DUODENAL INFUSION OF RAPESEED OIL IN MIDLACTATION COWS .6. INTERACTION WITH NIACIN ON DAIRY PERFORMANCE AND NUTRITIONAL BALANCE

Rapeseed oil and niacin, alone or in combination, were infused continu ously into the proximal duodenum of ruminally and duodenally fistulate d midlactation cows in a 4 x 4 Latin square design. Although niacin in plasma was higher for cows infused with niacin no significant effect occurred on milk production and composition. When cows were infused wi th oil, milk production and fat content were unchanged, milk protein c ontent was decreased, and lactose content was increased. The magnitude of decrease in the percentage of medium-chain fatty acids in milk of cows infused with oil was similar to the magnitude of the increase in the long-chain fatty acids of milk. Except for the decrease in milk C- 4:0 and the increase in C-14:0 and C-14:1 percentages, niacin did not exert a significant influence on the fatty acid profile of milk. Oil-f ree DMI tended to decrease in the cows that were infused with oil, but nutrient digestibility was unchanged with all infusates. Energy and p rotein intake and energy balance were similar, but protein balance ten ded to be higher with oil infusion. However, body condition score was decreased by oil infusion. For midlactation cows that received oil inf usion, the limitation in DMI or the direct effect of absorbed fatty ac ids limited milk protein synthesis and body condition score. Effective ness of postruminal niacin (alone or with oil) at midlactation is ques tioned.