PSX-B-4 - Effect of live yeast on the ruminal fermentation characteristics of growing steers in heat-stress conditions

The objective of this trial was to determine the influence of live yeast supplementation (LY) and temperature exposure (TEMP) on the ruminal fermentation characteristics of steers receiving a grower diet. The effects of LY and TEMP were investigated using a 2 x 2 crossover design that spanned five periods. Eight Angus crossbred steers (365 ± 32 kg) were randomly split into pairs and housed in four outdoor pens outfitted with an individualized feeding system. Animals were limit fed a grower diet (DIET) at 1.2% SBW with no live yeast supplementation (NOY) or a grower diet top-dressed with 10 g live yeast/d for 14 days (1.2 × 10¹² CFU/d). On days 13 and 14, animals were subjected to one of two TEMP conditions, thermoneutral (TN; 18.4 ± 1.1°C, 57.6 ± 2.8 % RH) or heat stress (HS; 33.8 ± 0.6°C, 55.7 ± 2.7 % RH), in two side-by-side, single-stall open-circuit, indirect respiration calorimetry chambers. Data were analyzed using a random coefficients model. Carryover effects were examined and removed from the model if not significant (P > 0.05). There was no effect of DIET, TEMP, or DIET×TEMP (P > 0.05) on ruminal pH, redox, ciliated protozoa count, acetate, butyrate, total VFA, and ruminal ammonia concentrations. Similarly, the acetate to propionate ratio was not influenced by DIET, TEMP, or DIET × TEMP (P ≥ 0.190). Propionate concentration was the greatest in animals in TN conditions receiving LY (P = 0.008). Compared to HS+NOY, HS+LY (P = 0.003) and TN+LY (P = 0.043) had greater ruminal enumerations of Fusobacterium necrophorum. This suggests LY (P = 0.010) provided a favorable environment for F. necrophorum during heat stress. Live yeast supplementation did not improve overall ruminal fermentation during heat stress. Additional research is required to better understand the dynamic relationship between live yeast and temperature exposure.