Fatty Acid Digestibility of Fat Sources Fed to Dairy Cows and Effects on Concentration of Fat in Milk

Milk fat yield affects the price received for milk by dairy farmers, and given the rising feed costs, maximizing diet digestibility is critical for the economical viability of dairy farms. Although feeding animal-vegetable fat or coconut oil to dairy cows can increase energy intake, they sometimes cause milk fat depression (MFD). Feeding monensin has been found to increase feed efficiency and milk yield; however, it sometimes causes a decrease in milk fat percentage. The objective of this research was to determine which sources of fat fed in conjunction with monensin may optimize yields of milk and milk fat. Six rumen-cannulated Holstein cows were fed 3 different diets varying in fat sources with and without the addition of monensin. The basal diet consisted of 50% concentrate, 33.5% corn silage, and 16.5% alfalfa hay. The control diet (C) contained 2.4% fat. This diet was supplemented with animalvegetable fat (AV) and coconut oil (CO) separately to increase energy density and concentration of total fatty acids to approximately 6.1%. These 3 diets (C, AV, and CO) were further supplemented with 260 mg/cow/day of monensin for the remaining 3 diets (CM, AVM, and COM, respectively). Compared to C, apparent digestibility of total fatty acids increased with the addition of fat in the diet. Total fatty acid digestibility was greater with CO than AV due to more digestible medium chain fatty acids in CO. Digestibility of C16 fatty acids were greater with the addition of fat. Monensin and CO increased digestibility of C18 fatty acids. The CO diet resulted in 3 about 5 kg/d less intake compared to C and AV. Addition of CO resulted in the greatest decrease in milk yield. Milk fat yield and percentage were decreased with fat addition, especially with the feeding of CO. Although CO is not a good alternative to AV to prevent MFD in dairy cows, its use can help identify the ruminal changes in biohydrogenation promoting MFD. Introduction: While research has been conducted on the effects of feeding animal-vegetable fat, coconut oil, and the ionophore, monensin (ELANCO Animal Health, Greenfield, IN), individually to dairy cows, little work has been done on the effects of these supplements fed together. Feeding animal-vegetable fat to dairy cows can cause a decrease in milk fat percentage due to incomplete biohydrogenation of unsaturated fatty acids during digestion (Pantoja et al., 1996), especially when monensin is fed. Monensin, however, has been found to optimize bacterial populations in the rumen and increase feed efficiency, as well as milk yield (Van Der Werf et al., 1998). Coconut oil, high in medium chain fatty acids, was used as an alternative fat source to help increase the energy density in the diet (Dohme et al., 1999). Objective: For this experiment, three different diets varying in fat sources will be fed with and without the addition of monensin to determine which combinations will be most favorable to dairy cows for milk yield and milk fat percentage. The basal diet was comprised of 50% concentrate, 16.5% alfalfa hay, and 33.5% corn silage. The diet had 3% fat and was labeled the control diet. This diet was supplemented with animalvegetable fat and coconut oil separately to increase energy density and fat to