Body compostion and abreeding value of coopworth sheep (Ovis aries L.) differing in selection for leaf and stem : A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agriculture Science with Honours at Lincoln University

Abstract

Herbivores are known to show preferences for some dietary components. The consequence of diet selection is that preferred components of the sward risk depletion and sustainability of the sward composition is compromised. Previous studies have identified variation in a population of Coopworth sheep in the degree of selection for leaf and stem. To investigate the variation in diet selection a study was conducted using Coopworth ewe hoggets, previously screeded as having either a higher (n= 14) or lower (n= 14) preference for leaf over stem in lucerne chaff. This involved the measurement of the body composition and the analysis of a variety of breeding values in order to explain reasons for the variation in the degree of diet selection between the two groups. The variation in diet preference was linked to the rumen volume, 4. 70 versus. 4.00 litres for high and low preference sheep respectably (P<0. 01). The level of diet preference was also found to be strongly linked between the various sires and the level of selection expressed by their progeny (P=0.05). There was found to be no relationship between the apparent carcass weight (on both a kg basis and a proportion of liveweight basis, P=0.1 and P=0.4 respectably) or the proportion of muscle (P=0.203), fat (P=0.324) and bone (P=0.284) in the animals between the more and less selective sheep. Overall there were no significant effects of dental arcade. More selective sheep had a slightly lower mean degree of protrusion (non significant), slightly wider arcade breadth at the widest point (non significant) and slightly wider arcade breadth (significant) on both the upper and lower jaw, while the various jaw measurements were unable to discriminate between the two groups and explain the variation in degree of selection. No difference was found between the means of the two groups and the associated BV's. A PCA analysis ruled out the various BV's as a way of discrimination between the two groups. These results show that more selective sheep have a greater rumen volume, with the precise mechanisms of this unclear. The trial demonstrated that there are good similarities between generations in their diet selection (between sire and progeny). This could potentially mean that diet preference is partly due to a genetic effect and is a heritable trait. If a genetic basis for diet preferences did exist and with no reduction in animal productivity and performance associated with selection, then it may be possible to select for animals that show a lower degree of selectivity. This will undoubtedly have huge implications for the New Zealand livestock industry through improving the overall efficiency and sustainability of pastoral livestock systems, ultimately improving annual productivity, production and profitability.