The effect on wool production and liveweight changes of four parasite control programs, differing only in the number and timing of anthelmintic treatments, was compared in two experiments. In one program (Salvage), only individual clinically affected sheep were treated, to avert death. In a summer rainfall environment, increases in wool production of 18.4, 25.0 and 34.2% above the Salvage control scheme were recorded, respectively, for Curative (4 thiabendazole, 1 rafoxanide), Preventive (5 thiabendazole, 5 rafoxanide) and Suppressive (11 thiabendazole, 5 rafoxanide) programs of control. In the area of dominantly winter rainfall, the increases in wool production over the Salvage scheme were 17.9, 26.4 and 45.9 % for the Curative, Preventive and Suppressive programs, involving 3, 3 and 11 treatments with thiabendazole, respectively. The three treatments in the Curative program were given 3, 0 and 1 month later than in the Preventive program. Liveweight gains followed a similar pattern to wool production, but with some compensatory gain in the spring at the lower levels of control in the summer rainfall environment. The final gains were 17.4, 20.7, 23.7 and 24.7 kg in the first experiment, and 12.3, 18.3, 19.5 and 22.6 in the second experiment, for the four schemes of parasite control. The high level of production resulting from the Suppressive program was attributed to the reduction of the contamination/reinfection cycle to a low level. The experimental design involved thorough replication and required that different treatment groups be grazed separately so as to facilitate the measurement of true production differences.

The effect of pre- and post-lambing anthelmintic treatment of the ewe on subsequent lamb production was studied. Sixteen groups of 25 maiden Border Leicester x Merino ewes were given combinations of a pre- and post-lambing 'drench' with thiabendazole. All groups were pastured on separate paddocks giving four replicates of each treatment program. Lamb liveweight changes were recorded for 19 weeks. A pre-lambing drench was found to increase lamb birth weights, and this advantage persisted until the lambs were drafted for slaughter in the nineteenth week from the beginning of lambing. The advantage was greater for twin compared with single lambs. The post-lambing drench had no effect on liveweight gain up to week 13, but had a significant effect over the next six weeks. This result seemed to have been related to the lower parasite burdens being passed from the ewe to the lamb, as a result of the post-lambing drench. This drench controlled the rapid rise in faecal egg counts which was observed in the lambs of untreated ewes after the eleventh week. At the time of lambing, pasture contamination was probably lower than under many practical field situations and results of the same magnitude may not occur regularly. The implications, however, are greatest in flocks with a high twinning performance where early drafting of a high percentage of twin lambs is important for economic or pasture management considerations.