Amelioration of heat stress in feedlot cattle by dietary means

Summary

Nutritional strategies may reduce the incidence and/or minimise the effect of high environmental heat load (EHL) in feedlot cattle. One such strategy would be to increase the dissipation rate of body heat during the cooler night hours, preferably without compromising growth rates. This could be implemented by managing feeding times or by altering the composition or the processing method of the dietary grain, to promote an increase in the proportion of metabolic heat that is evolved at night. 
The principle objective of this preliminary study was to determine if cattle fed a diet containing slowly-fermentable grain would exhibit less heat stress than when fed a rapidly-fermentable grain under conditions of a cyclical EHL. To maximise the chances of obtaining proof of concept, maximum EHL in the climate rooms was timed to coincide with peak fermentation of the rapidly-fermentable wheat-based feedlot diet. 
As a control, a group of cattle were fed a sorghum diet, characterised by slow fermentation and a more uniform dissipation of metabolic heat from the body after feeding. Both diets were sourced from a commercial feed supplier. Compared to levels in the sorghum diet, starch content of the wheat diet was 95%, and nitrogen content was 109%. Digestibility of starch and energy was about 15% higher in animals given wheat which resulted in 8% higher intake of digestible starch in these animals. 
In order to ensure the desired synchrony between EHL and metabolic heat production from the morning meal of the wheat-based diet, several techniques were used to characterise dietary fermentation patterns. Firstly, results using the dacron bag technique and rate of evolution of gas during in vitro fermentation of the diets, suggested that there were substantial differences in rates of digestion and fermentation in the rumen.  Secondly, in vivo assessment of patterns of aerobic and anaerobic heat production was made by measuring gas exchange of animals in respiration chambers. This indicated that metabolic rate, reflecting aerobic metabolism in cattle tissue, was 6% higher in wheat-fed animals over a 7-hour post feeding period, consistent with differences in intake of digestible starch. In addition, wheat-fed steers expired 14% (P