5 minute read

TL;DR:

Hot Australian summers reduce larval survival on pastures and crop stubbles. If you administered a first summer drench, in Nov/ Dec, there’s a good chance you might not need that late Jan/ early Feb drench.

Second summer drenches are also the drench that accelerates the development drench resistance the most- if you can avoid it, you should!

There is only one way to find out if you need a drench, and that’s with a worm egg count. Make it as easy as possible to monitor parasites with on-farm service.

What is a second summer drench?

A second summer drench is an optional drench usually administered in late January to February, following a first summer drench given at the beginning of summer.

Its purpose is to remove any residual worm burden if conditions over summer have allowed worms to survive or sheep have been re-exposed to infective larvae.

A second summer drench should only be given if supported by worm egg count (WEC) results.

Why a second summer drench is often unnecessary

Australian summer conditions are hot, dry and therefore unfavourable for worm larvae survival.

As a result, pasture contamination naturally declines over this period.

A well timed, effective first summer drench often provides excellent worm control over summer, and sheep usually do not re-accumulate significant worm burdens until temperatures begin to cool off and the growing season begins again.

The purpose of a second summer drench is to remove any worm burden that an animal may have picked up over summer.

Unnecessary second summer drenches provide little to no production benefit, accelerate the development of drench resistance, and add avoidable cost.

When a second summer drench may be required

A second summer drench should be administered only when worm egg count results indicate.

Sometimes, worm burdens can be reacquired over summer if:

• the first summer drench was not effective

• summer rainfall has occurred, resulting in green pick/ moisture on paddocks

• the animals are young (weaners and lambs are more susceptible to an infection developing)

• stock are grazing higher-risk paddocks (irrigated areas or green summer feed)

These factors increase the risk of an infection developing, but the only way to know if worm burdens have been reacquired is through a worm egg count.

How second summer drenches accelerate the development of anthelmintic resistance

Second summer drenches can be the most damaging in terms of accelerating drench resistance, especially when they are unnecessarily administered.

This is because pasture contamination is at its lowest after a long, extended period of hot and dry weather. Drenching in this environment will ONLY leave behind resistant worms.

In the absence of other, susceptible worms on pasture, these worms are left to continue the cycle, breed and soon dominate the worm population on your farm- this is a concept known as “refugia”. This means that the drench administered quickly becomes ineffective, if parasites are not appropriately managed throughout the year with proper drench group rotation and smart drenching practices.

Therefore, skipping an unnecessary second summer drench (when confirmed by a worm egg count):

• Maintains refugia/ susceptible worm populations

• Preserves drench efficacy

• Improves whole-of-year parasite control

A second summer drench should not be routine.

If you’re considering a second summer drench and need worm egg counts organised beforehand, give me a call or send me a text and I will organise a date and time that suits us both for on-farm testing.