Why are we so keen on combination drenches?
The recommendations around worm management and especially drenching of farm livestock is an area farmers have had to cope with a lot of change as we all learn more.
Many of us grew up with the theory that the annual rotation of single-action drench families was the best way to slow the development of drench resistance. Why didn't drench rotation work?
Remember that by drenching, you only knock about 10% of the total worm population - the rest are out on the paddock. When a worm population develops genes for resistance to white drench, killing some of the worms in that population with clear drench means that those white drench resistance genes are still sitting there. When you rotate back to a white drench, you pick up where you left off selecting for white drench resistance. Meanwhile, there may now be some clear drench resistance genes out there in the population, so when you rotate back to a clear drench...
Using drugs in combination has been shown to greatly delay this effect. Combinations are better at delaying resistance because the chances of a single worm in the population having genes for resistance to two chemicals is exponentially less than the chance of resistance for just one. When a third action family is added, this chance decreases by an even bigger exponential amount. In addition to this, experimental evidence has shown in many cases that drugs used in combination seem to achieve a greater potency than when used on their own.
Resistance will develop eventually to combinations. The power of combinations to delay resistance is much greater when there is little or no resistance to the individual actives. Based on the data we have, farms that adopted the use of double combinations early generally have lower levels of drench resistance than those who have stuck to using single actives. As alluded to above, using triple combinations would slow the process even more.
Drench resistance at the individual farm level?
Finding out the drench resistance status of the worms on your farm is really useful. It can change the recommendations made about drench choices and givesyou a picture of how far down the track you are to developing serious problems due to drench failure.
For sensible recommendations about combination drench choices, it pays to get a faecal egg count reduction test (FECRT) done to determine your farm's resistance status and go from there. If you have had FECRTs done, we can revisit them and see if there are newer combination product options available to you.
Reviewing management techniques, other than combination drenches, may also benefit you.
