Abstract：The purpose of this study was to investigate the effects of different ratios of rumen-protected soybean meal replacing conventional soybean meal on performance and plasma biochemical indexes of dairy cows. Sixty healthy Holstein dairy cows with similar parity, weight, average milk yield and lactation days were selected and randomly divided into 4 groups with 15 cows per group. Cows in the control group were fed a basal diet, cows in the experimental group Ⅰ were fed the diet which used 12.5% rumen-protected soybean meal instead of conventional soybean meal, cows in the experimental group Ⅱ were fed the diet which used 25.0% rumen-protected soybean meal instead of conventional soybean meal, and cows in the experimental group Ⅲ were fed the diet which used 50.0% rumen-protected soybean meal instead of conventional soybean meal. The pre-experimental period lasted for 45 days, and the experimental period lasted for 45 days. The results showed as follows:1) the dry matter intake of dairy cows of experimental groups was significantly higher than that of the control group (P<0.05), and increased with the rumen-protected soybean meal replacement ratio increasing, but there was no significant difference among the experimental groups (P>0.05). 2) The milk yield of dairy cows of experimental groups was significantly higher than that of the control group (P<0.05), and significantly increased with the rumen-protected soybean meal replacement ratio increasing (P<0.05). The milk protein percentage of dairy cows of experimental group Ⅲ was significantly higher than that of experimental group Ⅱ, experimental group Ⅰ and the control group (P<0.05). The milk urea nitrogen content of dairy cows of experimental group Ⅲ and experimental group Ⅱ was significantly higher than that of experimental group Ⅰ and the control group (P<0.05). There were no significant differences in milk fat percentage, lactose percentage, total milk solid and milk somatic cell count among all groups (P>0.05). 3) The contents of total protein and glucose in plasma of experimental groups were significantly higher than those of the control group (P<0.05), and significantly increased with the rumen-protected soybean meal replacement ratio increasing (P<0.05). There were no significant differences in urea nitrogen content and alanine aminotransferase, aspartate transaminase and alkaline phosphatase activities in plasma among all groups (P>0.05). Therefore, the application of rumen-protected soybean meal instead of conventional soybean meal in large-scale dairy farms in China can improve the dry matter intake of dairy cows, improve the milk composition and metabolism of dairy cows. Under the experimental conditions, the rumen-protected soybean meal instead of 50.0% conventional soybean meal in the basal diet has the best effect.
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