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Responses in rumen microbiomes of Bos taurus and Bos indicus steers fed rice straw and supplemented protein
Keywords: cattle, microbiome, supplementation
Bos indicus typically perform better than Bos taurus when consuming a low-quality diet; however, the response to supplementation is generally greater in Bos taurus. The underlying mechanisms supporting these responses have not been elucidated. Characterization of differences in rumen bacterial populations and their role in the two subspecies may provide insight. Five cannulated Angus and Brahman steers were used in concurrent 5 × 5 Latin squares with repeated measures. Animals were offered ad libitum access to rice straw (4.7% CP). Treatments consisted of an unsupplemented control and 2 levels (50 and 120 mg N/kg BW) of isonitrogenous supplements (30% CP), either high (H; 74%) or low (L, 26%) in UIP. Rumen sample were collected at 0 and 4h post-feeding, separated into liquid and solid fractions and frozen immediately in liquid N. Rumen bacterial taxa were sequenced utilizing a Roche 454 platform based on the 16s rRNA gene. At 97% sequence similarity, 97,826 OTUs were identified, which included 21 phyla, 108 families, and 255 genera. Each taxa was analyzed using PROC Mixed with period and animal as random effects. The top 7 phyla accounted for >98% of observed abundance. Six phyla differed as a function of fraction, time, and fraction × time interaction (P < 0.05). Bacteroidetes (65%) and Firmiucutes (28%) were dominant phyla, both tended to differ as a function of subspecies × treatment and treatment (P < 0.06). The top 14 families accounted for >93% observed abundance. Unlike phyla, families were influenced more by subspecies and treatments than fraction (P < 0.05). Prevotellaceae (>42%), Ruminococcaceae (>13%), Sphingobacteriaceae (>8%) and Lachnospiraceae (>7%) were the dominant families. Genus abundance also varied as a complex function of subspecies, time and various associated interactions with treatment and fraction (P < 0.05). Twenty genera were present at >0.8% average abundance and accounted for over 85% of total observed abundance. Dominant genera were Prevotella (29.5%), unknown Prevotellaceae (11%), unknown Ruminococcaceae (8.5%) and unknown Sphingobacteriaceae (7%). Overall bacterial community diversity was greater than expected because rice hay is recalcitrant to bacterial digestion. Similarly, consistent differences in family and genus taxa between B. indicus and B. taurus suggest important roles the symbiotic rumen microbiome may have in the capacity of B. indicus to utilize low-quality forage over a range of supplement types and levels.