• Wayne

Worm resistance thoughts

Updated: Dec 20, 2021

If a boxer has a knockout punch, he doesn’t only use that weapon. Of course.


When ryegrass started being resistant to herbicides, there were all sorts of ideas on how “we” created it and how best to manage it. There were many weird and wrong ideas!!! For example, sow susceptible ryegrass in with the resistant ones to dilute the resistance genes. Sigh. These sort of ideas forget the main idea about farming. Growing food. Profitably. No profits = no food. It is lunacy to sow a weed that is robbing your crop so that you can use a herbicide to control the susceptible ryegrass you just sowed. It is sad some today still can’t grasp the folly of this idea.


I was a cropping and pasture agronomist for 29 years. I was a cropping researcher for six years prior to that. Now I have graduated and am a farmer :-). A sheep farmer to harvest our pastures.


Of course in my work as a consultant, I would come across the worm problem in sheep, and worm resistance. Because I never needed to be involved in that facet of farming, I did not give it much time in my thinking about it.

However, when learning about sheep over the past few years, I voraciously read everything I could. I still do. But, I don’t agree with several practices in the livestock industry. I will write about those one day in these blogs, but for now, I want to discuss the issue of worm resistance in sheep.

Firstly a quick comment on herbicide resistance in ryegrass (it’s the same principle for any species). Never has a gene been created to be resistant to a herbicide by the use of the herbicide. Maybe read that again……..

There are wrong ideas about mutations and selection. Using a herbicide does not create a mutation so that it is resistant to the herbicide. When you use a herbicide, you are “selecting” for plants that are resistant to it. If they are susceptible, they die if they get a lethal dose. Of course. If there are no resistant genes to the herbicide in that species, that species will never ever become resistant to the herbicide. Never, no matter how many times you use that herbicide to control it.


By spraying just one herbicide to control a weed, if one plant in a gazillion has a gene to make it tolerate the herbicide, you will “select” for that resistance gene. After the first spray, it might be now one in a billion. Use the herbicide again, next year, and the ratio might now be one in a million and so on until one day, virtually all of the plants have the resistance gene(s).


Whatever control method you use, you are inadvertently selecting for genetics that are tolerant or resistant to that control method.


It is very similar to intestinal worm resistance in sheep. Using a drench does not create resistant genes. They are already there. By using any drench, you are killing the susceptible ones and selecting for the resistant ones. This is where some get confused. They think we should not use a drench because it creates resistance. “Creates” is the wrong word, but the problem of course is that until we have sheep that are completely resistant to worms, we will at times need to drench them to keep them alive.


Using just one mode of action on worms (intestinal worms in sheep) is unwise. Let’s assume you use moxidectin but it fails because it only controls 50% of the worms. Now if you use another mode of action drench with the moxidectin, such as levamisole and it only controls 80% of worms if used by itself, that would control another 40% of the worms giving 90% control in total.


Now if you use another drench, perhaps a top shelf one like Zolvix, it only needs to control the remaining 10% that the other two drenches failed to control. That is a much easier task than if it had to control 100% of the worms.


If you only used Zolvix, then if it had to control 10 million worms in a sheep, there is a high probability that one of those 10 million worms has a gene that will tolerate Zolvix. However, if you have already controlled 90% with other modes of action, then there is a much lower risk that amongst the remaining 10% of the worms, there will be one that is resistant to Zolvix. There’s a 90% probability that it was controlled by the other two modes of action - in my example.


It is a numbers game. It is always a numbers game. Get the numbers down very low in the sheep and less eggs are returned to the pastures in the dung. Those eggs will now be exposed to weather and predation that can reduce the numbers even lower. And hopefully we have sheep that have some tolerance to worms as well, which provides another mode of action against the worms.


People tell me using 3-4 modes of actions in a drenching program just causes the remaining worms to be resistant to ALL of those 3-4 modes of action. It doesn’t. Remember, drenching does not create a resistant gene, you are only selecting it (finding that needle in a hay stack). Those few surviving worms might be resistant to the three modes of action, but they might just have missed being exposed to a lethal dose. Now those remaining worms are still exposed to other modes of action - heat, dry, predation (everything in nature is eaten by something else) and hopefully whatever it is in a sheep that has tolerance to worms.


It’s a numbers game. It is always a numbers game. What we know from managing herbicide resistant ryegrass is not to use one mode of action and then a different one next time, but to use 3-4 modes of action. Next time, use 3-4 modes of action again, but at least one of them is a different one that you didn’t use the previous time. It is possible to make ryegrass extinct on a paddock using this strategy. By the way, mode of action is not just a herbicide. It can be growing a more competitive crop to shade and starve the weed. It can be machinery that pulverises the weed seeds into dust as it passes through the harvester. It can be a change in sowing time.

In management of sheep worms on Caluka Farms, I believe we should be choosing genetic resistance as much as possible, and when drenching, to use 3-4 modes of action each time.


Where we farm on Caluka Farms, we do not have long, hot, dry summers. Those of you who do have a very strong control option. Large numbers of worm eggs will not survive 6 months of hot and dry conditions. Therefore it is possible for you to use 2 modes of action in a drench just before the sheep go into dry stubble paddocks and control can last for 12 months. This is the system many of you use who farm with crops and livestock, and have long hot summers.


We don’t have that benefit, so need to rely on genetic resistance and multiple modes of actions when we drench. We change one of more modes of action every time we drench, but always use 3-4 modes. The aim is to keep the egg numbers so low on the pastures that it is easier for genetic resistance to drop the numbers even lower.


It is a numbers game. It is always a numbers game. Our aim is to have genetic resistance in our sheep that does >90% of the control so that one day, we rarely if ever have to drench. We have a long way to go to reach that level of genetic resistance.


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