Comparison of genetic and nongenetic strategies for control of gastrointestinal nematodes of sheep

Genetic selection for resistance to Haemonchus contortus, an experimental vaccine and protein supplementation were compared with a strategic anthelmintic drenching program in a factorial grazing experiment to determine their effect on nematode faecal egg count (FEC) of young Merino sheep. Averaged over a 224-day period, FECs were reduced 69% by genetic selection (P < 0.001), 35% by protein supplementation (P < 0.001), 28% by drenching (P < 0.01) and were unaffected by vaccination. FECs in undrenched selected sheep were lower than strategically drenched unselected sheep. Liveweight gain was decreased by vaccination (13%, P < 0.05) and increased by supplementation (44%, P < 0.001). Fleece traits were significantly different (P < 0.01) between the two genotypes with the selected line having a lower fibre diameter (0.6 mum, P < 0.01) and producing 9% less clean wool than the random line (P < 0.001). Supplemented animals grew more clean wool (17%, P < 0.001) with a broader fibre diameter (1.1 mum, P < 0.001). No important interactions between the various treatments were detected. Monitor sheep, run in the plots after the termination of the experiment, had significantly lower FECs (P < 0.001) when run in plots previously grazed by supplemented sheep and selected sheep (on average 35% and 46% lower, respectively, compared to plots previously grazed by unsupplemented and random sheep). The results indicate that the largest and most persistent effect on FEC in the host, and worm contamination on pasture, was achieved by genetic selection. (C) 2002 Published by Elsevier Science B.V. All rights reserved.