Suplementação de metionina protegida da degradação ruminal para vacas leiteiras alimentadas com dietas de alta ou baixa proteína

Abstract

The supplementation of dairy cows with rumen-protected methionine (RPM) can increase milk protein yield and content, but the response may be dependent on the protein profile of the basal diet. The objective of this experiment was to evaluate the supplementation of RPM to soybean meal based diets varying in protein profile. Treatments were protein levels low 15.6 (LCP) or high 17.2 (HCP) in factorial arrangement with no supplementation (CTL) or supplementation with RPM (0.07% of Smartamine M in diet DM). All diets had 0.3% of urea in DM. The predicted lysine to methionine ratio in MP was 2.9 for diets with RPM and 3.5 for CTL. Sixteen Holstein cows (96 days in milk) were assigned to four 4×4 Latin Squares, with 21-day periods, and 14 days of adaptation. The statistical model contained the random effect of cow and the fixed effects of period, protein (PT), methionine (MET), and the interaction between PT and MET. Significance was declared at P ≤ 0.05 and trends at P ≤ 0.10. Milk yield (37.2 kg/d) and DM intake (24.7 kg/d) did not differ. The yields and concentrations of casein and protein were increased by HCP. The RPM reduced milk lactose concentration, tended to increase fat concentration, and increased milk protein and casein concentrations only with diet HCP. High protein increased milk and plasma urea-N, increased the proportion of N intake in urine and reduced the proportion in milk, increased rumen microbial yield, but had no effect on total tract nutrient digestibility. The RPM increased neutral detergent fiber and OM digestibilities and had no effect on rumen microbial yield and the partitioning of N intake into milk, feces, and urine. The total tract digestibility of starch and plasma glucose concentration were increased by RPM only in diet HCP. The protein content of the diet did not affect plasma amino acid (AA) concentrations. The RPM increased the concentrations of plasma methionine and reduced the ration of lysine to methionine and tended to increase the concentrations of total of essential AA, arginine, and lysine. Branched chain AA, leucine, and histidine were increased by RPM only in HCP. High protein in the diet increased milk protein secretion but reduced the efficiency of N usage for lactation. The marginal efficiency of converting RPM protein to milk protein was higher with the high protein diet. Plasma AA concentration did not reflect the change in diet protein profile and plasma methionine concentration was a plausible marker of the capacity of a RPM source to supply digestible methionine, independently of dietary protein content. The consideration of the ruminal requirement for true protein may favor rumen function when low protein diets are adopted.