Most life cycles of equine parasites involve a period of development outside the horse and on the pasture; with eggs produced by the adult worms in the horse's gut being passed out in its dung. These eggs then develop into infective larvae on the pasture.
A single horse is capable of passing tens of thousands of small redworm eggs each day. Because a horse is mainly infected by ingesting infective larvae as it grazes, reducing its exposure to infective larvae on the pasture is paramount.
Horses' grazing behaviour
The way horses are kept dictate their feeding habits. Horses are fastidious feeders and prefer to eat young, immature plants and will graze some areas of a pasture down to the bare ground. In other parts of the pasture, plants are avoided and allowed to grow to maturity, which lessens palatability and nutrient availability. In addition, horses will not graze around droppings, so pasture plants around dung are also mature and less palatable.
A typical horse, weighing 450kg produces five to 12 pats or about 24kg of dung a day, that's 10 tonnes a year. Up to 50pc of grazing can be lost due to fouled areas, resulting in the characteristic "lawns" and "roughs" as well as an increase in weed infestation.
In the wild, horses are free to graze over very large areas so they can easily avoid eating from pasture that has been contaminated with potentially infective droppings. For domestic horses, however, grazing is often limited, thereby increasing their exposure to infection and action is required to reduce the number of infective larvae on the pasture. This reduction of pasture contamination is achieved by a combination of worming and pasture management.
Pasture management involves a range of actions, including:
Not over stocking pastures, ie no more than one or two horses per acre, as horses lower down the pecking order will be forced to graze the rough pasture where worm burdens will be higher;
Grazing horses alongside sheep, goat or cattle since parasites that affect horses are host specific. Any larvae eaten by other species are destroyed, reducing worm larval contamination of the pasture. Sheep and cattle will also help to improve pasture quality by eating the rough grass rejected by horses;
Dividing paddocks into smaller areas so they can be alternatively grazed and rested to reduce the pressure on the pasture and make it easier to remove droppings
Resting pasture for at least five months, although a good idea is not always an option. Besides, worm larvae can live for many years both on pasture and in horses, so simply resting pasture does not guarantee it will be worm free;
Worming horses 48-72 hours (product choice affects this timing) before moving to new pasture;
Not turning out young stock onto small turn out paddocks as pasture will develop extremely high larval counts particularly if droppings are not removed each day;
Avoiding grazing foals alongside older horses, as foals are a major source of pasture contamination;
Only harrow in very dry conditions, as in damp conditions harrowing simply spreads worm eggs and larvae over the pasture;
Regularly removing dung at least twice weekly during the grazing season, and once a week between November and March. (further research into this issue is on going and perhaps this advice may change one day)