Supplementation of ruminally inert fat in heat-stressed dairy cows.

Abstract

Three experiments (two lactation trials and one metabolic trial) were conducted to determine effects of supplemental fat and protein quality or supplemental fat and evaporative cooling on lactational performance and nutrient digestibilities; and to test the effect of supplemental fat and protein quality on ruminal fermentation and nutrient flow to the duodenum in lactating cows. In the first lactation study, milk and milk fat yields, milk fat percentage, and efficiency of feed utilization (FCM/DMI) were increased by the supplementation of 2.5% prilled fatty acids (PFA). Cows fed a fish-blood protein combination (FB) had lower milk yield and DMI than those fed cottonseed meal. In the second lactation study, milk yield, 3.5% FCM, and fat yield were increased by evaporative cooling. Supplementation of 3% PFA rendering dietary of FA to 7.4% appear to be excessive and resulted in only slightly increased (.7 kg) milk yield. Supplementation of PFA, increased efficiency of feed utilization, but decreased FA digestibility. In the third study (metabolic trial), both high quality (HQ) protein and a high level of supplemental fat gave similar positive responses in milk production. Compared to low quality protein (LQ), HQ increased yields of milk, 3.5% FCM, milk protein, milk lactose, and milk SNF. Compared to LQ, concentrations of lysine (Lys) in coccygeal plasma were higher for HQ and leucine was lower. Arteriovenous difference of plasma concentrations of Lys also was higher for HQ than LQ. Supplementation of PFA increased DMI, yields of milk, 3.5% FCM, milk protein, milk lactose, and milk SNF. Cows fed HQ diet had higher concentration of rumen ammonia N and rumen butyrate was increased by the supplemental fat. Concentrations of total VFA, propionate, butyrate, and valerate were increased by supplementation with HQ protein, thus providing more energy for the high milk production. The HQ diet decreased ratio of acetate:propionate, but increased intakes of OM, and CP, particularly in the MF diet. Also, the HQ diet increased total tract digestibility of CP. Digestibilities of ADF, NDF, and FA were not different among treatments.