Milk fat response and milk fat and urine biomarkers of microbial nitrogen flow during supplementation with 2-hydroxy-4-(methylthio)butanoate.

2-Hydroxy-4-(methylthio)butanoate (HMTBa) is a methionine analog that has been observed to attenuate biohydrogenation (BH)-induced milk fat depression (MFD), possibly through reducing the shift to altered BH pathways. It has also been suggested that HMTBa increases microbial protein synthesis in the rumen. Our objectives were to stimulate BH-induced MFD and (1) verify HMTBa inhibition of BH-induced MFD and changes in milk fatty acids (FA) associated with altered rumen BH (i.e., trans-10 C18:1); and (2) determine the effect of HMTBa on milk fat (i.e., odd- and branched-chain FA) and urine biomarkers related to microbial N flow. Twenty-four multiparous cows (45.6 ± 8.5 kg of milk/d; mean ± standard deviation) and 12 primiparous cows (32.8 ± 3.1 kg of milk/d) were arranged in a crossover design. Treatments were unsupplemented control and HMTBa fed at 0.1% of diet dry matter intake. The experiment was 80 d and included a 10-d pretrial covariate period. Each experimental period included 2 phases that differed in risk for BH-induced MFD, including a 28-d low-risk phase (31.6% neutral detergent fiber, 21.8% starch, and no oil) and a 7-d moderate-risk phase (28.7% neutral detergent fiber, 28.1% starch, and 1.0% soybean oil). We found no interaction of treatment and parity. Milk fat yield (1.43 ± 0.51 kg/d) and milk fat trans-10 C18:1 (0.42 ± 0.08 g/100 g of FA) did not differ between treatments during the low-risk phase. However, during the moderate-risk phase, HMTBa maintained higher milk fat concentration (3.91 vs. 3.79%), tended to maintain higher milk fat yield (1.44 vs. 1.38 kg/d), and decreased milk fat trans-10 C18:1 (0.61 vs. 0.93% FA) compared with control. Additionally, HMTBa increased milk fat concentration and secretion of odd- and branched-chain FA by 5.3 and 10.2%, respectively, but urinary biomarkers of microbial N flow (i.e., purine derivatives) did not differ between treatments. However, rumen bacterial samples were not available to provide cow- or treatment-specific microbial protein-to-marker ratios, which is a critical source of variation. Additionally, transfer of odd- and branched-chain FA to milk is dependent on several factors that may affect interpretation of these biomarkers. In conclusion, HMTBa decreased absorption of alternate BH intermediates and maintained higher milk fat when feeding a diet with moderate-risk for MFD.