Can Hot Weather Make Harrowing Your Horse Paddock More Effective?
- Horses Inside Out
- 1 day ago
- 4 min read
Many of us may be wishing the recent heatwave would end, temperatures would drop, and rain would arrive to improve ground conditions and help scorched pasture recover, so that we can enjoy doing a bit more with our horses. There is, however, something this prolonged hot weather has created: an unusual and rare opportunity in the UK, where harrowing can substantially reduce the level of parasites in the pasture
“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,” explains Diane.
Diane Gilby, Parasitologist and AMTRA-Qualified Prescriber from Intelligent Worming, explains this in more detail in this article to help you understand the science behind harrowing and the impact it can have when done correctly.
The Three Conditions that Make Harrowing Work Effectively
Let’s look at what the research tells us. 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.
Here’s 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.
How Hot Does it Need to Be?
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).
This is 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 were not high enough.
Why Mowing First is so Important
Mowing before you harrow is one factor 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 (grass), 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.
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.
Mowing or topping your paddock to below 10 cm before harrowing is the key step most people miss. After harrowing, the paddock should be rested for at least a week. The mowing step is what makes harrowing actually work.
It Takes More Than One Sunny Day
There’s no published study giving a definitive "X degrees for Y hours" answer, but the evidence points to this:
You need sustained hot, dry conditions, not just a warm afternoon. Aim for 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.
A Practical Checklist for Harrowing
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 after harrowing. 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
Cool, damp, or overcast weather is the wrong time to harrow. At temperatures below 25°C, the evidence shows it simply spreads viable larvae across a wider area of pasture, contaminating clean grazing that horses would otherwise avoid.
The same applies if the grass is too long. If it is above 15 cm and you cannot mow first, contamination may be spread 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.
Diane Gilby Parasitologist and AMTRA-Qualified Prescriber from Intelligent Worming will be joining us at the Anatomy Exhibition on 22nd August and will be available to answer any questions you may have about worming and pasture management. Make sure you book early to avoid disappointment.
Hot Weather and Harrowing: What the Research Says
Intelligent Worming - Evidence-based guidance on pasture harrowing for parasite control during hot weather conditions. Download your free guide below!
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