Colin Trengove, Sheep Health Lecturer, UA Roseworthy campus:
The mild dry start to winter has not encouraged worm infestations to date as indicated by recent worm egg count (WEC) monitoring. However, drenching has still been indicated where the objective is to reduce worm burdens pre-lambing or for keeping winter pastures relatively 'clean'. Preparation of the weaning paddocks may also have required drenching where they have been grazed during the preparatory phase.
Internal parasites are usually ready to infect ewes and lambs at the start of winter as worm eggs deposited on pasture during late summer/autumn will emerge as larvae and infect animals grazing short green and dead feed. Worm larval emergence escalates around three weeks after significant autumn/winter rain or a month earlier on irrigated pasture.
Late pregnant ewes have lowered immunity and so are unusually susceptible to worm uptake and development. A pre-lambing drench is often given to protect against this development unless WEC monitoring in the lead-up to lambing indicates a low worm risk.
In contrast, adult dry sheep grazing worm contaminated pastures acquire immunity through long term exposure to worm larvae and are able to expel 90+% of the larvae consumed before they develop into adult egg laying worms. This usually has two outcomes: low to moderate worm burdens through much of the year; and winter scours in some of the mob due to gut irritation caused by the immune rejection of larvae.
Lambs are born with variable immunity to worms. Some are able to suppress worm development – a genetic trait that can be selected for especially in ram breeding programs. Others have little or no ability to resist worm development and become heavily infected on “wormy” pastures. A worm drench at weaning is always advised unless WEC pre-weaning indicates the absence of worms.
Worm monitoring is the best tool for assessing the need to drench rather than the often costly and wasteful strategy of drenching whenever sheep are ‘in the yards’. A WEC as low as 100epg may indicate a need to drench on dry feed, while a count over 200epg may be the trigger for drenching on green feed. Consult your animal health advisor to discuss local recommendations.