LAUSR

Abstract The objective of this study was to investigate the effects of dietary fat supplement rich in palmitic acid or stearic acid on milk yield and composition, fatty acid profile, energy partitioning, and metabolic parameters of dairy cows fed low or high starch diets from d 1 to 28 of the postpartum period. Forty-four multiparous Holstein cows were used in a complete randomized block (based on parity, and previous 305-d milk yield); 2 × 2 arrangement with two dietary starch levels [high starch (HS); 260 g/kg of diet dry matter (DM) versus low starch (LS); 210 g/kg of diet DM] with either palmitic acid (PA) or stearic acid (SA) supplementation at the level of 15 g/kg of diet DM. A basal close-up diet (6.19 MJ/kg of diet DM, NEL; 200 g/kg starch) was fed to animals prior to calving. Regardless of the fatty acid (FA) type, cows fed HS diets had a higher dry matter and NEL intake, milk protein and lactose content, and a more positive energy balance than cows fed LS diets during the postpartum period. Compared with HS diets, LS diets reduced energy toward body reserves and maintenance as a fraction of the NEL intake. Although there was no main effect of PA or SA supplement on intake and energy balance, the effect of fat supplementation depended on dietary starch levels for milk yields and milk energy as a fraction of the NEL intake. Stearic acid increased milk yield in HS diets while PA tended to increase yield in LS diets. Supplementation of PA partitioned less energy toward milk (and more energy for maintenance) in HS diets than LS diets. Moreover, the C16:0 FA concentration in milk fat was higher, and the polyunsaturated FA (PUFA) and C18:1 cis-9 FA were lower in PA supplemented cows fed HS diets but not cows fed LS diets. High starch rations increased de novo FA concentrations in milk and decreased C18:0 and preformed FA concentrations compared with LS diets. Also, HS diets tended to have higher levels of insulin and lower levels of beta-hydroxybutyrate (BHBA) than LS diets. In conclusion, the HS diet was more beneficial to improving the energy balance (EB) of dairy cows than LS diets during the early postpartum period. Also, interactions between dietary starch levels and FA supplement for the variables measured in the current study (e.g. milk yield or FA profile) indicate that the response to fat supplements rich in specific FA differed on the basis of dietary starch concentrations.