2 Measuring the Impact of Heat Stress Abatement Strategies on Fecal Microbiome in Finishing Steers During the Summer Months in the Southeast United States

Abstract This study evaluated the effect of heat stress abatement strategies on the faecal microbiome of finishing beef steers in the Southeastern United States. The study was designed as a completely randomized block using 32 steers (BW 453k 4 kg) stratified by weight and randomly assigned to one of four treatment groups: covered with fans (CWF), covered no fans (CNF), outside with shade (SHADE), and outside without shade (OUT). All steers were housed outside from June to July and were brought up on a common feedlot ration. After-which steers were placed into treatments on d -10 and covered steers were given 10 days to acclimate Calan-gate system. During the acclimation phase, both covered and outside steers were brought up at a similar rate. Steers were weighed and faecal samples were collected on d 0, 50, and 85. Microbial DNA was extracted from the samples using a combination of mechanical and enzymatic processes, and the DNA sequences were analyzed using QIIME v2.0. There was a treatment x day interaction for Actinobacteriota, Bacteroidota, and Patescibacteria phyla. Additionally, there was a day effect for all three phyla (P < 0.001), but there was only a treatment effect (P < 0.001) for Patescibacteria. There was not an interaction for Fibrobacterota, Firmicutes, or Proteobacteria (P > 0.226), but there was a day effect (P < 0.019) for all three phyla. There was a treatment effect (P = 0.036) for Fibrobacterota, however there was not a treatment effect for Firmicutes and Proteobacteria (P > 0.394). Moreover, there was no interaction, treatment, or day effect (P > 0.39) for the Firmicutes to Bacteroidota ratio. These results suggest that stress abatement strategies may influence the fecal microbiome over time, and they might help better understand how stress affects the microbiome and nutrient digestibility in the lower gut.