Níveis dietéticos de proteína bruta para bovinos de corte em pastagem tropical

Abstract

The objective of this study was to evaluate the effect of crude protein levels in the diet, adjusted with variable protein supplementation, on nutritional performance, grazing behavior and productive performance of young bulls kept in tropical pasture. Thirty-six young bulls (½ Angus × ½ Nelore) were used with a mean age of 240 days and mean body weight of 217.24 ± 23.90 kg, maintained on pasture Urochloa brizantha cv. Xaraés. The experimental design was completely randomized, with four treatments, with two group repetitions. The treatments were designed to test increasing levels of crude protein (CP) in the diet, with the supply of protein supplement with 30% CP, consisting of: a control treatment (diet with forage, with 9.9% CP) and three treatments receiving a protein supplement, where the supplement supply was defined as that necessary to complement the crude protein of the forage, in order to reach the levels of 12.4%, 14.1% and 15.7% CP in the diet. A positive linear effect (P<0.05) was observed in the intake of supplement, total dry matter, crude protein, organic matter and digestible organic matter as a function of crude protein levels in the diet. However, no differences (P>0.05) were observed in dry matter intake, neutral detergent fiber and dry matter, crude protein and organic matter digestibilities. A linear effect was observed (P<0.05) only in the digestibility of neutral detergent fiber. Dietary levels of crude protein did not influence ruminal pH (P>0.05); however, provided increases (P<0.05) in ruminal ammonia nitrogen, serum urea nitrogen and urine nitrogen excretion. The grazing behavior observed during the diurnal period presented a decreasing linear effect as a function of dietary levels of crude protein (P<0.05). In addition, the crude protein level in the diet exerted an increasing linear effect on the performance of young bulls (P<0.05). The increasing crude protein level in the diet of beef cattle kept in tropical pasture, with the supply of protein supplement, does not affect the voluntary intake of forage and improves the nutritional and productive performance of the animals.