By Lewis Kahn, ParaBoss Executive Officer

March 2016

A feature article in the January edition of ParaBoss News outlined the easy steps to breeding worm resistant sheep. In this article, the benefits of worm resistance for reducing the number of infective worm larvae on pasture are discussed. 

Worm resistant sheep have lower worm egg counts and it makes sense that, when compared with more susceptible sheep, this would result in fewer infective larvae on pasture over time.  But how big is this effect?

A research project conducted near Armidale in northern NSW compared the performance of Merino ewes from a worm-resistant selection line against ewes from an unselected control line. The two groups of ewes (and their lambs) were grazed in separate paddocks from 10 weeks prior to 8 weeks after lambing and no drench was given at any stage.

The average worm egg count of worm-resistant ewes peaked at 300 epg, while the unselected ewes had a peak worm egg count of 1,000 epg.  Over the entire trial period of 18 weeks, the average worm egg count of resistant ewes was 80 epg compared with unselected ewes at 420 epg: a five-fold difference.

How did this translate to the worm-risk of paddocks? The effect of worm resistance on the number of infective larvae on pasture was determined by grazing young “tracer” sheep on these paddocks prior to and after the 18-week trial. The egg counts of these tracer sheep were compared before and after the trial.

As expected, at the start of the trial, there was no difference in worm egg counts from the tracer sheep that grazed the paddocks to be used by the resistant and unselected sheep. After the trial, results were different. The tracer sheep that grazed paddocks used by the worm-resistant ewes and lambs had worm egg counts that were half the value of those from the tracer sheep that grazed paddocks used by unselected sheep. A larval culture of the faeces indicated that the difference was greatest for barber’s pole worm. The barber’s pole worm counts from the tracer sheep on paddocks used by unselected ewes were three times as high as in tracer sheep grazing the paddocks used by the worm-resistant ewes.

There are a number of ways to create low worm-risk paddocks, which are detailed in WormBoss. In barber’s pole worm areas, the reduction in worm larvae from worm-resistant sheep will not be enough for dedicated low worm-risk lambing and weaning paddocks for susceptible stock. However, for other pastures throughout the year, worm-resistant sheep significantly reduce the number of infective larvae on pasture and provide an ongoing benefit.