Influence of feed intake and source of dietary carbohydrate on the metabolic response to propionate and glucose challenges in lactating goats.

The influence of type of nutrients in the diet (rapidly degraded starch plus rapidly degraded N or highly digestible fiber plus slowly degraded N) and amount of feed offered (2.40 +/- 0.10 or 2.00 +/- 0.10 kg of dry matter/d) on plasma constituents after a glucose tolerance test or a propionate challenge test were studied using 12 multiparous dairy goats. Type of diet and interaction between type of diet and amount of feed offered had no effect on the concentrations of any plasma metabolite measured during propionate challenge. During propionate challenge, goats fed at the high dry matter intake (DMI) had a higher clearance of propionate and glucose and a higher ratio of plasma insulin to glucose. During glucose challenges, goats fed diets that were rich in highly digestible fiber tended to have lower peak concentrations of glucose, and goats fed at the high DMI had a higher clearance of glucose. The plasma insulin response to the glucose test was not altered by treatment. The ratio of plasma insulin to glucose was numerically, although nonsignificantly, higher for goats fed at the high rate of DMI. Most of the differences concerning glucose metabolism, insulin response, and tissue responsiveness to insulin appeared to be primarily due to the amount of feed offered and not to the type of diets fed to dairy goats in midlactation.