Long-chain saturated fat supplements are used to increase yield of milk fat or improve energy balance in dairy cows. However, production responses to saturated fat supplements vary greatly, and therefore, it is currently not clear when these supplements should be fed. Our central hypothesis is that saturated fat supplements will have different effects on production responses depending on milk yield of dairy cows and composition of diets. In the first two experiments we evaluated the effects of palmitic (99% purity) and stearic (98% purity) acids on production and metabolic responses of past-peak dairy cows, compared with a diet with no supplemental fat. In both experiments, cows with a wide range of milk production were used to determine if cows at different levels of production responded differently to treatment diets. The first experiment showed that palmitic acid increased yields of milk (46.0 vs. 44.9 kg/d, P = 0.04), milk fat (1.53 vs. 1.45 g/d, P < 0.01), and 3.5% fat-corrected milk (3.5% FCM; 44.6 vs. 42.9 kg/d, P < 0.01) compared with a control diet with no supplemental fat, and that all cows responded similarly to treatment. The second experiment showed that stearic acid increased DMI (26.1 vs. 25.2 kg/d, P = 0.01) and yields of milk (40.2 vs. 38.5 kg/d, P = 0.02), milk fat (1.42 vs. 1.35 g/d, P < 0.01), and 3.5% FCM (40.5 vs. 38.6 kg/d, P < 0.01), with a greater response for high yielding cows (linear interaction P < 0.10). However, recovery of additional fatty acids consumed as additional yield of milk fatty acids was only 16.6% for palmitic acid and 8.2% for stearic acid supplementation. The third and last experiment evaluated the potential for a highly saturated fatty acid supplement (FAT; >85% saturated FA; 46% stearic acid and 37% palmitic acid) fed at two levels of dietary forage NDF (fNDF; 20 and 26%) to improve metabolic status and energy balance in fresh cows. Regardless of dietary fNDF content, supplementation of FAT increased DMI (23.6 vs. 22.2 kg/d, P = 0.04) and tended to decrease milk yield (46.6 vs. 49.7 kg/d, P = 0.10), improving energy balance (-12.0 vs. -17.3 Mcal/d, P < 0.01) and decreasing BCS loss (0.7 vs. 0.9, P = 0.02) when fed during the first 4 wk postpartum. However, postpartum supplementation of FAT interacted (P = 0.10) with fNDF concentration for 3.5% FCM yield when cows were fed a common diet from 5 to 10 wk postpartum: FAT decreased 3.5% FCM yield in the 20% fNDF diet (51.1 vs. 58.7 kg/d), but not in the 26% fNDF diet (58.5 vs. 58.0 kg/d). Supplementation of saturated fats might benefit lactating dairy cows in some cases but results are dependent upon fat supplements fed, diet, stage of lactation, and milk yield of cows. Further work is needed to clarify these situations as well as the marginal economic return, if any, of specific fat supplements for different situations.
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