Abstract
Forty cows were supplemented with concentrate feed containing added fat with different degrees of saturation, offered individually twice daily at a rate of 0.5 kg/kg of milk produced. For two weeks, beginning the second week of lactation, they received a pre-treatment diet to obtain data for use in subsequent analyses of covariance. The cows were assigned to blocks according to parity (1st or ≥ 2nd), date of calving, previous milk production (multiparous) or milk production during the pre-treatment period (primiparous). For the 6-wk comparison period the cows were randomly assigned to one of the following treatments: concentrate with no added fat (C); or with 5.9% fat from added tallow (T, low saturation level); a 50:50 mixture of tallow and partially hydrogenated tallow (T-PHT, intermediate saturation level); or all partially hydrogenated tallow (PHT, high saturation level). A forage dry matter intake of 5 kg/day was assumed for all treatments. The fat-supplemented diets partially replaced ground corn. Estimated net energy content of the experimental diets was 1.54 and 1.63 Meal NEL/kg for the control and fat supplemented diets, respectively. Daily intake of concentrate feed tended to be higher in the cows supplemented with fat (P = 0.09; 12.4, 13.1, 12.5 and 13.1 kg for C, T, T-PHT and PHT, respectively) and responded in a quadratic manner to the level of fat saturation (P < 0.05). The treatments did not significantly affect milk production (25.5, 24.9, 25.1 and 25.6 kg/day). Milk fat percentage was in general low and not affected by treatments (2.71, 2.69, 2.79, and 2.69%), whereas milk protein percentage tended (P = 0.09) to be lower in the fat-supplemented cows than in the control (2.87, 2.75, 2.74, and 2.69%). The partial efficiency of 4% fat-corrected milk production was not affected by fat supplementation (P > 0.05), but responded in a quadratic manner (P < 0.05) to level of fat saturation, reflecting the lower daily concentrate consumption in the cows supplemented with the T-PHT (1.61, 1.55, 1.77, 1.56 kg of milk/kg of concentrate intake, respectively). No differences were detected in visual body condition score of the cows on different treatments. Body weight change of the cows was not affected by fat supplementation; however, among those supplemented it tended (P = 0.07) to vary inversely with level of fat saturation. The inclusion of fat in the concentrate feed of cows under grazing conditions showed evidence of exerting beneficial effects on consumption of concentrates with minor effects on performance.