Effects of genotype on the performance and intake characteristics of sheepgrazing contrasting hill vegetation communities

The five-weight responses, voluntary herbage intakes and diet compositions of two breeds of sheep of different mature live weights, grazing two contra sting hill vegetation communities, were studied over three consecutive graz ing seasons. A total of 228 Latxa and 255 Gallega non-lactating ewes, with mature live weights of 42.4 and 33.1 kg respectively, grazed two replicates , each of two 5-ha plots comprising by area either 0.3 Calluna vulgaris and 0.7 grass species (Agrostis capillaris, Festuca rubra and Nardus stricta) (treatment C-3) or 0.7 C, vulgaris and 0.3 oft he above grass species (trea tment C-7). The Latxa ewes achieved higher rates of live-weight gain than the Gallega e wes under conditions of high availability (sward surface height >3.0 cm) of the preferred grass species (A. capillaris and F. rubra) but had lower rat es of live-weight gain in the later part of the grazing season when the ava ilability of the preferred grass species was low. In general, the Latxa ewe s had higher voluntary herbage intakes per unit live weight and selected a diet of higher digestibility than the Gallega ewes (P < 0.001). The composi tion of the diet was significantly affected by treatment (P < 0.001) and wa s of a higher digestibility in the earlier part of the grazing season (P < 0.001). The results indicate the existence of a significant (P < 0.001) gen otype X environment interaction affecting diet selection and animal perform ance. Between-breed differences in diet selection are likely to be importan t in determining grazing behaviour and thus the suitability of different ge notypes for particular environments. Breeds with a small body size are like ly to be better suited to the poorer land resources where the availability of preferred species is low.