How forage crops are changing the drench resistance game
Drench resistance is no longer a problem sitting somewhere else. It is here, widespread, and on many New Zealand sheep properties it is quietly costing money every season through slower growth rates, more frequent drenching, and lambs that never quite hit their potential.
The frustrating part is that drenching more is not the answer. Every time the drench gun comes out, it applies selection pressure that favours resistant worms. The more often it happens, the faster resistance builds. The shift has to come from somewhere else.
The good news is that most farmers already have a tool that helps manage this, and it is already sitting in the rotation.
Why grass works against you
Ryegrass pasture creates the ideal environment for worm larvae. It is dense, holds moisture, and sits close to the ground, which is exactly where larvae thrive. Research shows that 50 to 70 percent of infective larvae live in the bottom 2.5 cm of the pasture sward and the top centimetre of soil, which means every time a lamb grazes tight, it is entering the highest risk zone on the farm.
Forage crops change that dynamic. Crops such as brassicas, chicory and plantain grow taller and more upright, allowing more sunlight into the base of the canopy and creating a drier, less favourable environment for larvae. Lambs are grazing higher in the canopy, which naturally reduces their exposure to the areas where worm challenge is highest.
The data backs it
Multi-year research trials across several New Zealand sites, including commercial farms, have shown that lambs grazing summer forage crops carry significantly lower parasite loads than those on pasture. In many cases, lambs reached processing weight before needing another drench, which translated to a saving of two to three drenches per animal.
That is not just an animal health improvement, it is a system win. Fewer drenches means lower input costs, less labour and yarding, and importantly, less selection pressure driving resistance across the farm.
Building your forage toolkit
Brassicas such as rape, kale, leafy turnip and raphnobrassica are some of the most effective options. Single season brassica crops are particularly valuable because they are regularly resown, which exposes or disturbs the soil and helps break the worm lifecycle. Grazing them out and re-establishing the crop resets the system and reduces carryover risk.
Herb and clover pastures, including chicory, plantain and red clover, offer a strong multi-year option. When these crops are first established, worm challenge is typically very low, which creates an opportunity to reduce drench use while stock are grazing them. Their more open and upright structure reduces humidity at the base of the plant, limiting the microclimate that larvae rely on.
The key with these systems is keeping them clean. If grass and weeds creep in, the canopy starts to behave more like pasture again and the advantage is quickly lost, so maintaining crop integrity is critical.
Managing refugia properly
Forage crops often start with very low larval populations, which changes how drenching needs to be managed. Drenching stock onto a clean crop without thought can create a resistance hotspot, as the only worm eggs deposited are those that survive treatment.
For short term crops that are grazed out and resown, this risk remains low. For longer term herb and clover pastures, a more measured approach is needed. Monitoring faecal egg counts and extending drench intervals based on actual need will protect the effectiveness of drenches while still taking advantage of the lower challenge these crops provide.
The bottom line
Forage crops have always been a strong feed decision, but the value goes well beyond that. Lower larval intake means slower worm build up, longer intervals between drenches, and less pressure on the tools you rely on to manage parasites.
If weaned lambs are not already being routed through crops as part of your summer system, it is worth looking at how that fits. The combination of feed quality and parasite control is hard to ignore, particularly as resistance continues to build across the country.
Talk to your Catalyst adviser about what crops fit your system and rotation, and start using what is already in your toolbox to stay ahead of the problem.