A change of system

Veterinarian Sara Sutherland reports on the financial impact of farm management changes in response to drench resistance.

In Livestock11 Minutes
Sara Sutherland

How we manage worms is changing. The focus of advice around preventing drench resistance is shifting from “what do I drench with” or “when do I drench” to farm systems changes which allow farmers to use less drench, while maintaining profitability.

Farmers can do many things to reduce the impact of worms or the impact of drench resistance. There are many examples of farm systems changes, lots of potential tools in the toolbox. What works for your neighbour might not work for you. Farmers need to understand the impact of their farm management on the worm larvae on pasture, the worm life cycle, the animals, the grass growth, and the profitability of their farm. It’s a lot to think about.

A farmer at a recent worm discussion group commented “we need to make sure the changes we make aren’t costing more than the cost of having drench resistance!” So, at the next meeting we had a look at the financial impact of drench resistance, and the financial impact of a couple of these farm systems changes.

We invited a prominent local agribusiness consultant to put some of those changes into a computer model, using one of our group member’s farms as the base for the model. This was a successful hill country Wairarapa breeding farm.

The first question we asked was how much does it cost to have drench resistance and not know it (or not do anything about it)? One scientific study, done in 2012, compared the performance of lambs using an effective drench or a drench that was only 40-50% effective. In this study, lambs that had an ineffective drench were on farm longer (17 days longer for 50% of lambs to reach target weight, 34 days longer for 90% of lambs to reach target weight). They had a lower growth rate (110g/d instead of 180g/d), 9kg lower liveweight, 4.7kg lower carcase weight, and a lower carcase value. They had more dags, so more labour was required for dagging.

How would that look on your farm’s bottom line?
When we plugged in the lower growth rate into the model, this resulted in a decrease in gross farm revenue (GFR) by 4% and an increase in farm expenditure by 6% for an overall reduction in profit of 8%. On that particular farm it made a net loss compared with the base model of almost $100,000 per year. One of the group members commented that those numbers really make you take it seriously.

The cost of a novel drench (Zolvix Plus or Startect) is about $1 per dose more than the triple combination drench. Using a more-effective drench would improve the growth rate, and that would be cheaper than the cost of having resistance. If you do not change the farm management that allowed triple resistance to happen, changing to a novel drench is only a short-term fix. The first cases of drench resistance to the novel drenches are out there. And there are no new drenches coming. By using some farm management tools, you can have a farm where you don’t need to use as much drench, so you can still farm profitably despite drench resistance.

Our group looked at a number of potential farm management changes that group members might do to manage drench resistance. I’ll go over four of these – having fewer ewes, selling lambs store instead of finishing them onfarm, switching to terminal, and increasing your cattle ratio. 

What these strategies have in common is that they reduce the number of lamb days onfarm during summer. This reduces the risk of drench resistance because it reduces the number of young animals requiring regular drenching.

Lamb days onfarm over summer is one risk factor for developing drench resistance. For this exercise, we only changed one thing at a time, and compared to the base model (how the farm is performing) rather than the model of the farm with drench resistance. In reality, of course, you could change multiple things at one time.

When you reduce the number of ewes onfarm, you have more grass. Assuming you are putting that extra feed into feeding ewes (instead of keeping more lambs over summer), ewe body condition and mating weight increases. That increase in ewe body condition increases the number of lambs born per ewe, lamb and ewe survival, and lamb weaning weight.

Because lamb weaning weight increases, we can get more lambs off farm earlier (sold prime off mum). This means lambs require fewer drenches.  Prices will be higher when lambs are sold. Cull ewe weight increases so they are worth more. Risks and challenges with this change include making sure you don’t lose control of grass quality.

In this model, having fewer ewes on this farm improved the GFR by 2% and reduced the expenses by 5% for an improvement over the base model of about 6%. 

The next model was to have more ewes but sell all lambs store at weaning instead of finishing them onfarm. Again, this could help deal with drench resistance by having fewer lambs onfarm over summer. The computer told us this model decreased revenue by about 3% and decreased cost by 1% for an overall lower profitability. On this farm, this tactic did not make financial sense.

Increasing the cattle ratio was worthwhile according to this modelling. Increasing the cattle ratio had all the benefits of reducing the number of ewes in terms of ewe body weight, but without the risk of losing feed quality over summer.

Disadvantages of this tactic would include pugging and managing cattle on steep hills over winter. The model worked out that on this farm it could increase GFR by 2%, decrease costs by slightly more, and overall improved profitability by 6.5%.

Switching to a terminal flock is a huge move that wouldn’t suit everybody. It means losing control of your genetics, and you have the risk of buying in diseases. You have to source good quality two-tooths each year, which can be hard to do at scale. Advantages are paying less for rams and having bigger lambs at weaning which you can sell for a higher price. This model worked out an improvement in profitability of more than 5% compared to the base.

What do the three profitable models (fewer ewes, increasing cattle ratio, and terminal flock) have in common? They all increase lamb weaning weight. This means you can get lambs off farm earlier, at a higher weight, for a better price.  What about other tactics (subdivision, fertiliser) that increase lamb weaning weight? Will they have the same effect on drench resistance?

I’m sceptical of computer models. They are a starting place and need to be real world tested. They are only as good as the numbers you put in. They miss things. They are a way of comparing very specific changes. Remember that these results will be specific to this farm. What would they look like on your farm?

I’m not suggesting everyone should farm with fewer ewes.  Again, the tools you use will be different from what your neighbour uses.  Attend a Wormwise workshop, run through some scenarios with an advisor you trust, understand how the worms are affecting your farm. Do a FECRT or at least some drench checks to find out whether drenches are still working on your farm. 

When you have more knowledge, you can make changes that improve things on your farm. And at the end of the day, the best change is the one you want to implement. If you don’t believe that the change is the best for your farm then you won’t stick with it.


Sara explains the above table: The percentages are a percentage of the base model (how the farm is performing now). I was presented with the actual numbers, which I don’t want to share publicly as they are confidential to the farm (so you will have to trust me that this is a well-run, high-performing, commercial operation). The variance was calculated separately from the other two lines, which is why they don’t just add up.