Varying degradation rates of total nonstructural carbohydrates: effects on ruminal fermentation, blood metabolites, and milk production and composition in high producing Holstein cows.

Twelve multiparous Holstein cows (6 ruminally cannulated and 6 intact) at 56 to 77 d of lactation were used in an experiment with a 3 x 6 Latin square design. Cows were fed three total mixed rations that varied in degradation rates of total nonstructural carbohydrates (6.04, 6.98, and 7.94%/h). No interactions between treatment and square were detected. Higher ruminal degradation rates of dietary total nonstructural carbohydrates increased the nonammonia N (NAN) pool of the liquid fraction but did not alter the dry matter or volume of fractional pools, microbial NAN pools, or microbial composition. Treatment did not affect dry matter intake, total concentrations of volatile fatty acids, or fiber digestibility. As ruminal degradation rates of total nonstructural carbohydrates increased, the following effects were observed: 1) the turnover rates of solids increased linearly; 2) ruminal NH3 N concentrations and degradabilities of organic matter and N decreased, but propionate concentrations, bacterial efficiency, and total NAN flows tended to increase; 3) blood glucose and insulin concentrations were not affected, but blood urea N and nonesterified fatty acids were decreased; 4) intestinal and total tract digestibilities of organic matter and total nonstructural carbohydrates increased; and 5) milk production and milk true protein content and yield increased, but energetic efficiency of milk production did not change. The highest dietary ruminal degradation rate of total nonstructural carbohydrates increased the amount of nutrients digested in the intestine and increased milk production.