Reducing Protein Content with and Without Yeast Probiotic Actisaf Sc 47 Supplementation in the Diet of Dairy Cow: Effects on Nitrogen Use, Digestibility, and Rumen Microbial Protein

This study aimed to evaluate the effects of lowering crude protein (CP) levels in the diet of dairy cows alone or in combination with a yeast probiotic on nitrogen utilization, digestion, and rumen microbial protein synthesis. In total, six cows were included in a 3 × 3 Latin square design. Each study period consisted of 23 days of diet adaptation and 5 days for sampling and data collection. Cows were randomly assigned to one of the three diets according to protein level and supplementation: control diet with 16.5 CP%DM (CTR), a diet with 14.5 CP%DM without Actisaf Sc 47 supplementation (LCP), and a diet with 14.5 CP%DM with Actisaf Sc 47 supplementation at 5 g/cow/day (LCPActisaf). Reducing protein content from 16.5 to 14.5 CP%DM alone or in combination with Actisaf Sc 47 significantly reduced nitrogen intake. Regarding fecal nitrogen (FN), no difference was observed between the CTR and LCP groups. However, FN was significantly lower in the LCPActisaf group compared with that of the CTR group. There was a tendency for lower FN in the LCPActisaf group compared with the LCP group. Compared with CTR, urinary nitrogen (UN) was not impacted by LCP but tended to decrease with LCPActisaf (p = 0.1). Compared with CTR and LCP, supplementation with Actisaf Sc 47 increased nitrogen use efficiency (NUE) (p < 0.05). A tendency for higher NUE was observed in the LCP group compared with the CTR group (p = 0.07). Crude protein digestibility was similar between the CTR and LCP groups and tended to increase with Actisaf Sc 47 supplementation. Compared with CTR, neutral detergent fiber (NDF) digestibility was not impacted by LCP but increased by LCPActisaf (p = 0.05); LCPActisaf did not impact NDF digestibility compared with LCP. Organic matter (OM) digestibility was similar among treatments. Although there was no difference between the three groups on rumen microbial protein synthesis (RMP), supplementation with Actisaf Sc 47 increased RMP. Reducing protein content in combination with Actisaf Sc 47 appears to be a good strategy, enabling both the environmental footprint linked to nitrogen losses to be reduced, and fiber digestibility and rumen microbial protein synthesis to increase.