Open mind to drench resistance

 The spectre of triple drench resistance strikes fear in many farmers, but Wairarapa farmers Sam and Sarah Johnston have a positive story to tell, showing it is possible to overcome drench resistance with an open mind, and careful management. Rebecca Greaves spoke to them.

In Livestock22 Minutes

Outwardly, Tinui sheep and beef farmers Sam and Sarah Johnston had no indication their farm had a drench resistance issue. Regular drench reduction tests were undertaken as part of best practice, but it was a shock to the couple when the test showed triple drench resistance in 2018.

Through dedication and willingness to tackle the issue head on, Sam worked with his vet, Sara Sutherland, to put a plan in place, as well as seeking outside expertise. He wasn’t afraid to question his practices on drenching stock and made a number of management policy changes. In February this year, another drench reduction test showed the Johnstons had overcome triple drench resistance.

Not only that, but through reducing stock numbers and reducing the cost structure involved with worm management, the Johnstons have sustained stock health and maintained a profitable business.

Sam says what he did is specific to his farm. There are many variables, and what worked for him might not necessarily work for others.

“I started out as a shepherd, then went to Massey. This drench thing has really stuck with me. I had spent years drenching for people and never questioned it. I pride myself on stockmanship but you really rely on the technology, the drench gun or scales,” Sam says.

Over 15 years at Reata the Johnstons had undertaken three drench reduction tests. 

“I thought it was good information to have, but I couldn’t tell it was going to be a problem, and that’s my question – is it a problem that triple drench resistance is around? I think it is.”

Importantly, the fact they had undertaken regular tests meant they found out about the problem before it was too late, and it was a matter of tweaking policies to get on top of it. Had they not undertaken regular testing and not found out until the wheels fell off, the road back would likely have been longer and more difficult.

 ‘I started out as a shepherd, then went to Massey. This drench thing has really stuck with me. I had spent years drenching for people and never questioned it. I pride myself on stockmanship but you really rely on the technology, the drench gun or scales.’

“It was about looking at my own farm system, looking at the risks. I said straight away, no more trade lambs bought in. I stopped drenching tail end ewes.”

When faced with the reality of drench resistance Sam says it was fortunate he was in a place where he felt able to cope with tackling the problem. Having completed farm succession and having a young family, and knowing Sarah was at home with the kids, gave him the ability to put all of his energy into finding a solution.

Sarah grew up on a farm and has a background in human resources. While she’s not so involved with day-to-day farm life due to young children, her support and ability to hold the fort at home has allowed Sam to spend time off-farm and take advantage of learning opportunities. 

“This hit me at a time when I was able to deal with it, but I can see how this could add to anxiety or stress for other farmers. I had a young shepherd here I had trained up and we were able to attack this in our everyday life onfarm – it became top priority,” he says.

Sam hadn’t heard a lot about triple drench resistance prior to discovering it on his farm, but by working with vet, Sara Sutherland, they formed an RMPP Action Group. Sam was the lead farmer and Sara acted as facilitator. 

A pragmatic approach

“I understood they believed triple drench resistance could not be reversed. We got some expertise in, in a friendly environment to talk to the farmers in the group. We had funding and resources.”

Sam’s approach to tackling drench resistance was pragmatic. “I got put in a place where I had to try and make it better. Even if the results stayed the same after I changed things, I would still be happy, because I tried.”

Knowing his numbers, Sam knew exactly what economic farm surplus (EFS) he needed to break even, and was willing to do so to fix the problem.

“You control the things you can control. With the support of the action group I committed to every meeting, no excuses. It was all on me and my worker, who I trusted, and the advice we were getting. Sheep eat grass and shit out worms – and you can’t see worms. We are farming worms, whether we like it or not.”

Sam made a number of policy changes at Reata, outlined below.

