With temperatures climbing across the UK this week, I’ve had several calls from stud managers asking: “Is now a good time to harrow?”
It’s a great question. And the answer is more nuanced than most people realise.
The short version: harrowing CAN be an effective tool for reducing parasite larvae on pasture, but ONLY when the conditions are right. Get it wrong and you will spread viable parasites across a wider area, making the problem worse.
So what does the research tell us?
IT’S NOT JUST ABOUT TEMPERATURE
A lot of people think heat alone kills larvae. It doesn’t. The real killing mechanism is the combination of three things: desiccation (drying out), UV radiation from direct sunlight, and heat.
A 2008 study by Krecek and colleagues found that when you expose cyathostomin larvae directly to dry conditions, preinfective stages (L1 and L2) die within minutes. Eggs die within 5 hours. But infective larvae (L3) are much hardier. They can survive over a month in a desiccated state.
The game changer? When desiccation and UV are combined, even L3 larvae reach total mortality within 5 days.
This is exactly what harrowing does. It breaks up the faecal pat, which normally acts as a protective moisture reservoir, and exposes the eggs and larvae inside to sunlight and dry air.
THE TEMPERATURE THRESHOLDS
Research by Nielsen, Kaplan and colleagues (2007) and Mfitilodze and Hutchinson (1987) established that:
No larval DEVELOPMENT occurs above 40°C
Above 35°C with dry conditions, free-living stages begin to die off
In hot Australian summers, only 1 to 10% of larvae survived to the infective stage, compared to over 80% in spring and autumn (English, 1979)
But here’s the critical finding that most advice overlooks:
A 2022 Swedish study (Osterman Lind et al.) tested harrowing directly at temperatures between 14°C and 25°C in dry conditions. It did NOT reduce larval counts on pasture. The temperatures simply weren’t high enough.
SWARD HEIGHT MATTERS: MOW BEFORE YOU HARROW
This is one that almost nobody talks about, but the research is clear.
The whole point of harrowing is to expose broken-up faecal material to UV and desiccation. If your grass is tall, the canopy creates a shaded, humid microclimate at ground level that PROTECTS larvae rather than killing them.
English (1979) found that the majority of strongyle larvae stay within the lowest 10 cm of the sward, close to the soil surface. Langrova and colleagues (2003) confirmed this, finding 89% of cyathostomin L3 remained within 10 cm of the faecal pat. A 2021 Brazilian study found significantly more larvae survived in shaded areas compared to direct sunshine.
Tall grass provides exactly the shade and moisture that larvae need to survive at the base of the sward.
Here’s the practical guide:
Below 10 cm (4 inches): OPTIMAL. Good UV penetration and airflow to ground level.
10 to 15 cm (4 to 6 inches): WORKABLE in very hot, dry, sunny conditions.
15 to 20 cm (6 to 8 inches): UNLIKELY TO BE EFFECTIVE. Too much canopy shade.
Above 20 cm (8 inches+): DO NOT HARROW. You will just spread viable parasites under a grass canopy with no kill.
The key step most people miss: mow or top your paddock to below 10 cm BEFORE harrowing. Then harrow. Then rest the paddock for at least a week. The mowing step is what makes harrowing actually work.
DURATION MATTERS
There is no published study giving a definitive “X degrees for Y hours” answer. But the evidence points clearly to this:
You need sustained hot, dry conditions. Not just a warm afternoon.
At least 3 to 5 consecutive days of dry weather above 30°C after harrowing, with strong sunshine.
No rain forecast. If it rains within a few days of harrowing, you rehydrate the larvae and they survive.
Faecal pats in intact form dry out in 6 to 8 days in summer heat (English, 1979). Breaking them up through harrowing accelerates this dramatically.
PRACTICAL ADVICE FOR YOUR PADDOCKS
If conditions are right (and with this heatwave, they may well be):
- Check the forecast. You need 5+ days of dry, hot weather (above 30°C) ahead.
- Mow or top the paddock to below 10 cm (4 inches) first.
- Remove horses from the paddock BEFORE harrowing.
- Harrow to break up all faecal pats and expose contents to sunlight.
- Keep horses off the paddock for at least one week. Longer is better.
- Do NOT harrow if rain is forecast within 3 days. You will make things worse.
- This works best alongside regular poo-picking and diagnostic monitoring, not as a replacement.
WHEN NOT TO HARROW
Do not harrow in cool, damp, or overcast conditions. At temperatures below 25°C, the evidence shows it simply spreads viable larvae across a wider area of pasture. You contaminate clean grazing that horses would otherwise avoid.
Do not harrow if your grass is above 15 cm and you cannot mow first. You will spread contamination under a canopy where UV cannot reach it.
This is one of those areas where well-intentioned advice can cause real harm if the conditions aren’t understood.
A technical bulletin from Intelligent Worming guiding how to implement ideal harrowing can be found here