Feed Restriction Increases Mammary Epithelial Cell Exfoliation Rate in Dairy Cows

In dairy cows, feed restriction is known to decrease milk yield by reducing the number of mammary epithelial cells (MEC) in the udder, due to an increased rate of MEC apoptosis. The exfoliation of MEC from the mammary epithelium into milk is another process that could participate in regulating MEC number. We hypothesized that the decreased number of MEC in the udder of feed-restricted cows might also result from an increase in MEC exfoliation rate. Nineteen Holstein dairy cows producing 40 ± 0.7 kg at 80 ± 4 DIM were divided into two groups, control (n=9) and feed-restricted (n=10). Ad libitum dry-matter intake was recorded during a pre-experimental period of 2 weeks. For 29 days (Period 1), cows were fed either 100% or 80% of their dry-matter intake measured during the pre-experimental period. Then, all cows were fed ad libitum for 35 days (Period 2). Feed intake and milk production were recorded daily. Blood and milk samples were collected once during the pre-experimental period and 5, 9 and 27 days after the beginning of periods 1 and 2. MEC were purified from milk samples using an immuno-magnetic method. Lipid mobilization was assessed by plasma NEFA concentration. Mammary epithelium integrity was assessed by plasma lactose concentration, milk Na and Na:K ratio. Mammary tissue samples were collected by biopsy at the end of each period in order to analyze the rate of cell proliferation and apoptosis. Data were analyzed using the mixed procedure of SAS. Compared with control feeding, feed restriction induced a decrease in dry-matter intake by 20% (P<0.001), an increase in plasma concentration of NEFA (P<0.001) and a 3 kg/d milk yield loss (P<0.001) but had no effect on the rate of proliferation and apoptosis in the mammary tissue. Daily MEC exfoliation rate was 65% greater in feed-restricted cows compared with control cows (P<0.05). Feed restriction had no effect on plasma lactose concentration and milk Na:K ratio but increased milk Na concentration, not allowing concluding about the integrity of the mammary epithelium. In period 2, when all cows returned to ad libitum feeding, no significant difference was observed on dry-matter intake, milk yield, MEC exfoliation rate and mammary epithelium integrity between feed-restricted and control cows. These results show that the exfoliation process could play a role in regulating the number of MEC in the udder of dairy cows during a feed restriction without carryover effect on their milk production.