Abstract
Abstract The aim of the present study was to evaluate the effects of reducing dietary crude protein (CP) levels and supplementing rumen-protected methionine (RPM) on blood parameters and ruminal metabolites in lactating Holstein dairy cows. A total of 30 lactating Holstein dairy cows (60 ± 7 d in milk; mean ± SD) were randomly assigned to 1 of 2 treatments: diet containing 17.3% CP without RPM (control group; CON); diet containing 16.4% CP with supplementing 15.0 g/d of RPM (treatment group; RPM). All repeated, continuous data were subjected to PROC MIXED procedure of SAS (SAS version 9.2, SAS Institute Inc., Cary, NC.). The results related to blood showed that cows in RPM group exhibited lower concentration of blood urea nitrogen than that in CON group (P < 0.001). Moreover, there were no differences among treatments on concentrations of aspartate transaminase, alanine transaminase, alkaline phosphatase, globulin and albumin (P > 0.05). In ruminal metabolites, microbial CP (MCP) of dairy cows in RPM group was higher compared with CON group (P = 0.006). The concentrations of butyrate, valerate and isovalerate of RPM group were higher than that of CON group at 2h after feeding (P < 0.05). In conclusion, lower dietary CP with RPM supplementation could improve nitrogen utilization of dairy cows and synthesis of MCP in rumen, as well as change volatile fatty acids production at 2h after feeding.
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