Colin Trengove, Sheep Health Lecturer, UA Roseworthy campus:
Weather patterns appear to have been inverted in SA over the past 3 months with a surprisingly dry spring/summer in the south east and significant rainfall in the mid north and peninsulas. This pattern was contrary to that desired with rainfall interrupting harvest and downgrading grain quality in many cases while the relatively dry spring and summer in the SE is already limiting feed supply.
Worm egg counts in the SE have been in 100s despite the “dry” necessitating a first summer drench in November/December in many instances. Some producers are concerned about the efficacy of drenches they have used for several years and have been using the new “Zolvix” as it is the only drench group that we can be confident is a 100% effective. Stockowners who have performed a drench resistance test in the last couple of years are able to drench with confidence knowing which drenches are effective and what to use during the year. Speak to your worm advisor if you wish to know more detail on how to set up a drench resistance test.
Recent WECs in the mid north have been less predictable coinciding with the irregular rainfall episodes. Areas that have received over 75mm since November can anticipate significant worm larvae emergence in following weeks and this may have contributed to abnormally high worm burdens for this time of year.
As a general rule all young sheep should be sampled every 4-6 weeks to assess the potential for worm pickup and older sheep should be checked at least quarterly to monitor worm burdens. The reason for the different monitoring programs is that young sheep have not acquired immunity and so most worm larvae consumed develop into adult worms. In contrast, the acquired immunity in healthy adult sheep prevents larval development and so most worm larvae consumed fail to develop into adult worms. Consequently, young sheep will show high WECs while the worm count in adults will remain low despite similar exposure to infective worm larvae on pasture.
The forecast hot remaining summer will limit the development of worm burdens but a WEC on a few mobs in late January / early February will indicate if there is significant carryover infestations from last year and alert to any need for a second summer drench in February. Second summer drenching is strongly discouraged as it is a major factor in developing drench resistance. However, it is better to deal with a significant worm burden tactically at this stage than to avoid drenching for fear of developing resistance.