INFLUENCE OF PROCESSED CORN GRAIN IN DIETS OF DAIRY-COWS ON DIGESTIONOF NUTRIENTS AND MILK-COMPOSITION

Five primiparous Holstein cows (55 d in milk) that were fitted with ru minal and duodenal cannulas were used in a 4 x 5 incomplete Latin squa re to determine the effects of blends of steam-flaked and dry-rolled c orn on site and extent of nutrient digestion and milk yield and compos ition. Diets were fed as total mixed rations and consisted of 45% fora ge and 55% concentrate; each diet contained 27% corn grain. Dietary tr eatments were composed of blends of dry-rolled and steam-flaked corn i n ratios of 100:0, 67:33, 33:67, and 0:100. Intake of dry matter; dige stibilities of dry matter, organic matter, acid detergent fiber, cellu lose, neutral detergent fiber, fatty acids, and N; and microbial effic iency were unaffected by diet. Ruminal, postruminal, and total tract d igestion of starch increased linearly, and starch passage to the duode num decreased linearly, as the proportion of dry-rolled corn in the di et decreased. Ruminal propionate and valerate increased linearly, and acetate, butyrate, isovalerate, and the acetate to propionate ratio de creased linearly, as proportions of dry-rolled corn in the diet decrea sed; however, no changes in total volatile fatty acid concentrations i n ruminal fluid were observed. Ruminal fluid pH was similar across die ts. A decrease in dry-rolled corn decreased ruminal ammonia N and plas ma urea N linearly. Milk yield and composition, as well as milk N frac tions, were similar across diets. Although changes in fatty acid compo sition of milk fat were small, linear decreases in percentages of tran s-C-16:1 and cis-9- and cis-10-C-18:1, as well as a linear increase in the percentage of Cls,a occurred as the proportion of dry-rolled corn in the diet decreased. An increased proportion of dry-rolled corn in the diet decreased digestion of starch in the rumen, and patterns of v olatile fatty acid concentrations shifted accordingly. However, no eff ects on lactational parameters were observed.