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. 1992. Effect of prolonged thermal exposure on heat production, reticular motility, fumen-fluid and -particulate passage-rate constants, and apparent digestibility in steers. Can. J. Anim. . Six steers fitted with permanent rumen cannulae and fed alfalfa-grass hay were kept at either -10, l0 or 28"C for 21 d to evaluate the effects ofprolonged thermal exposue on heat production (HP) and digestive responses. HP was lowest at 28'C and increased significantiy (P < 0.05) as the ternperature 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, orga4c matter, neutral detergent fibre and crude protein on average were 17% (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 prolong6e ir un regime thermique sur la production calorifique et sur la digestion. La production calorifique 6tait au plus bas d 28"C et augmentait significativement (P < 005) ir mesure que les temperatures descendaient jusqu'd -10"C. La dur6e de la contraction r6ticulaire biphas6e au repos 6tait significativement (P < 005) raccourcie d -10"C (5,4 s) par rapport )r 10 et d 28'C (5,7 s), mais durant la rumination elle 6tait significativement plus 6lev6e (P < 0,05) ir -10"C. La digestibilit6 apparente de la matidre sdche, de la matidre organique, des fibres au d6tergent neutre et des prot6ines brutes 6tait en moyenne ll% (6,cart 9{6) plus forle (P < 0,05) ir 28'C qu'h 10'C. La constante de transit des particules solides (par heure) 6tait plus haute d -10'C (P < 0,05) qu'd 10 et 28"C (0,046 contre 0038 et 0,034 respectivement). La constante de transit des liquides montrait une r6ponse quadratique ir la temp6rature et 6tait inversement reli6e ir la digestibilit6. L6tude porte d penser que les temp6ratures d'ambiance influent non seulement sur la cin6tique des matibres solides et des liquides mais 6galement sur la forme et sur la p6riodicit6 des contractions r6ticulaires. En outre, les modihcations thermo-induites de la dur6e et de l'amplitude des contractions r6ticulaires durant la rumination, par rapport d la phase, de repos 6taient associ6es ir des modifications des constantes du taux de transit des matibres solides.

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The effects of intragastric arterial infusions (1 mL min−1) of physiological saline, vasoactive intestinal polypeptide (VIP) or VIP-antagonist [4Cl-D-Phe6, Leu17] VIP (1 nmol mL−1) on electromyographic (EMG) activities of the reticulum and reticulo-omasal orifice (ROO) in conscious, solid, meal-fed ewes kept in either warm (18 °C) or cold (1 °C) environments were studied. Infusions were of 15 min duration before, during and after feeding in both environments. In each ruminoreticular cycle the ROO showed long bursts of EMG activity coincidental with reticular EMG activity. Quiescence of the ROO was never fully observed until the reticulum relaxed. The frequencies of reticular bursts in both environments were higher during the period of active eating than during the post- and prefeeding periods. Infusion of VIP in the warm but not in the cold environment reduced frequency of reticular EMG. Compared with saline and antagonist, VIP increased the mean duration of the ROO quiescence to 13 s in the cold environment and to a significantly lower mean of 9 s in the warm environment. We conclude that 1) VIP mediates relaxation of the ROO in solid, meal-fed sheep, 2) ROO EMG activity is influenced by different environments and by phases of the feeding cycle, and 3) the inactivity of the ROO (relaxation) is not coincidental with the reticular EMG bursts in solid-meal-fed sheep. Key words: Sheep, VIP, reticulo-omasal orifice, EMG activity, warm and cold environments