Milk performance and oxidative status responses to rumen protected methionine supplementation in genotyped α-S1 casein lactating dairy goats fed two levels of metabolizable protein diets

In dairy goats, responses to rumen-protected Methionine (RPM) supplementation on milk performance and milk protein synthesis were inconsistent. The objective of the study was to evaluate the effects of supplemental RPM in diets differing in CP content on milk performance, milk composition, plasma concentration of amino-acids (AA) and parameters of oxidative status in goats genotyped for their genetic variant for alpha-S1 casein (CNS1). Sixteen goats (Body weight = 66.6 ± 8.5 kg, days in milk = 89 ± 5) were grouped into 2 blocks (High vs Low) according to their secretion of CNS1 in milk, and within each CNS1 block, goats were selected to make blocks comparable for milk yield, milk protein content, body weight and days in milk. Goats within CNS1 blocks were allotted to 2 periods cross-over (4 wks for each period with one washout week) nested in 2 × 2 factorial design with dietary CP (LP vs HP, 14 vs 16 % CP in TMR fed ad libitum) and RPM supplementation (CTL vs RPM, 0 vs 4 g/d). Dry matter intake and milk yield were individually recorded daily, whereas milk samples for milk composition determination, and blood samples were obtained individually twice weekly, and once weekly, respectively. Blood was sampled for glucose, NEFA, BHB, urea, insulin, total or oxidized glutathion, superoxide dismutase, catalase, malondialdehyde, and protein carbonyl. At the end of each period, blood was sampled during two consecutive wks to determine plasma AA profile. Goats fed HP diet had higher CP, PDIE, and LysDi (lysine digestible in intestine) intake than those fed LP diet, whereas goats supplemented with RPM had higher MetDi (methionine digestible in intestine) intake (P < 0.001) than those fed the CTL diet. Goats fed HP diet had similar milk performance, milk composition and yields, and similar feed, energy and PDI efficiencies, but higher milk urea concentration than those fed LP diet. Goats supplemented with RPM had higher milk yield only when fed the LP diet (RPM x dietary CP: P < 0.02). Compared to goats fed CTL diet, those supplemented with RPM had higher milk protein content, but this was observed only in those fed HP diet (RPM x dietary CP: P < 0.008), especially for Low CNS1 goats (RPM x CNS1: P < 0.05; RPM x dietary CP x CNS1: P < 0.11). Efficiencies of PDI, feed and energy were not altered by RPM supplementation, but CNS1 x dietary CP x RPM interaction was significant for these efficiencies. Goats supplemented with RPM tended to have higher plasma MDA (P < 0.07) and total glutathione concentrations (P < 0.09). In conclusion, increasing dietary CP failed to improve DMI, milk performance, milk protein content and protein yield, which suggest that Net Energy for lactation was a limiting factor of our experimental diets. With RPM supplementation, increase in milk production in the low CP group, and increase in milk protein content in the high CP group suggests that changes in metabolic partition of nutrients between milk secretion vs milk protein synthesis occurs according to the level of limitation of methionine in diet, which may affect the methionine requirement for goat milk production.