EFFICIENCY OF CONCENTRATE UTILIZATION IN MILK PRODUCTION BY GYR COWS SELECTED FOR MILK UNDER FEEDLOT OR PASTURE CONDITIONS

Abstract

Two experiments were performed with milking Gyr cows with the objective of evaluating the efficiency of supplements use in milk production. EXP 1 was carried out with 20 cows confined, being distributed in random blocks design, with for treatments and five replicates. The experimental treatments were composed by four diets containing four levels of concentrate (LC) and four levels of crude protein (CP, dietary dry matter basis): 11.7% LC and 11% CP; 23.2% LC and 12% CP; 35.2% LC and 14% CP; and 46.8% LC and 16% CP. Sorghum silage was used as forage. In EXP II, 21 cows were evaluated with increasing levels of concentrate, being the animals maintained in pastures of elephant grass (Pennisetum purpureum). The treatments were based on three levels of concentrate ration (2, 4, and 6 kg/cow/day) and two levels of CP in the total dietary dry matter (12 and 15%), plus an control treatment, in an 3 x 2 + 1 factorial arrangement. The animal response to supplementation was analyzed by Lineweaver-Burk model, using linear regression of the reciprocal of milk production as a function of reciprocal of nutrients supply. The theoretical maximum production was 12.94 and 12.77 kg of milk/cow/day and the amount of supplement to reach half of theoretical maximum production (Ks) was 0.34 and 0.17 kg/cow/day for the animals maintained in confined and pasture. Thus, the efficiency of concentrate utilization decreases and for the evaluated conditions it is lowest for the pasture fed animals.