Effects of environmental temperature and ammoniation on utilization of straw by sheep

Eighteen sheep were used to compare diets based on ammoniated wheat straw (AWS) or untreated wheat straw (WS) at three thermal environments: hot (HE, 36 degrees C, 70% relative humidity (RH], thermoneutral (TNE, 24 degrees C, 70% RH) and cold (CE, 4 degrees C, 70% RH). Six sheep were housed in each environment for at least 21 d prior to the trial. Two periods of 28 d were used in a crossover design. Within each period, digestibility was measured both under ad libitum intake (collections on d 16 to 20) and at restricted intake (collections on d 24 to 28). Diets consisted of 59% straw, 20% alfalfa hay and 21% concentrate. Urea was added to the untreated straw diet (.75%) to compensate partially for the N present in the AWS. Rectal temperature and respiration rates indicated that sheep in the HE were heat-stressed. Sheep housed in the CE had lower rectal temperatures and respiration rates than sheep in the TNE, but they did not shiver or show other visible signs of cold stress. Voluntary DM intake increased from 604 to 806 g/d, and ad libitum DM digestibility increased from 55.5 to 58.2% when AWS was fed. At restricted intake, ammoniation increased diet digestibility from 58.8 to 61.4%. Voluntary intake was lower in the HE (588 g/d) compared to 763 g/d in the TNE and 764 g/d in the CE. Digestibility of DM at ad libitum intake was lower in the HE (54.6%) than in the TNE (57.5%) or CE (58.6%); respective results at restricted intake were 58.5, 61.0 and 60.7%.(ABSTRACT TRUNCATED AT 250 WORDS)