Effects of Heat Load and Active Dry Yeast Supplementation on Ruminal Parameters

An active dry yeast (ADY) supplement (Vistacell, AB Vista, UK) was proposed to favorably influence ruminal parameters of heat loaded cattle. Eight British-cross steers (365 ± 27 kg) were fed a 90 % concentrate diet (1.8 % of BW intake, DM basis) and supplemented with either ADY (3.0 g/d; providing 6 × 10¹⁰ CFU/d) or no yeast (0.0 g/d; CON) for 14 d. Rumen boli were inserted through the esophagus on d 0 for continuous monitoring of pH and rumen temperature. On d 13 and 14, steers were housed in an open circuit calorimetry chamber for 48 h in either thermoneutral (TN) (18 ± 0.55°C; 35% RH) or heat loaded (HL) (35 ± 0.55°C; 35% RH) temperature. After 48 h, rumen fluid (500 mL) was extracted via esophageal pump and immediately processed for further analysis and in vitro fermentation using a 2×2 factorial arrangement with the in vitro gas production technique (IVGP). The IVGP revealed an effect of ADY on DMD versus CON (61.7 % vs. 32.8 %, P = 0.031) but there was no effect of temperature. No effects were observed on the fermentation dynamics of the DM. There was a significant interaction of supplement and temperature on estimated ME in which ADY+TN, CON+HS, and ADY+HL were greater (P = 0.0307) than CON+TN (2.97, 2.84, 2.72 vs. 1.95 Mcal/kg, respectively). The IVGP of methane (CH₄) g/L or CH₄ g/L/g of DM digested was not affected. Mean rumen pH of steers tended to be affected by temperature and supplement: HL steers had lower pH than TN (P = 0.064), and CON steers had lower pH than ADY (P = 0.077). Time spent below pH of 6 was affected by supplement: ADY was much less than CON (390.0 vs. 882.5 min/d, P = 0.05). Time spent below pH of 5.0 was affected by temperature: TN was much less than HL (1.75 vs. 590.75 min/d, P = 0.034, respectively). Mean rumen temperature and time spent above 40°C was affected by temperature but not by supplement. As expected HL steers had greater rumen temperature and spent more time above 40°C than TN steers (40.51 vs. 39.21 °C, P = 0.0165; 1,216.25 vs. 27.5 min/d, P = 0.002). Our results indicated that supplementation of 3.0 g/d of an ADY in the diets of steers consuming 90 % concentrate diets under severe heat loads may increase DMD, increase available ME, and relieve subacute ruminal acidosis.