Colin Trengove, Sheep Health Lecturer, UA Roseworthy campus:
Despite lean spring rain, worm egg counts (WEC) have crept up over the last month, with approximately half of recent counts indicating a need for a drench. Normally it would be expected for counts to fall during spring, but the lack of rain and corresponding pasture growth is possibly leading to increased exposure to worm larvae. Abundant spring feed has the double benefit of diluting access to worm larvae, plus the high quality spring feed enhances the immune system. A net reduction in worm egg output normally follows.
Spring drenching is typically reserved for weaners, while older sheep have their WEC checked prior to summer to see if a drench is required. This is because young sheep do not have an acquired immunity to worms and so easily become increasingly burdened by worm exposure.
The intent is to minimise all drenching and so adult sheep are only drenched as the pastures hay off if a significant worm burden is present. This may constitute 100 eggs per gram if sheep are in good condition and going onto adequate feed quality and quantity. It will be lower in weaners and if these conditions are not met.
A WEC in excess of 250 in spring will provide a good opportunity to test the efficacy of various drench groups. By treating groups of 15 young sheep with various drench groups and then checking the WEC from each group 10-14 days later, the effectiveness of each drench group can be calculated when compared to a group that received no drench. This then allows a drench rotation program to be worked out for the next 3+ years. Such drench trials are essential to ensure that the drench you intend to use is effective. Discuss this with your sheep health advisor if you have any concerns.