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Each quarter of the bovine mammary gland is an anatomically and functionally distinct gland. However, mastitis in one quarter may affect function of adjacent, uninfected glands. To investigate the mechanisms and potential mediators of these effects, we quantified early responses of the mammary gland to intramammary lipopolysaccharide (LPS) challenge, distinguishing between local and systemic effects. Ten multiparous cows over 70 d in milk were blocked into pairs by breed, cow-level somatic cell count (SCC), and milk yield. Within block, one cow was assigned to LPS treatment (T) such that both the front and the rear quarter of a randomly selected udder half received an infusion of 50 µg of LPS in 10 mL of saline (T-L); the contralateral quarters received only 10 mL of saline (T-S). Similarly, each paired control cow (C) received either 10 mL of saline (C-S) or no infusion (C-N) into udder halves. Cows were quarter milked twice daily, with foremilk samples (∼30 mL, front quarters) taken at -24, 0, 3, 6, 12, and 24 h relative to infusions. At 24 h, average milk yield in T-L and T-S quarters fell to 23 and 32% of pre-infusion levels, respectively. For T cows, systemic effects were observed by 3 h post-infusion as rectal temperature was elevated and foremilk fat concentration was reduced in both T-L and T-S. However, SCC and concentrations of l-lactate and total protein in foremilk indicated a local response to LPS: protein was transiently higher at 3 h, whereas SCC and lactate were higher at 6 h in T-L compared with T-S. Lactose concentration showed a local effect at 6 h, being lower in T-L than in T-S, and then a systemic effect at 12 h, being lower in both T-L and T-S than C quarters. Concomitant with changes in milk, systemic effects were also observed in blood. Plasma antioxidant potential and glucose concentration were lower in T cows than in C cows at 6 or 12 h, respectively, although neither variable remained different at 24 h. In summary, unilateral LPS infusion induced distinct, time-dependent effects on each milk component. Depending on the component, effects were local, systemic, or both, suggesting involvement of multiple different mediators that collectively result in systemic inhibition of milk production.

The mammary gland undergoes distinct periods of growth, development, and secretory activity. During bovine lactation, a gradual decrease in the number of mammary epithelial cells largely accounts for the decline in milk production with advancing lactation. The net decline in cell number (approx. 50%) is due to cell death but is simultaneously accompanied by cell renewal. Although the rate of cell proliferation is slow, by the end of lactation most cells in the gland were formed after calving. Typically milking is terminated when cows are in the final 2 mo of pregnancy. This causes regenerative involution, wherein extensive cell replacement and mammary growth occurs. We hypothesized that replacement of senescent secretory cells and progenitor cells during the dry period increases milk yield in the next lactation. Analysis of global gene expression revealed networks and canonical pathways during regenerative involution that support cell turnover and mammary growth, and reflect oxidative stress, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress. Immune responses consistent with influx of neutrophils, macrophages, and lymphocytes, and processes that support mammary differentiation and lactogenesis were also evident. Data also suggest that replication of stem and progenitor cells occurs during the dry period. Relying on long-term retention of bromodeoxyuridine-labeled DNA, we identified putative bovine mammary stem cells. These label-retaining epithelial cells (LREC) are in low abundance within mammary epithelium (

Ergot alkaloids in endophyte-infected grasses inhibit prolactin (PRL) secretion and may reduce milk production of cows consuming these grasses. We investigated the effects of consuming endophyte-infected fescue seed during late lactation and the dry period on mammary growth, differentiation, and milk production. Twenty-four multiparous Holstein cows were randomly assigned to 3 treatment groups. Starting at 90±4 d prepartum, cows were fed endophyte-free fescue seed (control; CON), endophyte-free fescue seed plus 3×/wk subcutaneous injections of bromocriptine (0.1mg/kg of body weight, positive control; BROMO), or endophyte-infected fescue seed (INF) as 10% of the diet on an as fed basis. Although milk yield of groups did not differ before treatment, at dry off (-60 d prepartum) INF and BROMO cows produced less milk than CON. Throughout the treatment period, basal concentrations of PRL and the prepartum increase in plasma PRL were reduced in INF and BROMO cows compared with CON cows. Three weeks after the end of treatment, circulating concentrations of PRL were equivalent across groups. In the subsequent lactation milk yield was not decreased; in fact, BROMO cows exhibited a 9% increase in milk yield relative to CON. Evaluation of mammary tissue during the dry period and the subsequent lactation, by quantitative histology and immunohistochemical analysis of proliferation markers and putative mammary stem or progenitor cell markers, indicated that feeding endophyte-infected fescue seed did not significantly affect mammary growth and development. Feeding endophyte-infected grasses during the dry period may permit effective utilization of feed resources without compromising milk production in the next lactation.