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Methionine (Met) is one of the first limiting amino acids (AA) for milk and milk protein production in many dairy diets. Before dietary Met can be utilized by the cow, it is first exposed to microbial degradation in the rumen. Met that bypasses ruminal degradation, is absorbed in the hindgut, and is available for metabolism by the cow is termed ‘bioavailable’ Met. Met flow to the hindgut can be increased by adding supplemental Met to the diet or by supporting microbial protein synthesis (MPS) and subsequently increasing the flow of microbial protein (MCP) to the small intestine. The majority of protein, 50 to 80%, that reaches the hindgut of ruminants comes from MCP, whose AA composition closely resembles that of milk. Two studies were conducted to better understand the effect of Met supplementation on Met bioavailability and the impact on MPS in the rumen. In the first study, Se substitution for Met was used to estimate the change in the flux of methionine at the mammary gland. This method was used to compare the bioavailability of five Met treatments: control, DL-Met, Smartamine (SMA), 2-hydroxy-4-(methylthio)-butanoic acid (HMB), the isopropyl ester of HMB (HMBi), and an experimental rumen protected supplement (ERPM). Met supplements were fed to twenty Holstein cows as a top dressing that provided 18g of Met derivative/d, in a truncated Latin square experiment. In the second study, the effect of supplementation of Met, from Met and Met analogs, on the synthesis of Met de novo versus the incorporation of preformed Met into MCP was determined by dosing 15NH3 and 13C-Met in batch cultures assigned to a randomized complete block experiment. Treatments consisted of control, 0.097% L-Met, 0.097% D-Met, 0.125% HMBi, 0.098% HMB, 0.250% HMBi (2×HMBi), 0.063% HMBi + 0.049% DL-Met (HMBi+MET), and 0.098% HMB + 0.039% isopropanol (HMB+ISO) and treatments were inoculated with faunated or defaunated inocula. Unless otherwise specified, treatments within experiments were equal molar. The calculated metabolizable Met flow in the first study for cows supplemented with DL-Met, SMA, HMB, HMBi, and ERPM was 42.4, 47.0, 52.8, 48.7, and 50.3 g/d respectively. Metabolizable Met estimates were adjusted for reduced DMI with HMBi supplementation (20.4 kg/d versus 21.9 kg/d for control, P < 0.01). Mean milk production for all cows was 30.3 kg/d and was not affected by treatment. In the second experiment, both 13C and 15N enrichment of the pellet differed over time (P < 0.001). L-MET decreased (P < 0.01) 13C enrichment of bacterial Met compared with HMBi+MET and both had lower (P < 0.03) enrichment than all other treatments. D-MET tended to have lower enrichment than HMBi (P = 0.16) and 2×HMBi (P = 0.12). In order to make definitive comparisons regarding the synthesis of Met de novo versus incorporation from pre-formed (treatment) Met into MCP, free 13C-Met in the supernatant needs to be determined. It is possible that lower enrichment are due to dilution of labeled Met by an increased incorporation of Met into MCP provided from L-MET, HMBi+MET and D-MET compared to other treatments. |
