Dietary preference of sheep for perennial ryegrass and white clover at contrasting sward surface heights

Non-pregnant, non-lactating ewes grazed adjacent monocultures of white clover and perennial ryegrass with three sward surface height (SSH) combinations [6 cm white clover: 6 cm perennial ryegrass (c6gG), 3 cm white clover: 6 cm perennial ryegrass (c3g6), 3 cm white clover: 9 cm perennial ryegrass (c3g9)] at two stocking densities (21.3 or 29.8 ewes ha(-1)). Immediately Drier to the experiment, all ewes grazed a c6g6 sward. Grazing time on each plant species was recorded during daylight over two 48 h-test periods. Subsequently, herbage intake rates for each species at each SSH were measured allowing intakes of each species to be calculated. For the first 24 h of both test periods (D1), ewes on treatment c3g6 spent less time grazing white clover than those on treatment c6g6 (228 vs. 362 min) and more time grazing perennial ryegrass (360 vs. 182 min). Total grazing rime on treatment c3g6 was more than on treatment c6g6 (587 vs. 544 min) but the difference was not significant. Perennial ryegrass intake was higher (895 vs. 452 g), and white clover intake (814 vs. 1687 g), total intake (1719 vs. 2140 g) and proper lion of white clover in the diet (0.460 vs. 0.794) were lower for treatment c3g6 than treatment c6g6. There were no significant differences in total grazing time, grazing rime on either species, proportion of grazing time on while clover or proportion of white clover in ther diet between trearment c3g6 and treatment c3g9. However, the higher intake rate of perennial ryegrass in treatment c3g9 led to higher perennial ryegrass and total intakes. For the second 24 h of both test periods (D2), ewes on treatment c3g6 again spent more time grazing perennial ryegrass than on treatment c6g6 (270 vs. 161 min) bur time spent grazing white clover was similar (318 vs. 308 min). Total grazing time was significantly higher on treatment c3g6 than on treatment c6g6 (588 vs. 469 min) but proportion of grazing time on white clover was similar (0.554 vs. 0.668). Perennial ryegrass intake was significantly higher for treatment c3g6 than for treatment c6g6 (672 vs. 402 g) while white clover intake was significantly lower (1140 vs. 1435 g) bur total intake was similar (1812 vs. 1836 g). The proper tion of white clover in the diet was significantly lower for treatment c3g6 (0.628 vs. 0.785) than for treatment c6g6. The only significant differences between treatments c3g6 and c3g9 were in perennial ryegrass intake (672 vs. 906 g) and in total intake (1812 vs. 2287 g). intake of perennial ryegrass on treatment c3g9 was also significantly greater than on treatment c6g6 (906 vs. 402 g) and total intake was higher (2287 vs. 1836 g). At the higher stocking density, time spent grazing perennial ryegrass and perennial ryegrass intake were significantly lower on D1 and D2 while total grazing time was also significantly lower and proportion of time grazing white clover and proportion of white clover in the diet were significantly higher at the higher stocking rate on D2. The results indicate that behaviour changed over the 48 h observation period for treatments c3g6 and c3g9 bur behaviour remained relatively constant for animals on treatment c6g6. Ewes traded off dietary preference against total intake by altering grazing times on perennial ryegrass and while clover to achieve maximum net benefit.