EFFECT OF AMOUNT AND LENGTH OF ALFALFA HAY IN DIETS CONTAINING BARLEY OR CORN ON SITE OF DIGESTION AND RUMEN MICROBIAL PROTEIN SYNTHESIS IN DAIRY COWS

Eight lactating Holstein cows fitted with simple cannulae in the rumen and proximal duodenum were used in a 2 × 2 × 2 factorial experiment in a randomized complete block design. The factors studied were: (1) forage length (full bloom alfalfa hay, long or chopped); (2) forage level in the total ration (25 or 75%); and (3) type of grain in the concentrate mixes (barley or corn). Organic matter (OM) intake was not affected by level of forage or grain type but was greater (P < 0.05) with chopped hay than with long hay. Apparent total digestion of OM increased (P < 0.01) from 59.7 to 68.1% as forage in the diet decreased from 75 to 25%, but site of digestion was unaffected. Flow to the duodenum of nonammonia-N, feed N, and microbial N were unaffected by forage level, forage length, or grain type. No differences in main effects were observed between corn and barley but interactions between grain type and forage length or amount indicate that type of grain had an influence upon site of OM digestion and flow of nonammonia-N and microbial N from the rumen. Efficiency of microbial protein synthesis (MPS) was positively associated with turnover rate of rumen solids regardless of liquid dilution rate. When no change in turnover rate of solids occurred, efficiency of microbial protein synthesis increased in association with increases in turnover rate of liquid. Key words: Lactation, cattle, bacteria, alfalfa, duodenum, rumen