Policy pre March 2018 (Before drench resistance was discovered)

Lambs: pre-wean drench, then every 28 days after weaning until May

  • Hoggets: capsule after scanning
  • Two-tooths: pre-tup drench, triplets capsule, tail end drenched throughout the year
  • Mixed age: tail end pre-tup drench, triplets capsule, tail end drenched throughout the year
  • Rams: triple quarantine triple on arrival, drenched pre and post tup
  • Trade stock: quarantine triple on arrival, every 28 days after that

“I was doing ‘best practice’ and it cost a lot of money. Now I know capsules are a risk, drenching sheep that might not have worms is a risk. Before, everything got drenched and ran on the flats on beautiful grass. They got fed so well it could have hidden the problem. 

“Was I buying in triple drench resistance? There’s no way to know. 

“By doing routine drench reduction tests I found it, and it wasn’t too late.”

“So I blamed myself, and I said ‘you can fix this yourself, it’s your farm, there are things you are doing that have created this’. But what worked for me might not work for others.”

Sam says there are many variables and no hard and fast formula to fix triple drench resistance.

What changed? 

Policy post-2018 until now.

Lambs: no pre-wean drench, drenched every 28 days to stop contamination, not many left after March. Ewe lambs drenched every 28 days until March

  • Hoggets: only drenched when required
  • Two-tooths: no drench
  • Mixed age: no drench
  • Rams: Zolvix on arrival, then no drench
  • Trade stock: Zolvix on arrival, look to sell before they get too many drenches

These days it’s all about growing young stock fast and getting them out of the danger zone when it comes to worms. The philosophy is to drench as little as possible. Now, 70% of the kilograms of liveweight on the farm does not get drenched.

All stock are mixed through the flats, medium and steep hill country to help control contamination.

“With lower inputs, animal health costs have gone down. From what I’ve learned, why do you need to drench adult sheep?”

Throughout the process Sam says he has learned a lot from AgResearch’s Dave Leathwick, and remembers his mantra – you won’t fix this problem with a drench gun.

“My ewe lambs now get seven drenches in their lifetime, and I’m only keeping them if they get in lamb. They’ve got to rear a lamb without a capsule.”

Since implementing the new policy of not drenching two-tooths, scanning has still been between 165-180%. 

“It ties in with body condition score (BCS) as well, but we haven’t seen a drop in performance. We don’t capsule 3000 ewes now either, that’s a $9000 saving, and less workload. We’re still following all the same farming principles as before, just without drench.

“We need animals that we can put under pressure and still come through. At tupping, any ewe that I know is not going to make it, which I would previously have drenched to lift it up, I kill. Hopefully over time that should weed itself out. I would say my ewe deaths have gone up through what I’ve done though, maybe a percent or two.”

Sam does get rid of more lambs, earlier, selling 27-28kg lambs in October. Previously, he would have put in a crop and aimed to put another $20 on them. Now, he sows a clean crop with no worm burden.

“We’re following the feed curve. I take out a lot of area in spring on the flats to get crops in for when it goes dry. With the new (drench reduction) test I could go back to my old way of doing things, but I don’t know if I want to – I could create the same problem again.”

On the ram front, Sam wonders if there is an opportunity for breeders to look at a parasite resistant ram.

When it comes to trade stock, trade lambs are gone and the only stock bought in, occasionally, are in-lamb ewes and some cattle.

His advice to other farmers is to do a drench reduction test and know where you stand. 

The goal at Reata now is to have good condition mixed -age ewes or cows that need low input. “I’m keeping my sheep in very similar condition (as before) without drenching them, by keeping the stocking rate lower and feeding them better. Often people hold on to lambs to try to make more money, but put their ewes under pressure, and that’s a double whammy.”

Since 2008 they have moved from 3500 ewes down to 2900, due to a range of policy changes over that time. Reata now runs about 550kg LW/ha in winter, and has gone from 12 SU/ha to sit at about 8 SU/ha today. Sam says the effect is a better lambing percentage, and higher killing carcase weight ewes and lambs, creating a more efficient system overall.

A vet’s perspective

Sheep and beef vet with Vet Services Wairarapa, Sara Sutherland, worked with Sam on his drench reduction tests and was instrumental in helping put a plan in place when resistance was discovered.

She would encourage other farmers to undertake testing, saying it is better to know and be able to do something than to bury your head in the sand.

