Effect of prolonged thermal exposure on heat production, reticular motility, rumen-fluid and -particulate passage-rate constants, and apparent digestibility in steers

Abstract

Six steers fitted with permanent rumen cannulae and fed alfalfa-grass hay were kept at either −10, 10 or 28 °C for 21 d to evaluate the effects of prolonged thermal exposure on heat production (HP) and digestive responses. HP was lowest at 28 °C and increased significantly (P < 0.05) as the temperature was reduced to −10 °C. The duration of the biphasic reticular contraction during resting was significantly (P < 0.05) reduced at −10 °C (5.4 s) compared with that at 10 °C (5.7 s) and 28 °C (5.7 s) whereas duration and amplitude of biphasic reticular contractions during rumination were elevated at −10 °C (P < 0.05). Apparent digestibilities of dry matter, organic matter, neutral detergent fibre and crude protein on average were 11% (range 9–16%) higher (P < 0.05) at 28 than at 10 °C. The particulate passage-rate constant was higher at −10 °C (P < 0.05) than at 10 and 28 °C (0.046 vs. 0.038 and 0.034, respectively). The fluid passage-rate constant showed a quadratic response to temperature and was inversely related to digestibility. This study suggests that thermal exposure not only affects rumen particulate and fluid kinetics but also influences the form and pattern of reticular contractions. Further, the temperature-induced changes in duration and amplitude of reticular contractions during rumination were associated with changes in particulate passage-rate constants. Key words: Environmental temperature, reticulum, motility, fluid and particulate passage rates, digestibility, steer.