Influence of feed intake and source of dietary carbohydrate on milk yield and composition, nitrogen balance, and plasma constituents of lactating goats.

The influence of diet (synchronized or not synchronized for the degradation rate of the carbohydrate and N fractions) and amount of feed offered [2.40 +/- 0.10 or 2.00 +/- 0.10 kg of dry matter (DM)/d] on milk yield and composition, N balance, and some plasma constituents was studied using 32 multiparous dairy goats (100 +/- 16 d in milk) that were fed a mixed diet for 9 wk. Diets were 40% concentrate with rapidly degraded starch and rapidly degraded N or highly digestible fibers and slowly degraded N. Nitrogen balance was determined at wk 4 and 8. Goats that were fed greater amounts of DM had higher yields of milk, fat-corrected milk, and protein, but lower milk fat concentrations during the whole trial. Raw milk yield was higher at wk 4 and from wk 6 to 7 for goats consuming the rapidly degraded diet than for goats fed the slowly degraded diet at a high feed intake. Concentrations of milk fat tended to be greater for goats fed the rapidly degraded diet at wk 5, 7, and 8. At wk 5 and 6, an interaction between feed intake and diet was observed for milk protein concentration. Nitrogen digestibility, milk N, and N balance were increased for goats fed at high intakes. The output of N in urine and the efficiency of N use for milk output was greater, and N balance was lower, for goats fed the rapidly degraded diet. Plasma concentrations of nonesterified fatty acids, beta-hydroxybutyrate, and urea (wk 2 to 6) were lower, and insulin concentrations were transiently increased, for goats fed at high intakes. Goats fed the rapidly degraded diet compared with goats fed the slowly degraded diet had higher plasma concentrations of urea, which may indicate inefficient use of ruminal N.