These days, sheep producers picking up a drum of drench are faced with a huge range of options to choose from. Fifty years ago you had pretty limited options, which wasn’t great for worm control but did at least make the decision slightly simpler.
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As the available options increase, the decision becomes more complex. Do you buy the cheapest drench? Or the one with the best ad on TV? Or the one your neighbour reckons works on his place? Or the one that comes with a free drench gun/vaccine pack/hat/football/steak knives? (Admittedly the latter is always tempting).
The truth is, there are better ways to decide which drench to use. Some drench products aren’t cheap, so it pays to think through your selection so that you’re using something effective and appropriate to the situation. As the saying goes, the most expensive drench is the one that doesn’t work.
There are lots of things to consider when selecting a drench. You’ll need to think about what worm species you’re targeting, the resistance status of your flock (i.e. which drenches have lost effectiveness), whether you need long-acting protection, what length of meat withhold period you can tolerate, and whether you want to treat other parasites like liver fluke or tapeworm.
Something to keep in mind when making a selection is that a “combination” drench of some sort has a couple of significant benefits.
Combination drenches are those which contain two or more “active ingredients” (which are the specific chemicals responsible for killing worms). For example, a drench which contains a BZ (“white” drench), Levamisole (a “clear” drench), and an ML (“mectin” drench) is a “triple combination”, since it contains three separate active ingredients.
The first benefit combination drenches have is that they’re often more effective at killing worms. To explain why this is the case, imagine a combination drench containing two separate chemicals – A and B. If (due to resistance) chemical A only kills 90% of worms, then using it on its own would leave 10% of worms behind. If chemical B kills 80% of worms, then using it on its own would leave 20% of worms behind. But if we use them together, in a combination drench, then drench B will kill 80% of the 10% left behind by chemical A, leaving only 2% of the original worms remaining. This is a much better result than you’d get by using either drench A or B on their own.
This higher kill rate is a good short term benefit. But there are also big wins in terms of slowing the development of drench resistance on your farm. Research has shown that using combination drenches significantly reduces the rate at which resistance develops to each of the individual active ingredients.
You could think of it as each active ingredient “protecting” the others in the mix. Worms which are resistant to one active can be killed off by another, so that they don’t survive to perpetuate genetic drench resistance.
