Metabolism of nitrogenous compounds in beef cattle fed tropical forage supplemented with protein in the rumen, abomasum or both
Four Nelore steers, averaging 280 ± 10 kg BW, fitted with ruminal and abomasal cannulas were used in a 4 × 4 balanced Latin square design to evaluate the effect of protein supplementation in the rumen and/or abomasum on N metabolism in cattle fed tropical forage. The treatments were: 1) control (without supplementation); 2) ruminal supplementation (250 g/d casein); 3) ruminal plus abomasal supplementation (125 g/d casein in the rumen and 125 g/d in the abomasum); 4) abomasal supplementation (250 g/d casein). Supplements and hay were provided twice per day at 0600 h and 1800 h. The animals were fed with Tifton-85 hay (9.9% CP; 71.5% NDFap) for ad libitum intake. Each period lasted 20 d, comprising 15 d of adaptation and 6 d for sampling. On d 16 through 19 of each period, eight spot samples of abomasal digesta and feces were collected, oven-dried, composited and subsequently analyzed. On d 20 total urine collection was performed. On d 21 blood samples and ruminal fluid were taken every 6 h (beginning at 0600 h) and composited on a daily basis. Supplementation increased (P < 0.10) N intake, N total digestibility, N balance, ruminal ammonia-N (RAN), serum urea-N (SUN), renal urea clearance (RUC), RUC proportion excreted, and urinary N and urea excretion. However, there were no differences (P > 0.10) between sites of supplementation, except for RAN, SUN, RUC, and urinary N and urea excretion which presented a negative linear effect (P < 0.10) by the displacement of supplementation rumen to the abomasum. Ruminal N digestibility, microbial N flow, as well as N retained/N intake were not affected by supplementation (P > 0.10). Intestinal N digestibility was increased (P < 0.10) by supplementation. Moreover, there was a positive linear effect (P < 0.10) on N intestinal digestibility when supplement was changed from the rumen to the abomasum. Fecal N excretion was not affected (P > 0.10) by treatments. These results indicate that protein supplementation either in the rumen or in the abomasum, exerts similar effects on efficiency of N utilization, but with different metabolic events.
Keywords: beef cattle, metabolism, nitrogen