Influence of varying protein and starch degradabilities on performance of lactating cows.

A milk production and digestibility trial with 32 cows (2 X 2 factorial with 8 cows per treatment) was conducted to test animal response to varying rumen degradation of dietary starch and protein. Four rations (barley-cottonseed, barley-brewers dried grains, milo-cottonseed, and milobrewers dried grains) were compared in early lactation cows fed 65% concentrate and 35% forage for 60 d. Digestibilities of starch were higher for barley than milo diets; whereas, those for organic matter were higher for cottonseed meal than brewers dried grains. Milk production was highest on barley-cottonseed with mean adjusted values of (kg/d): 37.4, 34.9, 34.2, and 34.6 for respective treatments. Dry matter intakes were not different among rations. Milk fat was higher on milo than barley with respective diets averaging (%): 3.1, 2.9, 3.4, and 3.6, and resulting in lowest FCM for the barley-brewers dried grains diet. Milk protein was not altered by treatment. These data suggest that responses to varying protein degradability can be altered by rate of starch breakdown in the rumen.