Effect of feeding isoacids with urea on growth and nutrient utilization by lactating cows.

In the growth study 10 g each of isobutyrate, isovalerate, 2-methylbutyrate, n-valerate, and phenylacetate were mixed with 80 g of urea, 50 g of molasses, and 270 g of ground hay and fed to nine dairy heifers for 90 days. The control group was fed the same diet minus isoacids and phenylacetate. All animals were fed ad libitum timothy hay as the sole roughage. In the digestion trial, two mixtures of isoacids and a control ration were fed to nine lactating cows in a 3 X 3 Latin square design with 28-day feeding periods. Corn silage was the sole roughage, and concentrate was fed in proportion to milk production. Collection of feces and urine was for the last 5 days of each period. Mixture 1 of isoacids contained on a molar basis 28, 24, 24, and 24% of isobutyrate, isovalerate, 2-methylbutyrate, and n-valerate, respectively. Corresponding for mixture 2 were 36, 17, 17, and 30%. Addition of isoacids increased growth rate of younger animals but not older ones. The isoacid mixtures improved utilization of nitrogen by lactating cows. Urinary nitrogen decreased, and retention of absorbed nitrogen increased. Digestibility of dry matter and nitrogen were not affected by isoacid additions.