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

Using the natural abundance of nitrogen isotopes to identify cattle with greater efficiency in protein-limiting diets

The difficulty in selecting cattle for higher feed and nitrogen use efficiency (NUE) is an important factor contributing to poor growth and reproductive performance in dry-tropics rangelands. Therefore, the objectives were to examine the cattle variation in retaining nitrogen in a protein-deficient diet and the natural abundance of stable isotopes in body tissues as a practical alternative for the detection of more efficient cattle. In experiment 1, feed efficiency parameters were determined in 89 Brahman steers fed a protein-limiting diet for 70 days, followed by 7 days in metabolism crates for total collection of urine and faeces and calculation of nitrogen retention and NUE. The diet-animal fractionation of nitrogen isotopes (D15N) was quantified in tail hair and plasma proteins using isotope-ratio MS. There was a large variation in growth performance, feed efficiency and nitrogen losses among steers. Quantifying D15N in tail hair (D15Ntail hair) resulted in stronger correlations with feed efficiency and nitrogen metabolism parameters than when quantified in plasma proteins. D15Ntail hair was positively correlated with nitrogen losses in urine (r = 0.31, P < 0.01) and faeces (r = 0.25, P = 0.04), leading to a negative correlation with NUE (r = 0.40, P < 0.01). The group of steers with lower D15Ntail hair had greater feed efficiency, lower nitrogen losses, and greater NUE. In experiment 2, for evaluation of isotope fraction as a predictor of reproductive performance, 630 Brahman-crossed cows were classified for reproductive performance for 2 years. From this group, 25 cows with poor reproductive performance and 25 cows with good reproductive perfor- mance were selected. Tail hair representing 7 months of growth were segmented and analysed for carbon (d13C) and nitrogen (d15N) isotope enrichment. Reproductive performance was not associated with diet selection, as there was no difference in tail hair d13C between groups. However, more productive cows had lower (P < 0.05) tail hair d15N during the dry season, indicating differences in N metabolism and pos- sibly lower N losses. In addition, cows with better reproductive performance and, therefore, greater nutri- ent demands, had similar body condition scores and a tendency (P = 0.09) for higher live weight at the end of the trial. In conclusion, the findings of the present study confirm that nitrogen isotope fractiona- tion in tail hair can be used as a predictor of nitrogen losses, NUE, and reproductive performance of Brahman cattle on low-protein diets

Using live yeast to improve forage digestibility and performance of steers

Northern Australia’s pastoral lands often experience short rainfall for extended periods of the year, hence not being uncommon for cattle to perform poorly, or even lose weight due to low quality of grasses in those conditions. To overcome this problem, producers engage in supplementary feeding, emphasizing the use of small amounts of protein. The addition of rumen specific live yeast (LY) to supplements may increase the use of forage by establishing a healthy gastrointestinal tract and increasing digestibility. The objective of this experiment was to evaluate LY added to a dry lick protein supplement. It was hypothesised that LY would enhance rumen forage digestibility and therefore increase dry matter (DM) intake and average daily gain (ADG) of growing steers consuming a low crude protein (CP) tropical forage