Effect of altering the non-structural: 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 synthesis and milk production of manipulating the non-structural (NSC):structural (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 pasture only (P), 0.85 P plus 0.15 NSC/protein mixture (PR), and P plus an additional 0.1 (trial I) or 0.15 (trial 2) NSC (PEI in a Latin-square arrangement. All diets were isonitrogenous and P and PR were isoenergetic. PE but not PX increased microbial protein synthesis and decreased ruminal ammonia and milk urea levels, compared with P. Efficiency of microbial synthesis (g N per kg digestible organic matter intake) was not altered by treatment. Treatments had minor effects on ruminal pH anti no effect on volatile fatty acid concentrations. PE and PX did not affect milk yield or protein yield and decreased fat yield compared with 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 than on P. The results suggest that increasing the ratio of NSC: protein by increasing total carbohydrate intake was more effective in improving nitrogen utilization in the rumen than was increasing the NSC:SC ratio without increasing carbohydrate intake.