Partial preference of sheep (Ovis aries L.) for leaf versus stem across generations: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours

Abstract

Selection by sheep for high quality components in pasture results in high degrees of heterogeneity arising in the sward, reducing pasture quality. A study was then undertaken to determine if the partial preferences of sheep for leaf over stem were representative of those displayed by their full siblings and dams. Forty Coopworth hoggets were screened for their selectiveness between leaf and stem in luceme chaff. They were then ranked on their selection index (SI) for leaf, and split into two-groups of the eight most-selective (highest SI) and eight least-selective (lowest SI). The full siblings (twins) and dams of these 16 hoggets were then identified and tested for their selectiveness between leaf and stem using the same methods. Each group of sheep were allowed an adjustment period of seven days, and were tested for selectiveness for 14 days. Of the forty hoggets that were screened for selectiveness, there was a highly significant (P<0.001) difference between the more-selectiveness and less-selective groups in their SI for leaf (1.10 vs. 1.06 respectively). The full siblings and dams of the more-selective hoggets produced an average SI for leaf of 1.10 and 1.09 respectively, and was significantly (P<0.001) different from the full siblings and dams of the less-selective hoggets, who produced an average SI for leaf of 1.07 and 1.06 respectively. There was no difference between the hoggets, their full siblings and their dams in SI. The amount of feed consumed as a proportion of feed offered accounted for 20% of variation in SI (P<0.001). When SI for leaf was adjusted for this as a covariate, there was still a significant (P<0.01) difference between the more-selective and less-selective hoggets (1.09 vs. 1.06 respectively). Although there was no difference in the SI for leaf of the full siblings (1.08), the dams of the more- and less-selective were significantly (P<0.01) different (1.10 vs.1.07 respectively). Again there was no difference between the generations. Overall, preferences displayed by the hoggets were also displayed by their full siblings and dams. The results of this trial suggest that the partial preferences of sheep for leaf over stem may in part, be due to genetic traits. If it is possible to manipulate the diet selection of sheep through breeding, it could have important implications for pasture and grazing management. The use of less selective sheep may allow the homogeneity of pasture to be maintained. The proportion of clover in the sward would increase, and reproductive stem and seed head components would be more easily removed. The long term result would be increased livestock performance as pasture quality improves, driving pastoral livestock systems to become more efficient, sustainable and profitable. Diet selection of livestock may also be manipulated for the purposes of weed control and ecological sustainability in natural grasslands.