparison with dairy and laboratory animals.     

Effects of long-term exogenous bovine somatotropin on glucose metabolism and the utilization of glucose by the mammary gland in different stages of lactation of crossbred Holstein cattle

The effects of long-term administration of recombinant bovine somatotropin (rbST) on glucose turnover and the utilization of glucose in the mammary gland using a continuous infusion of [3-³H]glucose and [U-¹⁴C]glucose in lactating crossbred Holstein Friesian(HF) cattle were investigated. Glucose turnover of rbST-treated animals was significantly higher than those of control animals ( P  < 0.05) in mid lactation, while plasma glucose concentrations were not affected. The utilization of glucose of non-mammary tissues of rbST-treated animals significantly increased ( P  < 0.05) as lactation advanced. The glucose taken up by the mammary gland in early lactation increased flux through the lactose synthesis and the pentose cycle pathway with significant increases in NADPH formation for fatty acid synthesis during rbST administration. The utilization of glucose carbon incorporation into milk appeared to increase in milk citrate and milk triacylglycerol but not for milk lactose as lactation advances in rbST-treated animals. The stimulant effect for milk yield by rbST treatment was transiently and significantly increased in early lactation and was decreased in late lactation even though there was a high level of udder blood flow. These findings demonstrate that the regulation of biosynthetic capacity within the mammary gland would be influenced more by local than by systemic factors. The proportion of glucose would be metabolized less for lactose synthesis, but metabolized more via the Embden-Meyerhof pathway and the tricarboxylic acid cycle as lactation advances.     

The effect of insulin and relaxin upon mitosis (in vitro) in mammary tissue from pregnant and lactating pigs

The rate of cellular proliferation in the mammary glands of pigs during late gestation and lactation was assessed by measuring the incorporation of ³H-thymidine (T₁) into the DNA of mammary gland explants in vitro. The T₁ showed a linear response over the first 9 hr in vitro, and was not affected by the addition of 500 ng insulin/ml medium. From day 100 to parturition the T₁ rose, reached a peak at 2 d after parturition and declined during lactation to the lowest levels seen at day 21 of lactation.

The inclusion of 0–1000 ng relaxin/ml medium on T₁ at 24–72 hr in vitro had no effect in stimulating T₁ in mammary tissue explants taken from either pregnant or lactating pigs.     

Mineral nutrition of the sow: a review

The sow varies greatly in her mineral requirements, which largely reflects the nutritional demands during different phases of her reproductive cycle. Minerals are required for conceptus product formation, mammary secretions, and growth and maintenance. Although the sow's mineral requirements are highest during late gestation and lactation, those that are supplied from the diet and tissue reserves both contribute in meeting these nutritional needs. The sow is fairly resilient to borderline mineral insults, but longevity in the herd may be compromised under these conditions. Specific mineral deficiencies vary, but the reproducing sow will deplete the pool of tissue minerals before litter size, congenital abnormalities of developing fetuses or milk mineral composition are affected. Inadequate mineral supplies may prolong the duration of parturition, increase the number of stillbirths and result in a higher occurrence of skeletal problems. Tissue mineral reserves can be increased in various tissues with higher dietary fortification, but subsequent mineral availability from various pools differ. Because feed intake during lactation is low, daily mineral intake during lactation often is below NRC (1988) requirements.     

Leptin and ghrelin levels in colostrum,milk and blood plasma of sows and pig neonates during the first week of lactation

Radioimmunology was used to determine leptin and ghrelin levels in sow colostrum and milk in relation to those in sow and neonatal pig blood plasma and to the body weight of piglets during the first week of lactation. The highest concentration of leptin was found in colostrum on the second day of lactation (69.3 ± 6.3 ng/mL). Leptin concentrations in sow plasma were significantly lower than in colostrum/milk (2.19 ± 0.9 ng/mL, P = 0.7692) and were stable in the first 7 days of lactation. Total and active ghrelin concentrations in colostrum/milk were stable in the measured time points (6734 ± 261 pg/mL, P = 0.3397; 831 ± 242 pg/mL, P = 0.3988, respectively). Total ghrelin concentrations in sow plasma were lower than in colostrum/milk. These results indicate that pigs follow a unique species‐specific pattern of leptin and ghrelin synthesis, release and existence, and that the mammary gland is an important source of leptin and ghrelin contained in colostrum/milk.     

Shedding of porcine reproductive and respiratory syndrome virus in mammary gland secretions of sows.

OBJECTIVE: To document shedding of porcine reproductive and respiratory syndrome (PRRS) virus in mammary gland secretions of experimentally inoculated sows, to evaluate effects of vaccination during gestation on virus shedding during the subsequent lactation, and to evaluate shedding of PRRS virus in milk of sows in commercial herds. ANIMALS: 6 sows seronegative for PRRS virus were used for experiment 1, and 2 sows were retained for experiment 2. For experiment 3, 202 sows in commercial herds were used. PROCEDURE: In experiment 1, 2 sows were inoculated with PRRS virus, 2 sows were vaccinated with modified-live PRRS virus vaccine, and 2 sows served as control pigs. Mammary gland secretions were assayed for PRRS virus. In experiment 2, pregnant vaccinated sows from experiment 1 were vaccinated with another modified-live PRRS virus vaccine. Mammary gland secretions were assayed in the same manner as for experiment 1. For experiment 3, milk collected from 202 sows in commercial herds was assayed for PRRS virus. RESULTS: In experiment 1, PRRS virus was detected in mammary gland secretions of both vaccinated and 1 of 2 virus-inoculated sows. In experiment 2, virus was not detected in samples from either vaccinated sow. In experiment 3, all samples yielded negative results. CONCLUSIONS AND CLINICAL RELEVANCE: Na?ve sows inoculated late in gestation shed PRRS virus in mammary secretions. Previous vaccination appeared to prevent shedding during the subsequent lactation. Results for samples obtained from sows in commercial herds suggested that virus shedding in mammary gland secretions of such sows is uncommon.