Effect of a blend of palmitic and stearic acid on transfer of preformed fatty acids into milk fat using an 18 C n-3 fatty acid perturbation model

The transfer of dietary fatty acids (FA) into milk fat is variable and has been reported to be higher on lower fat diets, which may be a homeorhetic adaptation to support milk fat production. The objective of this study was to determine the effect of increasing dietary fat on the kinetics of transfer of preformed FA into milk fat using a perturbation model based on abomasal infusion of a bolus of n-3 FA. Eight ruminally cannulated multiparous Holstein cows (115 ± 32 DIM and producing 41.5 ± 4.43 kg of milk; mean ± SD) were used in a switchback design with three 21-d periods. Treatments were a low-fat basal diet (CON) and the basal diet supplemented with a blend of palmitic and stearic FA fed at 2% DM (Fat; 33.5% 16:0, 53.1% 18.0, and 6.2% 18:1n-9). Increasing dietary FA tended to increase milk yield by 2.2 kg/d and increased milk fat yield by 170 g/d. Yield of preformed FA in milk was increased 119 g/d but yield of <16 C and 16 C FA was not changed compared with CON. Apparent total-tract digestibility of DM, OM, NDF, and FA were not changed by FA supplementation. Transfer of dietary 18 C FA to milk fat was 110% of dietary fat consumed in CON and was decreased to 82% in the Fat treatment. On d 15 of each period, cows were abomasally infused with a free FA stock that provided 44 g of α-linolenic acid (ALA) and 20 g of stearidonic acid (SDA), and milk FA profile was observed over the following 7 d. Maximal enrichment of ALA and SDA in milk fat occurred at the second milking following the bolus (∼14 h) and returned to baseline concentrations by 74 h. Increasing dietary fat decreased total transfer of n-3 FA by 5.4 percentage units. Total transfer of infused ALA was 51.9% and SDA was 27.9% in the CON and decreased to 43.3% and 22.7% in the high-fat diet, respectively. Similar to previous work, the majority (81%) of the n-3 FA transferred to milk fat occurred in the first 24 h, which is expected to represent direct transfer of absorbed FA to milk fat and was not influenced by dietary fat. The greater than 100% transfer efficiency of 18 C FA to milk fat and the higher transfer efficiency of n-3 FA to milk fat when feeding a lower fat diet indicates dynamic partitioning of absorbed FA that is responsive to dietary FA supply.