Characterizing the Influence of a Bacillus Direct-Fed Microbial Supplement on Low Quality Roughage Digestibility

Abstract

Four experiments were conducted to evaluate direct-fed microbial (DFM) supplementation as an alternative to ionophores for improving nutrient utilization of low-quality roughages in beef cattle. Total collection trials were used to evaluate the effects of a Bacillus-based DFM product and Rumensin in beef cattle consuming corn stalks (Experiment 1) and grass hay (Experiment 2). Sixteen rumen-cannulated crossbred Angus steers (831 ± 89.4 kg) were used in a randomized block design with a 2 × 2 factorial arrangement of treatments for each experiment. Treatment included: 1) negative control (NC); 2) Bacillus-based DFM (B; 3 g∙animal-1∙d-1/d); 3) Rumensin (R; 250 mg∙animal-1∙d-1/d), and Bacillus-based DFM + Rumensin (BR). For experiments 1 and 2 digestion experiments were replicated twice, with 30 d washout periods between treatments. Apparent nutrient digestibility and retention were determined through total fecal and urine collections and daily rumen samples were obtained 2 hours post-feeding for short chain fatty acid (SCFA) analysis. Rumensin supplementation increased ADF digestibility of the corn stalk diet compared to the controls (P < 0.04), but DFM supplementation did not improve fiber digestibility or nitrogen retention compared to the control treatment on either diet (P > 0.05). Molar proportions of acetic, propionic, and butyric acids were similar across all treatments for cattle consuming the GH diet (P > 0.05). Acetic acid made up a greater proportion in the RB treatment than in the B treatment (P < 0.05), and propionic acid made up a lower proportion in the RB treatment than in the R or B treatments (P < 0.05) when cattle were consuming the GH diet.In vitro fermentation was used to investigate the impact of Rumensin and a Bacillus-based DFM, alone and in combination, on in vitro dry matter disappearance and the molar proportions of SCFA. Rumen fluid was collected from steers fitted with rumen cannulae on the aforementioned treatments, two hours post-feeding and day 5 of the digestibility experiment. In vitro batch culture fermentations were conducted in triplicate using rumen fluid from steers exposed to their appropriate dietary treatment for 30 d with ground corn stalks during experiment 3 and ground grass hay during experiment 4. In vitro samples were analyzed after 0, 12, and 24 hours of incubation. Dry matter disappearance increased with incubation time but was not affected by treatment for either forage type. For fermentation corn stalk fermentations, R increased molar proportions (P < 0.001) of propionate and decreased (P < 0.01) the molar proportions of acetate and the acetate:propionate ratio, while B increased (P < 0.03) molar proportions of butyrate. When grass hay was used as a substrate, molar proportions of butyrate decreased (P < 0.05) in steers receiving R compared to all other treatments. Results demonstrate that R alters SCFA fermentation patterns in a forage-dependent manner, while Bacillus-based DFM selectively increases butyrate production. Under the conditions of these experiments, findings suggest differential effects of R and DFM on ruminal fermentation, with limited evidence supporting DFM as a direct replacement for ionophores. Additional research is warranted to elucidate mechanisms and optimize DFM efficacy for livestock production.