Dr Kirstie Pickles shares her to-do list for creating an effective pasture management plan.
In continuation of her three-part series on worming, Dr Kirstie Pickles, a European specialist in equine medicine, brings us Part 3 with this excellent how-to guide for good pasture management.
Her advice is designed to help our members tackle the issue of worms in grazing equines and the growing resistance to worming drugs.
By Dr Kirstie Pickles
Good pasture management, also known as pasture hygiene, is critical in order to decrease pasture infectivity and, therefore, the worm burden of grazing horses.
This approach, alongside targeted strategic deworming treatments (anthelmintics), is recommended as best practice to decrease the threat of anthelmintic resistance.
The best pasture management for an individual horse owner will be dictated by multiple factors, including time available. To ensure the system is achievable it is necessary to take all of these points into account.
Most importantly, however, parasite lifecycle, stocking density, and climate dictate certain key points which should be considered.
The main worm of importance to adult grazing equines is cyathostomins, so here are the facts to consider when creating your pasture management plan.
DOs
1. Poo picking
Infected horses pass eggs in their dung onto pasture. The removal of manure from the pasture, therefore, is probably the single most useful practice to establish. Poo picking at least twice weekly is necessary to reduce pasture infectivity due to the cyathostomin larvae migrating out of the faecal pat to increase their chance of being eaten and continuing their lifecycle. There is no point in collecting dung if the cyathostomin larvae have already migrated on to adjacent clean pasture! Eggs hatch in suitable weather conditions, above 10°C and moist climates, which generally includes spring, summer, and autumn in the UK. In winter, therefore, poo picking can probably be extended to once a week, unless the weather is mild. Manual collection is better, i.e. more effective, at reducing pasture infectivity than machines but it is obviously far more labour intense.
2. Separate dung heaps
There is no point collecting all that poo if subsequently it is stored adjacent to the field, allowing the larvae to migrate back onto the pasture. Manure piles should be stored remotely from the grazing pasture because, remember, larvae can migrate many metres.
3. Co-graze with ruminants
If possible, co-grazing land with ruminants is extremely useful in reducing pasture infectivity as horses and ruminants do not share any common gastrointestinal parasites. Grazing sheep after horses have moved off the grass will therefore clean up the pasture as cyathostomin larvae are ingested but cannot complete their lifecycle.
4. Stocking density
Ensure pasture is grazed at a suitable stocking density. High stocking densities allow large numbers of parasites to develop on pasture, particularly if young animals are grazing as they are likely to excrete higher numbers of eggs due to their lower immunity.
5. Use egg counts
Faecal worm egg counts (FWEC) should be used to identify and treat high egg shedders. Within a given population of horses, over 80% of eggs are produced by 20% of the horses, which is likely due to individual immunity factors. Targeting these high shedders will greatly reduce pasture infectivity.
DON’Ts
1. Harrowing
Cyathostomin eggs survive the cold and wet very well. Therefore, in the UK harrowing tends to achieve the opposite of the desired effect and actually increases pasture infectivity by merely spreading eggs and larvae all over the grazing land. In countries with predictable sustained periods of hot, dry weather harrowing can be useful, but not in the UK.
2. Resting pasture over winter
It is a common misconception that eggs and larvae will die over winter so moving horses off grazing for the winter will allow horses to return to the same pasture as clean grazing in spring. As previously mentioned, eggs and larvae survive cold and wet very well and so this practice can allow high levels of parasites to be maintained. If pasture is to be rested, it should be done during a prolonged spell of hot, dry weather (difficult in the UK!).
3. Pasture roughs
Don’t allow pasture to develop ‘roughs areas’ where horses regularly defecate because these can act as a reservoir for parasites.
4. Treat and move
Treating with anthelmintics and then moving the horse onto new, ‘clean’ grazing is no longer recommended for properties with reasonable management. The reason for this is due to wanting to protect refugia, the population of worms left untreated. If horses are moved to new grazing immediately after deworming treatment, the refugia is reduced, which increases selection pressure for anthelmintic resistance. Imagine a horse is treated with a wormer that kills all apart from a small number of worms which are resistant to that drug. The horse is then moved to clean pasture. The only worms now excreted onto that pasture will be resistant worms.
Read more on worming:
Worming Part 1: Evaluating the Small Redworm Blood Test
