A method to quantify changes in supply of metabolizable methionine to dairy cows using concentrations of selenium in milk

A method that accurately quantifies changes in the supply of metabolizable Met following a dietary change will allow accurate economic comparisons of various Met sources. This paper describes a novel method of estimating the relative supply of metabolizable Met based on changes in the concentration of Se in milk. Selenized yeast (Se-yeast) contains selenomethionine (Se-met) and because Se-met and Met are indistinguishable by cells, Se-met can be used as a tracer of Met. We hypothesized that if the size of the Met pool was increased but intake of Se-met was constant, the concentration of Se in milk relative to milk Met would decrease. To test this hypothesis, 6 Holstein cows were fed a diet that contained 0.3 mg of Se from Se-yeast/kg of diet DM and then in a 2-period crossover experiment, were abomasally infused with water (control) or an aqueous solution that provided 9 g of Met/d. Milk was sampled during the infusion and the specific activity (SA) of milk (Se concentration divided by Met concentration) was calculated for each treatment. The SA in milk from Met-infused cows was divided by SA in milk from control cows to calculate the change in supply of metabolizable Met. As hypothesized, infusing Met reduced the SA of milk (84.7 vs. 72.5 mu g of Se/mg of Met). The calculated flow of metabolizable Met was 17% greater when cows were infused with 9 g of Met/d compared with cows infused with water (essentially the same difference was measured using SA calculated with N concentrations of milk). Assuming the infused Met was 100% absorbed, the flow of metabolizable Met for control cows was 9/0.17 = 53 g of Met/d, which agreed well with literature data and estimates derived from common nutritional models.