Rumen bacteria and feed efficiency of beef cattle fed diets with different protein content

Context: Beef cattle feed efficiency is challenged in northern Australian production systems due to the limited dietary protein, leading to changes in rumen bacterial populations and fermentation outcomes. Aims: Two types of diets with different dietary protein contents were used to evaluate changes in rumen bacterial composition and diversity, aiming to correlate rumen bacterial populations with feed and rumen efficiency parameters. Methods: In total, 90 Brahman steers (341 ± 45 kg BW) were selected for this trial, but rumen fluid was collected from 85 Brahman steers, at 0 and 4 h after feeding, during a feed-efficiency trial. The steers were fed with a low-protein diet, including 70% rumen-degradable protein and 8.8% crude protein (CP) for 60 days, followed by a high-protein diet for the same period (13.5% CP). Liveweight and dry-matter intake measurements, as well as urine, faeces and rumen fluid samples, were collected to determine feed and rumen efficiency, and ruminal bacteria composition. Steers were clustered into groups using principal component analysis and Ward's hierarchical method, and differences in feed-efficiency parameters among clusters were compared. Key results: Rumen bacterial composition differed between diets (P < 0.01) and diversity changes were more related to bacterial richness (P < 0.01). In a low-protein diet, there were four distinct clusters of steers, on the basis of rumen bacteria, in which the most efficient steers, with a better residual feed intake (P = 0.06) and lower rumen ammonia concentration (P < 0.01) before feeding, had the highest relative abundance of Prevotella (P < 0.01). While in a high-protein diet, no differences were observed on feed or rumen fermentation parameters among steer clusters. Conclusion: In a low-protein diet, rumen bacterial shifting might contribute to upregulate nitrogen recycling, favouring feed efficiency. Implications: Identifying ruminal bacterial populations involved in nitrogen recycling upregulation might be useful to select the most efficient cattle fed low-protein diets.