Comparison of Rumen Function and Digestive Efficiency in Merino and Dorper Sheep

In recent years in Australia there has been a decline in wool production enterprises and an expansion of the sheep meat/prime lamb industries. Introduced in 1996, the Dorper has become one of the fastest growing (in terms of uptake by producers) meat breeds in Australia with their success attributed mainly to its adaptability and that it can produce marketable lambs under less than ideal pasture conditions. Four grazing and three animal house studies were undertaken to investigate potential differences in animal production by measuring nutrient digestibility, ruminal parameters, energy metabolism and diet selection (based on faecal n-alkanes) in Merino and Dorper sheep fed diets of varying quality. The diets investigated in the animal house studies were lucerne and wheaten chaff and two pelleted concentrate diets while those included in the grazing studies were winter wheat, lucerne/clover mix pasture, wheat stubble and canola. Throughout the grazing trials production differed between the Merino and Dorper sheep. For all grazing trials the average liveweight (LW) and body condition score (BCS) of the Dorper sheep was greater (p < 0.05) than that of the Merino ewes. When grazing a low quality diet (wheat straw), after an initial decline in condition, Dorper sheep maintained BCS and LW while both the BCS and LW of the Merino ewes declined over the experimental period. However, when ewes grazed high quality diets, responses in BCS and LW differed depending on the pasture base. When grazing on lucerne/clover mix pastures the BCS and LW of both the Merino and Dorper ewes increased over the experimental period. While grazing winter wheat the LW decreased but the BCS of the pregnant and then lactating Merino and Dorper ewes increased. The loss in weight was expected due to physiological status of the ewes and the inability to correct LW for conceptus weight. At the commencement of grazing canola, Merino ewes were significantly lighter than the Dorper ewes but after after 54 days of grazing there was no difference in the LW of the two breeds. From the animal house studies, when fed restricted roughage diets of differing quality and concentrate pelleted diets there were no differences (p > 0.05) in nutrient digestibility, rumen pH and ruminal ammonia concentrations but variable differences (p < 0.05) in volatile fatty acid (VFA) concentrations and proportions between Dorper and Merino sheep. This suggests that any production differences between these two breeds when when fed such diets are thus the result of either differences in feed intake and/or diet selection or differences in energy metabolism as a consequence of differences in the concentrations and/or proportions of VFA. When fed concentrate-based pelleted diets, breed differences existed in ruminal outflow rates as well as the concentrations and proportions (in particular acetic and butyric acids) of VFA. These differences could be due to differences in particle outflow rates from the rumen and/or microbial populations in the Merino and Dorper sheep. Metabolite/hormonal challenges were conducted on a high quality pelleted diet to see if differences existed between Merino and Dorper sheep. Baseline glucose concentrations were higher in the Dorper sheep, but the magnitude of the response to both the epinephrine and insulin challenges did not vary (p > 0.05) between the sheep breeds, therefore the stress response was the same. It is likely that it is not just fatness of the animal which influences basal blood glucose concentrations but also insulin resistance and/or physiological differences between meat and wool sheep breeds. When grazing on high quality feeds such as winter wheat and canola there were no differences (p > 0.05) in diet selection (based on faecal n-alkanes), and therefore presumably nutrient supply or microbial efficiency between the Merino and Dorper sheep. However, when grazing on lucerne/clover mix pasture and wheat stubble differences existed in diet composition and therefore potentially nutrient supply and microbial efficiency between the two sheep breeds. Future research needs to be conducted into establishing when (which types of pastures) and to what extent diet selection differs between Dorper and Merino sheep and how this may impact on nutrient utilisation and their subsequent performance. Undertaking this research in a much more complex grazing system, such as the rangelands, would be particularly useful given the increase in popularity of this breed in these areas of Australia.