Number one on the list is the EFFECTIVENESS of each drench group on YOUR property, because the most expensive drench is the one that doesn’t work.
As a reminder, a drench is considered effective if it reduces worm egg count by at least 98%. In other words, values less than this indicate the drench is ineffective. However, a drench can still provide reasonable protection as effectiveness falls below the 98% level, but at some stage it will no longer provide sufficient protection. At the same time, the speed at which drench effectiveness falls, will increase at lower levels of effectiveness.
For example, a trial conducted by Brown Besier in Western Australia (where scour worms are dominant) in the 1990s demonstrated that in relation to an effective drench, the use of drenches with 85% and 65% effectiveness led to Merino weaner wethers cutting 0.1 and 0.5 kg less greasy wool and being 1 and 6 kg per head lighter (respectively) after one year.
Ineffective drenches can lead to:
And it costs you much the same in time and labour to use an ineffective drench.
Unfortunately, it is often not obvious that a drench is losing effectiveness until there is an outright failure or obvious production loss.
The TYPE OF WORMS you are trying to control are accounted for here, as drenches may differ in their efficacy against particular worm types. For example, in the summer rainfall areas, it is common for macrocyclic lactone (ML, mectin) products to be 100% effective against black scour worm, but only 40–80% (or less) effective against barber’s pole worm.
The second criterion when choosing a drench is whether it is a COMBINATION OR SINGLE-ACTIVE product. Because commercial combinations contain two to four active ingredients, there is a much lower chance that the worms will survive this type of product compared to drenching with only one or other of the ingredients in the combination drench.
But just because a product is a combination, that does not ensure its effectiveness.
A Drench Resistance Test will assess the effectiveness of individual groups and actives, and with these results you will be able to predict the effectiveness of many combination products.
(See more information on the benefit of combination drenches and calculating efficacy of them using the WormBoss online Combination Drench Efficacy Calculator).
The best combinations are those with the most actives and where each active, individually, has high effectiveness.
So, is the third criteria price? No. The LENGTH OF PROTECTION is still more important. In most situations, short-acting drenches are the best choice as they contribute less to development of drench resistance. However, there are times when it is prudent to use a persistent drench (usually a intra-ruminal capsule or an injection).
Persistent products are useful in times of very high worm risk, on badly worm-contaminated paddocks, and are more susceptible sheep (such as weaners). They can also be of value to some producers when trying to prevent further contamination of paddocks when low worm-risk paddocks are being prepared.
But remember, a persistent product is giving up to three months comparative advantage for resistant worms compared to the day or so advantage following a short-acting drench.
What about choosing a drench group you didn’t use last time as the fourth criteria—DRENCH ROTATION? You may be surprised to find that this is of very little importance compared to the three criteria above. However, it is important in a few situations: the next drench after a persistent treatment or after using a prepared low worm-risk paddock (see more information about drench rotation).
In an effective integrated worm control program, drench price only accounts for about 20% of the costs—it is far more important to minimise production loss and deaths, which are a high proportion of the overall “worm costs”.
When you have decided the best combination of drench groups to use on the worms affecting your sheep and the length of protection required, now consider price.
The WormBoss Drench database shows an indicative price of a dose for a 50 kg sheep. You can find prices at www.wormboss.com.au and choose Drenches from the menu. Any of the searches will yield products with their price listed.
The GST-inclusive prices were obtained in early 2017 from Landmark Armidale and do not account for any specials, discounts or rebates. Prices may differ at your local supplier, but these will give you a fair idea to compare different drench types. But you may still want to do calculations of your own.
In combination with your Drench Resistance Test results, you may find that an older style 3-active combination that is just as effective on your property is much cheaper than a newer single- or double-active product. But remember there are times and occasions when changing drench groups is important.
There is no substitute for doing a Drench Resistance Test to find out the effectiveness of many drenches on your property.
If you have 1000 sheep and carry out this test (with 8 drenches tested and one control) each 3 years, it’s the equivalent of about 25c/sheep/year. The more sheep you have, the cheaper it is on a pro-rata basis (see more on the cost of a DrenchTest).
If you have not done a comprehensive DrenchTest of single actives in the last three years, plan to do one this year. The WormBoss website provides instructions to help you.