Effect of feed restriction and nature of forage on the mammary exfoliation rate in dairy cows
Résumé
Diet, in term of quantity and quality, is one of the main factors that affect milk yield and composition in dairy cows. The mammary epithelial cell (MEC) exfoliation process, together with the cell proliferation/apoptosis balance, participates in regulating the MEC number in the mammary gland and thus milk yield. Little is known about the effect of diet in the regulation of MEC number in the mammary tissue. This trial aimed to investigate the effect of feeding level and nature of forage on the MEC exfoliation rate. Sixteen multiparous dairy cows producing 37.0 ± 0.68 kg at 99 ± 4.6 DIM were divided into 4 groups of 4 cows each according to their diet. After a pre-experimental period of 3 wk, cows were fed either 100 or 75% of their ad libitum DMI with diets based on corn silage or conserved grass for 3 wk (Period 1). The feeding level was then reversed for the 3 following weeks (Period 2). Milk samples were collected at the end of the pre-experimental period and 15 d after the beginning of each period for milk MEC purification using an immunomagnetic method to assess
the daily MEC exfoliation rate. The percentage of apoptotic MEC in milk was determined by flow cytometry after annexin V/propidium iodide labeling. Mammary epithelium integrity was assessed by milk Na:K ratio determination. Data were analyzed using the mixed procedure of SAS. After 15 d of treatment, feed restriction decreased milk yield by 10% (P < 0.05) without affecting the MEC exfoliation rate. The Na:K ratio was greater with feed restriction, suggesting a loss of mammary epithelium integrity (P < 0.05). The nature of forage affected neither milk yield nor mammary epithelium integrity, but the daily MEC exfoliation rate was greater in
cows fed with the corn silage–based diet (P < 0.01). When cows were fed 100%, the percentage of apoptotic cells among exfoliated MEC was lower with the corn silage–based diet than with conserved grass–based diet (P < 0.05). These results show that the MEC exfoliation rate was not affected by a 15-d feed restriction whereas it was affected by the nature of forage. Mammary epithelial cell exfoliation rates did not vary
along with variations of milk yield. The effect of the duration of feed restriction on the MEC exfoliation rate and the effect of the nature of forage on the cell proliferation/apoptosis balance need to be investigated further.