This study assessed the impact of replacing soybean meal (SBM) with rapeseed meal (RSM) on the performance of lactating dairy cows with a focus on meals’ potential for utilizable Crude Protein (uCP) supply at the duodenum. Four samples of SBM and two types of RSM were used in an in vitro experiment and the uCP of the samples was estimated. In the second experiment, nine Holstein dairy cows were used in a 3 × 3 Latin square design in which three experimental diets (differing in SBM to RSM ratio) were fed to the cows for the duration of 21 days. There was a significant difference in the effective uCP between treatments in all rate of passages (p<0.001) and the effective uCP of pelleted RSM was significantly higher than the other protein meal samples. In case of soybean meals (except the outflow rate of 1% per hour), maximum and minimum of effective uCP were observed in one of the pelleted and powder SBMs, respectively. Dry matter intake, milk yield and compositions were not affected by the mentioned treatments. However, blood urea nitrogen was significantly greater (p<0.01) in the cows fed only by SBM than other groups. Cows consumed RSM significantly excreted lower urinary nitrogen compare to animals fed with SBM (p<0.01). Overall, lowering CP level in the diet of lactating dairy cows by using similar euCP supply from RSM as an alternative for SBM, decreased nitrogen losses with no influence on the animal performance.

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