The first summer drench is a strategic drench given to sheep and goats in the winter rainfall areas of Australia. It plays a major role in the preparation of low worm-risk paddocks for use during winter by weaners.
Weaner sheep and goats develop immunity to worms at about 12–18 months of age, before this, they are more vulnerable to worm infections.
Although often given at the same time, the weaning drench and the first summer drench shouldn’t be confused as being the same thing.
Lambs and kids have little natural immunity and are stressed by the process of weaning, so that they are highly susceptible to worms. This can be compounded if their nutritional needs are not met.
As such, winter-spring drop lambs/kids should always be drenched at weaning in the moderate to high rainfall areas. The exception may be those born much later in spring that receive a drench at marking to coincide with the first summer drench; these should then be WormTested at weaning to decide whether a drench is warranted.
If lambs and kids are weaned before October, then it is too early to make the one drench do two jobs. A weaning drench and a separate, later, first summer drench should be given. Delaying the weaning drench in this case may lead to wormy weaners and allows them to contaminate pastures for an extra month or two.
In Western Australia, a weaning drench is more often given mid- to late-spring.
The first summer drench is usually given in November/December. Its timing generally coincides with the period when the pastures are haying-off, but in better years this isn’t always the case. It serves two purposes: it removes worm burdens from the animals and works with weather conditions over summer to create safe grazing for young sheep or goats in the autumn-winter period.
As worms that have built up in stock over the winter and spring are killed by the first summer drench, the continued deposition of eggs onto pastures in late spring/early summer is halted and that, in turn, reduces the carry over of worm larvae into the autumn period. Low worm-risk paddocks are then available for weaners after the autumn break.
When the first summer drench coincides with haying off of pastures its period of protection is extended by the additional worm-free period of 3–4 weeks after the drench when little deposition of worm eggs (during the pre-patent period) onto pastures occurs, and this is often followed by an additional couple of months when it's too dry for worm larvae to develop. If the weaning drench coincides with haying off, only one drench is needed.
Western Australia, with its Mediterranean climate, typified by long, hot and completely dry summers, is a little different to the more eastern winter rainfall areas (where occasional summer rains are more likely). Adult sheep and goats should NOT generally receive a first summer drench; instead, they are treated in autumn. When pastures dry off completely or stubbles are available, all young sheep and goats up to 18 months old can be drenched.
It is important not to delay the first summer drench if the season is extended due to later rain. Wormy sheep or goats—particularly weaners—will continue to contaminate the pastures if not drenched.
The critical point is that a period of 3 months is required over summer to allow worm larvae deposited in spring to die. During this period you want no further worm eggs developing to larvae.
The principles of preparing a low worm-risk paddock are the same anywhere: stop further contamination and allow existing larvae to die over a period of 3 months during hot weather increasing to 6 months during very cold weather.
Usually, in winter rainfall areas, the haying-off period occurs at the right time to help this—in very dry years or regions (like WA) where summer rain is rare, eggs will not develop to larvae, but in regions with occasional summer rain, some eggs deposited on the pasture may develop to larvae.
If haying-off is delayed, sheep/goats should receive a first summer drench at the usual time and not be delayed, but you may need to consider a second summer drench if continued rain occurs.
A first summer drench is best given in November or early December, but for lambs and kids, can be brought forward into October if weaning occurs then. September is too early, January is too late.
A second summer drench is based on a worm egg count, rather than being routine. This drench is important to consider for sheep or goats that will graze paddocks to be used by weaners in the next autumn and winter.
Assuming the paddock has been prepared earlier with a first summer drench given to the stock using it, the sheep/goats might need a second Summer drench (generally in January or early-February, 6–8 weeks after the first summer drench) into this paddock to ensure no worm eggs are dropped onto the pastures. If adults are to be grazed during this period, Worm Egg Count first and only drench if >100 epg.