But now 60-80pc of Austra-lian farms have reported that their flocks possess some degree of resistance to ivermectin.
Ivermectin belongs to the macrocyclic lactones (ML) group of compounds, which were discovered in the 1970s.
NSW Primary Industries Department internal parasites coordinator Stephen Love said as a result, interest had grown in two other ML versions developed for sheep drenches: abamectin and moxidectin.
³Ivermectin was the flagship molecule in the mectin group of drugs when it was released in the Australian market as a sheep anthelmintic,² he said.
³Abamectin was initially vie-wed by some as something of a second cousin to ivermectin.
³There has been renewed interest in abamectin as it was found in research and in trials in New Zealand and Australia to be more potent than ivermectin.²
Sales of ivermectin-based sheep drenches have declined for years and sales of abamec-tin-based products continue to lift.
³Its potency means abamectin will give a better kill of worms, including some resistant worms,² Dr Love said.
³The general observation is that better potency means less selection for resistance in worm populations.
³The other issue is persisten-cy, which is a two-edged sword.²
Dr Love said there were many situations where farmers did not need a persistent drench.
³If possible it is better to use a short-acting rather than a longer-acting drench,² he said.
³High persistency means better worm control but with possible increased selection for resistance.²
Abamectin and ivermectin are relatively short-acting and moxidectin persists for longer.
Long-acting drench was more often needed in areas and seasons where barber¹s pole worm was a major problem and in situations where farmers did not have clean paddocks to move sheep to, Dr Love said.