Effect of 2-hydroxy-4-(methylthio)butanoate (HMTBa) on risk of biohydrogenation-induced milk fat depression.

Diet-induced milk fat depression (MFD) is a multifactorial condition resulting from the interaction of numerous risk factors, including diet fermentability and unsaturated fatty acids concentration, feed additives, and individual cow effects. 2-Hydroxy-4-(methylthio)butanoate (HMTBa) is a methionine analog that has been observed to increase milk fat in some cases, and interactions with MFD risk factors may exist. The objective was to evaluate the effect of HMTBa supplementation on milk fat synthesis in cows with different levels of milk production and fed diets with increasing risk of biohydrogenation-induced MFD. Sixteen high-producing cows (44.1 ± 4.5 kg of milk/d; mean ± SD) and 14 low-producing (31.4 ± 4.3 kg of milk/d) were used in a randomized block design. Treatments were unsupplemented control and HMTBa fed at 0.1% of diet dry matter (25 g/d at 25 kg of dry matter intake). The experiment was 70 d and included a 14-d covariate period followed by 3 phases whereby diets were fed with increasing risk of MFD to determine the interaction of treatment and diet-induced MFD. During the low-risk phase, the base diet was balanced to 33.5% neutral detergent fiber (NDF) and had no exogenous oil (28 d); during the moderate-risk phase, the diet was balanced to 31% NDF and contained 0.75% soybean oil (14 d); and, during the high-risk phase, the diet was balanced to 28.5% NDF and contained 1.5% soybean oil (14 d). An interaction of treatment, production-level, and dietary phase was observed. Low producing cows neither experienced substantial biohydrogenation-induced MFD nor a response in milk fat to HMTBa supplementation. In high-producing cows, HMTBa maintained higher milk fat concentration during the moderate- (2.94 vs. 3.49%) and high-risk (2.38 vs. 3.11%) phases. High-producing cows receiving HMTBa also had greater milk fat yield (0.94 vs. 1.16 kg/d) and lower trans-10 C18:1 (6.11 vs. 1.50) during the high-risk phase. In conclusion, HMTBa increased milk fat in situations with a high risk of biohydrogenation-induced MFD by decreasing absorption of alternate biohydrogenation intermediates.