Abstract: 【Objective】 In this experiment,the bioavailability of three types of rumen-protected methionines (A,B and C) was measured using the selenomethionine method and the modified in vitro three-step procedure,respectively,and the results obtained from the two methods were subjected to correlation and regression analyses aiming at exploring the relationship between the two methods. 【Method】 ① Selenomethionine method:Thirty-three healthy lactating dairy cows were used in the experiment,divided into three groups according to a randomised block trial design,with a total of 32 days divided into three phases,of which the first 7 days were pre-feeding (phase 0),the days 8 to 22 were phase 1,and the following 10 days were phase 2.In phase 0,cows were fed only the basal diet,and starting from phase 1,all cows were supplemented with selenium yeast at a dose of 0.3 mg Se/kg DM,in Phase 2,continued with the same dose of selenium yeast supplementation,all cows were supplemented with rumen-protected methionine supplementation A (33.3 g/d,group A),B (29.4 g/d,group B) and C (56.82 g/d,group C) based on the same amount of effective methionine (25 g/d).Milk samples were collected on days 22 and 32 for the determination of selenium,methionine and nitrogen concentrations,and the bioavailability of the three rumen-protected methionines was measured as the change in the ratio of selenium concentration/methionine concentration (selenium concentration/nitrogen concentration) in phases 1 and 2.② Modified in vitro three-step procedure:Three lactating dairy cows fitted with rumen fistulas were used in the experiment,and each of the three rumen-protected methionines (rumen-protected methionine A,B,and C) was set up in two parallels and three replicates,and the bioavailability of rumen-protected methionines were assessed by rumen incubation and in vitro simulation of gastric-small intestinal digestion of DM and CP.The correlation between the two methods was analysed and regression equations were established. 【Result】 The results of the selenomethionine method showed that the bioavailability of rumen-protected methionine A based on changes in the ratio of milk selenium concentration/methionine concentration and milk selenium concentration/nitrogen concentration were 83.02％ and 81.42％,respectively,while for rumen-protected methionine B,72.94％ and 72.47％,respectively,and for rumen-protected methionine C,51.77％ and 50.98％,respectively.② The results of the modified in vitro three-step procedure showed that the bioavailability of rumen-protected methionine A,B and C were 86.87％,75.47％ and 59.56％,respectively.The analysis of the results of the two methods showed that there was no significant difference (P＞0.05),and there was a highly significant positive correlation (r＝0.867-0.964,P＜0.01) between the results of the two methods. 【Conclusion】 Under the conditions of this test,both the selenomethionine method and the modified in vitro three-step procedure could be used for the determination of the bioavailability of the three types of rumen-protected methionines,and a strong positive correlation existed between the two methods.

Key words: selenomethionine method; modified in vitro three-step procedure; rumen-protected methionine; bioavailability