Effects of different levels of methionine and ruminally undegradable protein on the amino acid profile of effluent from continuous culture fermenters

Eight dual-flow continuous culture fermenters were used to study the effect s of feeding different levels of methionine and ruminally undegradable prot ein (RUP) on the amino acid (AA) profile of protein residue after microbial fermentation. Four isocaloric (1.76 Mcal/kg of NE1) and isonitrogenous (17 .7% CP) diets were formulated based on NRC recommendations for a 630-kg cow producing 42 kg/d of milk. Two dietary treatments, diets HmLr (high methio nine, low RUP) and HmHr (high methionine, high RUP), were formulated to mee t, but not exceed, more than 140% of each estimated individual AA requireme nt for milk production. Diets LmLr (low methionine, low RUP) and LmHr (low methionine, high RUP) were formulated to meet the estimated individual AA r equirement except for methionine. Diets HmLr and LmLr contained 35% RUP, an d diets HmHr and LmHr contained 43% RUP. Digestion of DM, OM, and NDF was n ot affected (P > .05) by treatments. Tyrosine, cystine, and threonine ranke d as the most resistant AA to microbial fermentation, whereas methionine an d lysine ranked among the most degradable. In general, microbial degradatio n of individual AA followed the same pattern as CP degradation, being highe r in diets HmLr and LmLr and lower in diets HmHr and LmHr. Because true mic robial digestion of methionine, cystine, tyrosine, and isoleucine was reduc ed in the presence of high levels of methionine, we concluded that suppleme ntation of dietary methionine could increase flows of these amino acids to the duodenum.