Hormonal and nutritional regulations of lactation persistency in dairy ruminants

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anglais. IntroductionIt has long been recognized that the lactation curve in dairy ruminants is shaped by the regulation of mammary tissuecell numbers throughout lactation1,2. However, the precise hormonal and nutritional factors driving lactation persistencyare not been fully elucidated. Therefore, a set of experiments was conducted to investigate the effects of hormonal andnutritional challenges on cell turnover in the mammary tissue.Materials and MethodsInitially, experiments on dairy cows were conducted to assess the effects of hormonal challenges. The effect of inhibitingprolactin release was assessed by administering daily injections of a dopamine agonist (quinagolide) to cows for 8 weeks,compared with control group receiving no injections. Additionally, the effect of sex steroids was studied by performingeither ovariectomy or sham surgery on dairy cows, followed by a monitoring period of 52 weeks. Furthermore, theeffects of feeding level were investigated in dairy cows assigned to either a basal diet or a severe feed restriction from 2wks before calving to wk 11 postpartum, or during a 4-wk moderate feed restriction (-20% DMI). The effects of differenttypes of diets were investigated by supplementing 32 dairy cows with either by pass starch or amino acids (rumenprotected lysine, methionine, and histidine) from 8 wks to 26 wks postpartum. Similarly, 2 levels of NEL (1.48 vs. 1.65Mcal/ kg DM) through concentrates and amino acid supply (rumen protected methionine) were tested on 24 midlacI14tatinggoats for 4 wks. Mammary tissue cell turnover was assessed through immunohistochemical analyses in allexperiments, and the exfoliation of mammary epithelial cells (MEC) into milk was examined in some cases.ResultsThe impacts on milk yield (MY) resulting from hormonal challenges (-14% during prolactin inhibition or +10% afterovariectomy) were associated with alterations in apoptosis/proliferation balance in the bovine mammary tissue and adecrease of MEC exfoliation (only for ovariectomy). The decreases in MY due to feed restriction (- 38 % and - 8%respectively with a severe and a moderate feed restriction) in cows were associated with either a higher level ofapoptosis or an increase in MEC exfoliation with no effect on cell proliferation. The increase in MY (+ 11%) induced bystarch supplementation in cows was accompanied by enhanced cell proliferation. AA supplementations did not affectmilk yield but resulted in increased cell proliferation in both dairy cows and goats and reduced cell apoptosis in dairygoats.ConclusionsTaken together, these experiments support the hypothesis that the regulation of lactation persistency in dairy cows andgoats, subjected to hormonal or nutritional challenges, involves changes in the number of mammary cells, mediated byadjustments in the apoptosis/proliferation balance and the rate of mammary cell exfoliation into milk. AAsupplementation can also affect cell turnover.