EFFECT OF HERBAGE SPECIES AND FEEDING LEVEL ON INTERNAL PARASITES ANDPRODUCTION PERFORMANCE OF GRAZING LAMBS

The effect of internal parasites on the performance of lambs grazing d ifferent forage species was examined in two experiments. In Experiment 1,300 Coopworth lambs grazed stands of Nui ryegrass, Wana cocksfoot, Au Triumph tall fescue, Puna chicory, or Otaio lucerne for 64 days bet ween February and April 1992. Lambs were either infected with 10000 mi xed species L(3) larvae at the start of the grazing or maintained free of parasites by fortnightly drenching with ivermectin. Herbage was of fered at either 2 or 4 kg DM/head per day (green). In Experiment 2, Nu i ryegrass was replaced with low-endophyte Ruanui ryegrass and lambs w ere infected with 19500 L(3) larvae 21 days before grazing which comme nced on 2 February 1993 and continued for 100 days. In Experiment 1, f ew herbage larvae were recovered during grazing and differences among forage species were not significant. In Experiment 2, more larvae per kg DM were recovered from the grass swards than from either chicory or lucerne (P < 0.05), differences among the grasses being small. Faecal egg counts averaged 1030 epg in Experiment 1, counts tending to be lo wer in lambs grazing ryegrass and chicory (P < 0.05). In Experiment 2, FEC averaged 730 epg, lambs grazing chicory or lucerne having signifi cantly (P < 0.05) lower counts (c. 320 epg) than lambs grazing grasses (c. 1100 epg). Numbers of adult nematodes in lambs at the end of the grazing period followed similar trends to FEC counts, Trichostrongylus sp. being the most common nematode present (67%) in both experiments. Parasitised lambs had higher dag scores than control lambs (P < 0.01) but, with the exception of endophytic Nui ryegrass, there was little difference among the forages when lambs were maintained free of parasi tes. There was no consistent relationship between dag score and faecal DM%, FEC, adult nematodes, or carcass weight gain (r(2) < 0.36). In E xperiment 1, control lambs offered a high allowance of chicory grew at 311 g/day, significantly (P < 0.01) faster than their counterparts on lucerne (222 g/day) or grasses (169-195 g/day). In Experiment 2, grow th rates of control lambs grazing chicory and lucerne were similar at c. 250 g/day and faster (P < 0.01) than those on grasses (120-148 g/da y). Parasites significantly depressed estimated carcass weight gains i n both years (P < 0.01), the effect being greater in Experiment 2 (2.7 kg) than Experiment 1 (0.4 kg). In Experiment 1, the impact of parasi tes was confined mainly to lambs grazing grasses at low herbage allowa nces, particularly ryegrass (P < 0.05). In Experiment 2, where the lar val challenge was higher, lambs grazing all species other than chicory were affected. It was concluded that the use of chicory can reduce th e effects of parasitism on the growth of grazing lambs relative to eit her ryegrass, cocksfoot, or tall fescue, but responses to lucerne are more variable.