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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 froman increase in MEC exfoliation rate. Nineteen Holstein dairy cows producing 40 ± 0.7 kg at 80 ± 4 DIM were divided into 2 groups, control (n = 9) and feed restricted (n = 10). Ad libitum DMI was recorded during a pre-experimental period of 2 wk. For 29 d (period 1), cows were fed either 100 or 80% of their DMI measured during the pre-experimental period. Then, all cows were fed ad libitum for 35 d (period 2). Feed intake andmilk production were recorded daily. Blood and milk samples were collected once during the pre-experimental period and 5, 9, and 27 d after the beginning of periods 1 and 2. Mammary epithelial cells were purified from milk samples using an immunomagnetic method. Lipid mobilization was assessed by plasma NEFA concentration. Mammary epithelium integrity was assessed by plasma lactose concentration, milk Na, andmilk 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 DMI 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, which does not allow conclusions about the integrity of the mammary epithelium. In period 2, when all cows returned to ad libitum feeding, no significant difference was observed on DMI, 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 a carryover effect on their milk production.

Caractérisation du processus d’exfoliation des cellules épithéliales mammaires au cours de la traite chez la vache laitière. 23. Rencontres autour des Recherches sur les Ruminants

The aim of this study was to compare the modifications in mammary gland metabolism by supplying an ideal versus an imbalanced essential AA (EAA) profile at low and high metabolizable protein (or PDIE, its equivalent in the INRA feeding system). Four lactating, multiparous Holstein cows received 4 treatments composed of 2 basal diets containing 2 levels of PDIE (LP or HP) and 2 different infusions of AA mixtures (AA− or AA+) in the duodenum. The AA+ mixture contained Lys, Met, Leu, His, Ile, Val, Phe, Arg, Trp, and Glu, whereas the AA− mixture contained Glu, Pro, and Ser. The infusion mixtures were iso-PDIE. The diet plus infusions provided 13.9 versus 15.8% of crude protein that corresponded to 102 versus 118 g/kg of dry matter of PDIE in LP and HP treatments, respectively. The treatments were designed as a 2 × 2 crossover design of 2 levels of PDIE supply (LP vs. HP) with 28-d periods. Infusions of AA in the duodenum (AA− vs. AA+) were superimposed to diet within each 28-d period according to 2 × 2 crossover designs with 14-d subperiods. Increasing the PDIE supply tended to increase milk protein yield; however, the efficiency of PDIE utilization decreased and the plasma urea concentration increased, indicating a higher catabolism of AA. The AA+ treatments increased milk protein yield and content similarly at both levels of protein supply. This was explained by an increase in the mammary uptake of all EAA except His and Trp. The mammary uptake of non-EAA (NEAA) was altered to the increase in EAA uptake so that the total AA uptake was almost equal to milk protein output on a nitrogen basis. The ratio between NEAA to total AA uptake decreased from 46% in LPAA− to 40% in LPAA+, HPAA−, and HPAA+ treatments. The PDIE efficiency tended to increase in the AA+ versus the AA− treatments because the NEAA supply and the amount of NEAA not used by the mammary both decreased. Nevertheless, our AA+ treatments seemed not to be the ideal profile: the mammary uptake-to-output ratio for Thr was higher than 1 in LPAA−, but it decreased to 1 in all the other treatments, suggesting that Thr was deficient in these treatments. Conversely, an excess of His was indicated because its uptake was similar in AA+ and AA− treatments. In conclusion, balancing the EAA profile increased milk protein yield and metabolizable protein efficiency at both levels of protein supply by increasing the mammary uptake of EAA and altering the NEAA uptake, leading to less AA available for catabolism.