Assessment of rumen and fecal short chain fatty acids profiles and microbial populations in rumen fluid and feces of beef cows under two different feeding regimes

For cow-calf production of beef industry in Canada, grazing on grasslands and seeded pastures
provide a chance for farmers to reduce feeding costs compared to feeding grain-based diets. Beef
cows usually graze on seeded pastures or native grasslands with highly diverse vegetation and
plant species from spring to fall and feed silage or hay in winter. However, the knowledge on the
relationship of grazing pattern, gut microbial activity, and feed efficiency of beef cows is limited.
In this study, rumen fluid and fecal samples were collected from beef cows (n = 40) to identify
the changes of short chain fatty acids (SCFAs) and microbial profiles between two different
feeding regimes (fall deferred native grass grazing and winter silage feeding) and feed efficiency
(efficient and inefficient) groups. The SCFA concentration was analyzed with gas
chromatography and microbial populations were identified by using qPCR with specific primers
for bacteria, archaea, protozoa, and fungi, respectively. The concentrations of acetate,
propionate, and butyrate in the rumen were higher (P<0.01) in fall grazing compared to winter
feeding and rumen butyrate molar proportion was higher (P<0.05) in low-residual feed intake
(RFI) cows (efficient group) than high-RFI cows (inefficient group). However, except for the
rumen butyrate proportion, there was no difference in both rumen and fecal SCFAs profiles
between RFI groups. Rumen microbial profiles were not significantly different between fall
grazing and winter feeding, but the populations of bacteria and archaea in feces were higher (P<0.001) in winter feeding compared to fall grazing. Rumen protozoal population was higher (P<0.05) in high-RFI group, while no difference between different RFI groups was observed for bacteria, archaea, and fungi. Furthermore, feeding management x RFI (P<0.05) interactions only affected rumen and fecal protozoal population but not the other three microbial groups. These results suggest that different feeding managements with different diets and environments may affect rumen microbial population and fermentation which are closely associated with feed
efficiency of beef cows. Further study investigating microbial changes during the whole grazing
patterns throughout the year might contribute to comprehensive understanding on grazing cattle
feed efficiency and the development of specific grazing strategies.