Abstract
Most cow-calf producers utilize pasture grazing during the summer months to maintain favorable growth at lower costs. Recent studies have demonstrated a relationship between the rumen microbiota and important performance traits such as feed efficiency and methane emissions in cattle. However, this has not been studied in pasture-grazed cattle due to the difficulty of collecting rumen samples and measuring production traits in these systems. The objectives of this study were to characterize variation in rumen microbiota composition and fermentation profiles in cattle grazed on different pastures, and to link this variation to feed efficiency and methane emissions. In this study, rumen fluid samples were taken from 60 heifers being tested for residual feed intake (RFI) in feedlot under a 100% barley silage diet. Similarly, rumen fluid samples were subsequently taken from 8 high-RFI (inefficient) and 8 low-RFI (efficient) heifers while grazing as separate herds on forage oats and tested for methane emissions using an open-path Fourier Transform Infrared (OP-FTIR) spectrometry method. Total DNA was extracted from all rumen fluid samples and quantitative real-time PCR (qPCR) was performed to estimate microbial populations. Volatile fatty acid (VFA) concentrations were assessed by gas chromatography. The qPCR results indicated that inefficient cattle had less rumen protozoa on grazing than when they were in the feedlot (2.5 × 10(6) vs 8.6 × 10(6), p-value < 0.05). Regardless of RFI, cattle had more total rumen methanogens during grazing than in feedlot (1.5 × 10(8) vs 0.6 × 10(8), p-value < 0.05). The VFA profiles showed that grazed cattle had lower acetate:propionate ratios versus feedlot animals (p-value < 0.05), indicating more efficient microbial activity while grazing. In conclusion, compared to feedlot barley diet with silage, grazing on pasture led to divergent rumen microbial composition and changes in VFA production, which may be associated with cattle feed efficiency and methane emission.