EFFECT OF ALTERING THE NONSTRUCTURAL - STRUCTURAL CARBOHYDRATE RATIO IN A PASTURE DIET ON MILK-PRODUCTION AND RUMINAL METABOLITES IN COWS IN EARLY AND LATE LACTATION

The effect on digestibility, ruminal metabolites, microbial protein sy nthesis and milk production of manipulating the non-structural (NSC):s tructural (SC) carbohydrate ratio in a predominantly pasture diet was investigated in cows in early (trial Ii and late (trial 2) lactation. Twenty-four cows in trial I and 15 cows in trial 2 were offered pastur e only (P), 0.85 P plus 0.15 NSC/protein mixture (PR), and P plus an a dditional 0.1 (trial I) or 0.15 (trial 2) NSC (PEI in a Latin-square a rrangement. All diets were isonitrogenous and P and PR were isoenerget ic. PE but not PX increased microbial protein synthesis and decreased ruminal ammonia and milk urea levels, compared with P. Efficiency of m icrobial synthesis (g N per kg digestible organic matter intake) was n ot altered by treatment. Treatments had minor effects on ruminal pH an ti no effect on volatile fatty acid concentrations. PE and PX did not affect milk yield or protein yield and decreased fat yield compared wi th P in trial 1. Milk yield was increased on PE and PX compared with P and was greater on PE than PX, in trial 2. Yields of fat and protein were higher on PE than on P and yield of protein seas higher on PR tha n on P. The results suggest that increasing the ratio of NSC: protein by increasing total carbohydrate intake was more effective in improvin g nitrogen utilization in the rumen than was increasing the NSC:SC rat io without increasing carbohydrate intake.