“I think the big thing it’s important for people to understand is Sam didn’t know he had a problem. People often don’t know they have an issue until the wheels fall off. The other thing is, he wasn’t afraid to let people know. I think there have been other high profile farmers who have been open about their status now, and people don’t need to be afraid to test.”

Sara says it occurs when the worm has a gene that allows it to survive the drench. Over the whole population of worms, some have a gene that allows them to survive. Those worms then mate, the offspring survive and, over time, the resistant population builds up.

Management factors that can contribute to that include over-drenching, using long acting products and not giving a high enough dose. 

“If you drench and then move stock to a clean pasture the only worms that will be there are the ones that have survived the drench, and they can build up very quickly.”

The other common way farmers can get a resistant population of worms is by buying them in, for example trade lambs. 

A full faecal egg count reduction test (FECRT) is the only way to tell which worm species are resistant to which drenches on your farm. Because these are expensive and take a bit of work to organise, the recommendation is to do these every three to five years.  

In between, farmers should do ‘drench checks’. A drench check is a worm egg count taken 10 days after drenching. Zero counts at this time indicate the drench is effective. Sara recommends farmers do these routinely once or twice a year, and whenever they suspect an issue.   

“As drench gets less and less effective, farm production will go down, but you don’t see it, don’t see it until all of a sudden you drench your lambs and it doesn’t work. It can be there, just under the surface.

How to prevent drench resistance

Sara says, in general, vets have moved away from drench plans and now recommend a parasite management plan, to maintain production while stopping drench resistance from developing. 

Having outside expertise, like vets, who have training and interest in worms look at your whole farm management and what steps you can take to minimise the risk has value. “I think it’s really important to sit down and make a plan.”

She also highly recommends attending a wormwise workshop. Wormwise is the national worm management strategy. It is aimed at helping farmers and their advisors manage worms, not just for today but sustainably for the future. See 

“They have a good website and the workshops are free through Beef + Lamb New Zealand. They give you the basic knowledge about the worm lifecycle and how drenches work. They have the most up-to-date information and you can have confidence in their principles.”

Best practice onfarm includes trying to avoid the use of capsules. If you do use them, make sure they are a targeted treatment. Think about the way you are grazing your farm. Each time a lamb is drenched, ask yourself, what are you using for refugia? 

It is still recommended that lambs be drenched every 28 days through summer to avoid a large worm build up come autumn.

“The more you drench the more times that worm is being exposed to the drench, but at the same time you don’t want to just stop drenching. Each lamb, at some stage, will develop its own immunity to worms at around 12-18 months. Try to avoid drenching adult ewes. Most of the time they can handle the worms.”

Try to have some worms in the system that have not been exposed to your drench, and that is your refugia.

Farmer and vet working together

As a vet, whenever Sara does a drench reduction test she always sits down with the farmer and talks through the results. This involves making a plan of where to go to from there, and the Johnstons were no different.

“We sat down and looked at the risks in Sam’s system that could have led to this developing. Capsules were one, drenching on to crops of clean pasture, drenching adult ewes. What’s important about him is that he looked at his risk factors, changed his policies and still has a profitable business. He has gone back to where triples are working 100% now.”

Having the Action Group also helped, as it gave them access to experts to talk to farmers in the group.

Sara says the Johnston’s case shows that people don’t need to be afraid of drench resistance, there are things that can be done. If you get drench resistance it is possible to manage your way out of it.

“I don’t take credit for it all. Sam came up with some forward planning himself, like having crop to put his hoggets on instead of giving a capsule. But I’m happy with this outcome, and to have been part of it.

“Anyone who doesn’t know their status, do tests, talk to your vet. Make a parasite management plan. What worked for Sam might not work for you, but there will be things you can do on your own farm.”

Key points


  • Triple drench resistance can be overcome
  • Get a drench reduction test – early detection crucial 
  • What works for one farm might not work for others
  •  Be proactive, seek outside help and resources

Farm facts

  • Reata – owned by Sam and Sarah Johnston
  • Location: Tinui, Wairarapa
  • 800 hectares (effective) – 155ha flat, 400ha medium hill and 245ha steeper hill
  • Sheep and beef breeding unit with a trade component