Grazing preference for pasture species by sheep is affected by endophyte and nitrogen fertility

The grazing response of ewe hoggets offered tall fescue, endophyte-infected (+E) or endophyte-free (-E) perennial ryegrass all sown with white clover into a fertile silt loam was examined in April 1990. In a second experiment 2 grazings were observed where 5 grasses had been overdrilled into old lucerne growing on stony infertile land. Pregnaht ewes (August) and ewes with young lambs (October) grazed 3 replicates of tall fescue, +E perennial ryegrass, cocksfoot, phalaris and prairie grass. After the August grazing nitrogen fertiliser was applied at 0 and 300 kg N/ha to 5 x 2 m subplots in each 30 x 9 m grass plot. April observations showed that on day one 76% of grazing time was on tall fescue. During the first 4 days of grazing grass leaf height decreased 60 mm in tall fescue, 43 mm in +E and 42 mm in -E ryegrass while in the last 5 days decreases were 5, 3 and 25 mm. Over the 9 days' grazing, pseudostem height declined 5.2, 1.5 and 5.0 mm, green grass cover from 62 to 30,65 to 44, and 68 to 33%, and herbage removal was 940, 1100 and 1300 kg DM/ ha from tall fescue, +E and -E ryegrasses. During the first 3 days of the August grazing, grass leaf heights of cocksfoot and +E ryegrass declined at a much slower rate than in the other 3 species. The October grazing showed a similar defoliation pattern on plots without N. Addition of 300 kg N/ ha resulted in very rapid defoliation of all 5 species during the first day of the 7-day grazing period. April grazing showed that initially sheep preferred tall fescue. Even though grazing time was similar for +E and -E ryegrasses the hoggets consumed less +E than -E ryegrass, possibly owing to reluctance to penetrate the +E pseudostem horizon. The August and October grazings demonstrated the ability of sheep to discriminate between grass species and strong rejection of cocksfoot and +E ryegrass. However, N at a rate similar to a urine patch produced leaf in all species which was equally attractive to sheep. The reduced intake of +E ryegrass measured in the April grazing may explain some of the lower animal performance of sheep on +E ryegrass pastures. Equally, grazing preference shown by rate of canopy height decline in any pasture is probably highly correlated with pasture intake and animal productivity.