Desempenho de novilhas Nelore à pasto e cinética da fermentação ruminal da Urochloa brizantha cv. Marandu utilizando virginiamicina e/ou monensina sódica

This study aimed to evaluate the inclusion of virginiamycin (VM), monensin (MON) and the association of these additives (VM+MON) in energetic mineral supplement on the performance of Nelore heifers on pasture and a the kinetics of ruminal fermentation of Marandu grass (Urochloa, synonymy Brachiaria brizantha). In the first experiment, forty Nelore heifers were used in a randomized block design with four treatments. The treatments consisted of energetic mineral supplementation (CONT, control group) added VM (33,5 mg/100 kg WB), MON (18,8 mg/100 kg WB) and the association VM+MON (26,3 mg/100 kg WB).The animals were kept on pasture from January to June. The food intake and performance data were analyzed and compared by Tukey test (p<0.05).The forage offer was not a limiting factor for the animals performance. There was no difference (p>0.05) at the intake of pasture and average daily weight gain (0.562 kg animal day-1) for the different treatments. The variability in supplement intake and daily intake dose of the additives may have influenced the performance of the animals. In the second experiment, four adult cows fistulated at rumen were distributed in a 4x4 Latin square, in factorial 4x6, four supplements of the first experiment with the inclusion or not of additives in supplement and six Marandu grass samples collected in the months from January to June to evaluate the ruminal fermentation kinetics. The rumen fluid, collected on the 16th day of each period, was mixed to the buffer for the formation of inoculum and for incubation of forage samples. Data were compared by Tukey test (p<0.05). The additives reduced the in vitro degradability of dry matter of Marandu grass in months of better forage quality, besides reducing by 10% the total gas production. The VM reduced the in vitro methane production (mL) by 9.3% compared to MON and VM+MON and 16% compared to the control group. The association between VM and MON provided less gas production of the fibrous carbohydrates fraction and lower rate of degradation of this fraction. Given the new scenario of sustainable production, the use of additives appears to be an alternative for cattle grazing, however strategies to minimize the variations in the daily dose of additives should be evaluated trying to improve animal performance.