Rumen parameters of highly supplemented Nellore cattle on Marandu grass pasture under two herbage allowances

The research aims to evaluate the effect of herbage allowances of Brachiaria brizantha cv. Marandu pastures on the rumen parameters of beef cattle receiving high levels of supplementation. Twelve rumen-cannulated cattle with an average BW of 390 kg were distributed into 12 paddocks, receiving supplementation of 2% BW. Different herbage allowances were obtained by keeping the same stocking rate in the paddocks, with different initial masses (3,370 and 8,470 kg DM/ha). The experimental period was 112 days split into four period. At the beginning of acclimation, the supplement was provided in increasing amounts, starting at 1% BW and increasing by 0.25% BW every four days, until it reached 2% BW. Rumen fluid was collected in four occasions, at six-hour intervals, in each experimental period. Samples representative of rumen content of each animal were collected via cannula and used to determine N-NH₃ and short-chain fatty acids (SCFA). The trial was completely randomized design and all data were analyzed using the MIXED procedure of the SAS 9.2 program with significance of 10%. Herbage mass and herbage allowance averaged 6,507 and 2,711 kg/DM/ha and 2.91 and 1.31 kg/DM/kg BW on the treatments with high and low herbage allowance, respectively. Regarding the SCFA, there was herbage allowance vs. evaluation period interaction on acetate concentrations (P=0.01), propionate, and acetate:propionate ratio (P<0.01). Acetate differed only in fourth period, in which it was 5% higher under high allowance (P=0.01). Propionate concentration and acetate: propionate ratio changed at first and fourth period. Higher allowance provided a 4% increase in propionate concentration at first period, but an 18% decrease at fourth. The high allowance reduced the acetate: propionate ratio by 5% at first period, but increased it by 20% at fourth period. N-NH3 was the only variable affected by the allowance vs. collection time interaction , and only six hours after supplementation, it was noticed difference, with high allowance providing a 37% increase in the animals compared to the low herbage allowance (P>0.10). Different herbage allowances cause alterations in rumen parameters of pasture-finished animals receiving high levels of supplementation. Supported by FAPESP 2014/05510-0 and CONNAN.