Comparative daily energy expenditure and water turnover by Dorper and Merino sheep measured using doubly labelled water

Details of the energy (food) requirements of domestic herbivores are essential for predicting grazing pressures and subsequent ecological impacts on rangelands. However, these details are lacking for some of the more recently introduced sheep breeds to Australia, such as the Dorper breed sheep, which are principally meat sheep, and it is uncertain how they compare with the traditional Merino, a wool-breed, sheep. We used the doubly labelled water method to compare the field metabolic rate and water turnover rate of Dorpers and Merinos grazing together in a small holding paddock in a typical rangeland environment. We found no significant differences in field metabolic rate (Dorpers 481 ± 125 kJ and Merinos 500 ± 109 kJ kg–0.73 day–1) or water turnover rate (Dorpers 397 ± 57 mL and Merinos 428 ± 50 mL kg–0.8 day–1). As such we conclude that under controlled conditions with limited movement and ready access to feed and water, dry sheep equivalent of 1 is appropriate for Dorpers (that is, one Dorper ewe had a grazing requirement equal to one standard, dry Merino wether). However, we also found that the field metabolic rate for Merinos under these conditions was only around half that measured in published studies for animals ranging freely in a large paddock system. This suggests that more work is needed to fully appreciate the energetic and grazing impacts of Dorpers versus Merinos under more realistic grazing conditions (e.g. large paddock systems) where feed and water are more spread. It also highlights limitations of the current dry sheep equivalent rating system, which has been derived from laboratory measures of sheep metabolic rates.