Ruminal Microbial Populations and Fermentation Characteristics in Beef Cattle Grazing Tropical Forage in Dry Season and Supplemented with Different Protein Levels.

We tested the hypothesis that supplementation with protein improves fermentation parameters without damaging the rumen microbial populations of beef cattle grazing Urochloa brizantha cv. Marandu during the dry season. Four rumen-cannulated Nellore bulls (571 ± 31 kg of body weight) were used in a 4 × 4 Latin square design. The treatments were not supplemented with concentrate (only free-choice mineral salt ad libitum) and supplemented (supplements with low-LPSU, medium-MPS, and high protein supplement-HPS), supplying 155, 515, and 875 g/animal/day of crude protein (CP), respectively. The abundance of each target taxon was calculated as a fraction of the total 16S rRNA gene copies in the samples, using taxon-specific and domain bacteria primers. There was no difference (P > 0.05) across treatments for intakes of dry matter (DM), forage and neutral detergent fiber (NDF), digestibility of DM and NDF, and ruminal pH. Animals supplemented with concentrate had greater (P < 0.05) intakes and digestibility of CP, ether extract and non-fibrous carbohydrate contents of the substrates (EE + NFC), and ruminal ammonia nitrogen (RAN) compared to control. Bulls that received only mineral salt had lower proportions of Butyrivibrio fibrisolvens and had greater (P < 0.05) proportions of Fibrobacter succinogenes, Ruminococcus albus, Ruminococcus flavefaciens, Methanogen archaea than bulls supplemented with concentrate. The MPS animals had greater (P < 0.05) intake and digestibility of CP, RAN concentration, and had lower (P < 0.05) proportions of Fibrobacter succinogenes, Ruminococcus flavefaciens, and Selenomonas ruminantium than LPSU animals. The HPS provided higher (P < 0.05) intake of CP, RAN and proportion of Ruminococcus albus when compared with MPS. In conclusion, supply of 515 g/animal/day of protein via supplement provides better ruminal conditions for the growth of cellulolytic bacteria of bulls on pasture during dry season.