Characterization of ruminal temperature and its relationship with ruminal pH in beef heifers fed growing and finishing diets.

This study characterized the relationship between ruminal temperature (T rum) and pH in beef cattle fed growing and finishing diets. In Exp. 1, 16 ruminally cannulated beef heifers (388.5 ± 34.9 kg BW) were fed 4 growing diets in a replicated 4 × 4 Latin square with four 21-d periods. Diets were (DM basis) grower control (CTLg; 35% barley grain plus 5% canola meal), 40% corn dried distillers grains plus solubles (CDDGSg), 40% wheat dried distillers grains plus solubles (WDDGSg), and 37.6% WDDGSg plus 2.4% corn oil (WDDGSg+O). All diets contained 55% barley silage (DM basis). Ruminal pH and T rum were continuously monitored for 4 d each period starting on d 18. In Exp. 2, the 16 heifers were gradually transitioned (529.1 ± 41.1 kg BW) from the growing diets to 1 of 4 finishing diets in a replicated 4 × 4 Latin square with four 28-d periods. Diets were (DM basis) finisher control (CTL f), 40% corn-based distillers grains plus solubles (CDDGS f), 40% wheat-based distillers grains plus solubles (WDDGS f), and 37.4% WDDGSf + 2.6% corn oil (WDDGS f+O). All finishing diets contained 8% barley silage (DM basis). Ruminal pH and T rum were measured from d 25 to 28. With growing diets (n = 64), maximum T rum was negatively related to minimum pH (P < 0.001, r = -0.53) and positively related to starch intake (P < 0.001, r = 0.55). Maximum T rum, T rum > 40°C (h/d), and area under the curve (AUC) T rum > 38°C (area × h/d) accounted for 28.3, 9.5, and 4.7%, respectively, of the variability in minimum pH (R(2) = 0.43, P < 0.001, n = 64). Mean T rum and maximum T rum were greater for CTL g than CDDGS g, WDDGS g, and WDDGS g+O. With finishing diets (n = 63), maximum T rum was negatively related to minimum ruminal pH (P < 0.001, r = -0.63) and positively related to DMI (P < 0.001, r = 0.62) and to starch intake (P < 0.001, r = 0.58). Maximum T rum, AUC T rum > 39°C, and duration T rum > 38°C accounted for 40, 17, and 3.6%, respectively, of the variability in minimum pH (R(2) = 0.60, P < 0.001, n = 63). Mean T rum and maximum T rum were greater for CTL f than CDDGS f, WDDGS f, and WDDGS f+O. When individual animal data were examined, heifers with duration T rum > 40°C did not necessarily have greater duration pH < 5.2 or pH < 5.5. Ruminal temperature has the potential to predict ruminal pH, likely owing to the biological relationship between acid production and the heat of fermentation. Exploitation of this relationship to predict pH could provide a means of overcoming the problems associated with long-term monitoring of ruminal pH using electrode-based approaches.