Mammary cell turnover: Relevance to lactation persistency and dry period management

Milk production is a function of the number of mammary secretory cells and the activity per cell. Declining milk yield with advancing lactation is due to a decline in the number of secretory cells, although a decline in activity per cell occurs during late gestation. By replacing older cells with younger cells, cell turnover is a mechanism whereby persistency of lactation can be enhanced. Management practices such as bovine somatotropin (bST) supplementation, photoperiod manipulation and increased milking frequency may enhance lactation persistency. Mastitis, stressors, concomitant pregnancy, and decreased milking frequency may decrease persistency by increasing apoptosis. Mammary epithelial cells with lightly staining cytoplasm appear to be the progenitor cells responsible for proliferation within the mammary gland. Renewal of these cells and senescent secretory cells prior to the next lactation may be critical events responsible for the impact of dry period length on milk yield in the next lactation. Data indicate that there is extensive cell renewal in conjunction with net proliferation during the dry period. Although the 60-d dry period is the current management standard, continued advances in genetic potential and dairy management indicate that a reassessment is in order. Recent studies have shown that multiparous dairy cows treated with bST can be milked continuously without significant milk loss in the next lactation. Indeed, total milk yields (pre plus postpartum yields) were increased. However, lactating heifers require a dry period of sufficient length for optimal milk yield in the next lactation. Neither bST nor increased milking frequency reversed the negative impact of an inadequate dry period on milk production by dairy heifers. However, these studies have provided additional insights and areas for future investigation.