Effect of Thermal Stress and Dietary Crude Protein on Digestive and Physiological Responses in Selected Polled Dorset Sheep

Abstract

Twelve Polled Dorset ram lambs from selected parentage, six adapted (AD) and six nonadapted (NA) to a Mississippi subtropical environment, were used in a metabolism trial to investigate effects of environmental temperature (10 vs. 32C), and dietary crude protein levels (10 vs. 20%) on selected responses. Response variables of dry matter digestibility (DMD), digestible energy (DE), grams nitrogen retention (NR), percent nitrogen retained (%NR), nitrogen digestion (NO), water consumption, rectal temperature, and respiration rate were analyzed as a completely random split-plot design with type as the whole-plot treatment and temperature and crude protein factorially arranged as sub plot treatments. There was no difference between AD and NA rams for DMD, DE, gNR, NO, water consumption, or rectal temperature and respiration rate. There was no difference in rectal temperature between AD (38.6 C) and NA (38.7 C) rams when compared at 10C. When exposed to 32C, NA rams (39.1 C) had higher (P<.05) rectal temperature than AD rams (38.7 C). Respirations per minute of AD (13.7) and NA rams (16.1) did not differ when subjected to 10C. Respiration rate in NA rams (94.4) was higher (P<.05) than AD rams (83.5) when exposed to 32C. There was an increase (P<.05) in DMD for rams exposed to 32C (71.8%) vs. 10C (70.4%). There was no difference in DE and NR for rams when exposed to either 32C or 10C. The DMD (73.7%), DE (72.9%), grams NR (47.3), and %ND (77.2) for rams fed 20% CP diets was higher (P<.05) than DMD (68.5%), DE (66.6%), gNR (23.1), and NO (61.3) for rams fed the 10% CP diet. Rams fed 20% CP diets had higher (P<.05) water consumption (3.4 vs. 2.8 L) than rams consuming 10% CP diets. These data suggest physiological differences in respiration and rectal temperature but not in digestive metabolism between AD and NA rams when exposed to heat stress.