Rumen microbial population dynamics during adaptation to a high-grain diet

Abstract

High-grain adaptation programs are widely used with feedlot cattle to balance enhanced growth performance against the risk of acidosis. This adaptation to a high-grain diet from a high-forage diet is known to change the rumen microbial population structure and help establish a stable microbial population within the rumen. Therefore, to evaluate bacterial population dynamics during adaptation to a high-grain diet, 4 ruminally cannulated beef steers were adapted to a high-grain diet using a step-up diet regimen containing grain and hay at ratios of 20:80, 40:60, 60:40, and 80:20. The rumen bacterial populations were evaluated at each stage of the step-up diet after 1 week of adaptation, before the steers were transitioned to the next stage of the diet, using terminal restriction fragment length polymorphism (T-RFLP) analysis, 16S rRNA gene libraries, and quantitative real-time PCR. The T-RFLP analysis displayed a shift in the rumen microbial population structure during the final two stages of the step-up diet. The 16S rRNA gene libraries demonstrated two distinct rumen microbial populations in hay-fed and high-grain-fed animals and detected only 24 common operational taxonomic units out of 398 and 315, respectively. The 16S rRNA gene libraries of hay-fed animals contained a significantly higher number of bacteria belonging to the phylum Fibrobacteres, whereas the 16S rRNA gene libraries of grain-fed animals contained a significantly higher number of bacteria belonging to the phylum Bacteroidetes. Real-time PCR analysis detected significant fold increases in the Megasphaera elsdenii, Streptococcus bovis, Selenomonas ruminantium, and Prevotella bryantii populations during adaptation to the high-concentrate (high-grain) diet, whereas the Butyrivibrio fibrisolvens and Fibrobacter succinogenes populations gradually decreased as the animals were adapted to the high-concentrate diet. This study evaluates the rumen microbial population using several molecular approaches and presents a broader picture of the rumen microbial population structure during adaptation to a high-grain diet from a forage diet.

FIG. 1.

PCA of T-RFLP data. A shift in bacterial population structure begins by diet 3 (60:40 grain/hay) and is more apparent by diet 4 (80:20 grain/hay).

FIG. 2.

Firmicutes/Bacteroidetes ratio during adaptation to high-concentrate diet based on T-RFLP analysis and 16S rRNA library analysis. Gray line, prairie hay; black line, high-concentrate diet; black filled circles, rRNA 16S libraries.

FIG. 3.

Comparison of 16S rRNA gene libraries at the phylum level. Populations that are significantly different at a P value of >0.01 are indicated by an asterisk. (A) Total population (prairie hay [gray bars] versus high-concentrate diet [black bars]); (B) distribution of organisms within the phylum Firmicutes among animals on prairie hay (gray bars) versus a high-concentrate diet (black bars).

FIG. 4.

Phylogenetic analysis of libraries constructed from hay-fed animals and grain-fed animals. A consensus phylogenetic tree constructed using the maximum-likelihood method is shown. The scale bars indicate the length of 1 substitution per 100 residues. Phylogenetic analysis was performed on a nonredundant set of OTUs identified from each treatment (diet). The inner ring shows the distribution of each clone at the phylum level (see the key); the white region represents unclassified sequences. The outer ring shows where each clone originated. Green, prairie hay; blue, high concentrate; black, both.

FIG. 5.

qRT-PCR-based population changes of some selected rumen bacterial species during adaptation to a high-concentrate diet. Assays included four biological replicates and two technical replicates. Changes in population are shown as the fold change compared to the size of the population when the animals were fed hay during the adaptation phase. rpoB was used for normalization, and fold changes in bacterial populations were calculated as described in Materials and Methods.