PRODUCTION AND ECONOMIC RETURNS FROM MERINO WEANER SHEEP SUBJECTED TO 4 FREQUENCIES OF ANTHELMINTIC ADMINISTRATION IN EAST-GIPPSLAND, VICTORIA

In East Gippsland, Victoria, 4 treatment groups, designated ''weekly'' (W), ''high'' (H), ''low'' (L), and ''salvage'' (S), each consisting of 3 subgroups of 10 Merino weaner sheep, were given different numbers of anthelmintic doses during 1978, 1979 and 1980. The doses were respectively 51, 51, 50 (W); 9, 9, 12(H); 3, 4, 6(L); and 1, 3, 2(S). Both anthelmintics were administered according to the manufacturers'' recommendations. Subgroups were grazed on separate 1 ha plots and rotated between plots within treatments at 2-week intervals in an attempt to eliminate between paddock differences. Sheep were replaced annually. These programs resulted in major differences in wool production and final liveweight between groups. Over the 3 years of the experiment, sheep dosed frequently grew heavier (P < 0.05) and produced more wool than did sheep in the L and S groups. Mean final liveweights for the W. H. L. and S groups respectively were 53.6, 51.4, 47.0, and 46.4 kg (l.s.d. [P = 0.05]= 3.5 kg). Corresponding figures for wool growth were 5.96, 5.62, 5.07, and 5.17 kg (l.s.d. [P = 0.05]= 0.60). Mortality in the W (5.6%) and H (2.2%) groups was also lower than in the L (18.9%) and S (27.8%) groups. The H group gave the highest economic return of $21.81 per sheep compared with $17.61 (W group), $16.31 (L) and $15.06 (S group). This advantage was maintained throughout a wide cost-price range. However, as both the W and H programs selected strongly for strains of nematodes resistant to anthelmintics, anthelmintic therapy alone cannot be relied upon for long-term parasite control.