Effects of protein concentration and beta-adrenergic agonists on ruminal bacterial communities in finishing beef heifers

Abstract

To improve animal performance and modify growth by increasing lean tissue accretion, beef cattle production has relied on use of growth promoting technologies such as beta-adrenergic agonists. These synthetic catecholamines, combined with the variable inclusion of rumen degradable (RDP) and undegradable protein (RUP), improve feed efficiency and rate of gain in finishing beef cattle. However, research regarding the impact of beta-adrenergic agonists, protein level, and source on the ruminal microbiome is limited. The objective of this study was to determine the effect of different protein concentrations and beta-adrenergic agonist (ractopamine hydrochloride; RAC) on ruminal bacterial communities in finishing beef heifers. Heifers (n = 140) were ranked according to body weight and assigned to pens in a generalized complete block design with a 3 × 2 factorial arrangement of treatments of 6 different treatment combinations, containing 3 protein treatments (Control: 13.9% CP, 8.9% RDP, and 5.0% RUP; High RDP: 20.9% CP, 14.4% RDP, 6.5% RUP; or High RUP: 20.9% CP, 9.7% RDP, 11.2% RUP) and 2 RAC treatments (0 and 400 mg/day). Rumen samples were collected via orogastric tubing 7 days before harvest. DNA from rumen samples were sequenced to identify bacteria based on the V1-V3 hypervariable regions of the 16S rRNA gene. Reads from treatments were analyzed using the packages 'phyloseq' and 'dada2' within the R environment. Beta diversity was analyzed based on Bray-Curtis distances and was significantly different among protein and RAC treatments (P < 0.05). Alpha diversity metrics, such as Chao1 and Shannon diversity indices, were not significantly different (P > 0.05). Bacterial differences among treatments after analyses using PROC MIXED in SAS 9 were identified for the main effects of protein concentration (P < 0.05), rather than their interaction. These results suggest possible effects on microbial communities with different concentrations of protein but limited impact with RAC. However, both may potentially act synergistically to improve performance in finishing beef cattle.

Fig 1. Alpha-diversity metrics for ruminal bacterial richness and evenness among the treatments described by observed, Chao1 and Shannon.

Colors represent different treatment diets grouped by treatment diets of control (CON), rumen degradable protein (RDP), rumen undegradable protein (RUP), and the presence or absence of ractopamine hydrochloride (RAC).

Fig 2. Beta-diversity measurement for bacterial communities using a Bray-Curtis PCoA grouped by treatment diets of control (CON), rumen degradable protein (RDP), rumen undegradable protein (RUP), and the presence or absence of ractopamine hydrochloride (RAC).

Ellipses and points with same color represent samples within treatments groups (P < 0.05). Clusters indicate similarities among treatments groups.

Fig 3. Overall phyla relative abundance identified in the ruminal bacterial communities, grouped by the treatment diets of control (CON), rumen degradable protein (RDP), rumen undegradable protein (RUP), and the presence or absence of ractopamine hydrochloride (RAC).

Fig 4. Taxonomic profile of the 99% genera relative abundance of the ruminal bacterial communities among the treatments.

The boxplot represents the mean across all samples and different colored points represent different treatments. Purple represents the control diet (CON), blue represents the rumen degradable protein diet (RDP), and yellow represents the rumen undegradable protein diet (RUP).

Fig 5. Taxonomic profile of the 99% genera relative abundance of the ruminal bacterial communities between the protein concentration.

The boxplot represents the genus-level abundances across all samples and different colored points represent different diets. Purple represents the control diet (CON), blue represents the rumen degradable protein diet (RDP), and yellow represents the rumen undegradable protein diet (RUP).

Fig 6. Taxonomic profile of the 99% genera relative abundance of the ruminal bacterial communities between the RAC concentration.

The boxplot represents the genus-level abundances across all samples and different colored points represent different diets. Purple represents no ractopamine hydrochloride (RAC) added to the diet and yellow represents RAC added to the diet.