An average 1000-pound horse (Jubal, here, is 1100) produces approximately 50 pounds of manure a day. (That's over nine tons in one year!) Multiply that by the number of horses in a particular enclosure and the numbers can be staggering. Now add the fact that many horses are kept at stocking rates higher than one on every two acres (recommended by the Virginia Cooperative Extension,) especially in urban and suburban areas, or on farms such as boarding facilities, and it becomes more obvious why dragging or harrowing manure in the field may not be enough.
Parasite control is the most often cited reason for keeping fields clean. Horses grazing in areas where manure has been allowed to lay are likely to ingest parasites such as stongyles or bots. External parasites find manure a very attractive breeding ground as well. One female stable fly can lay 20 batches of eggs in one month (that was just one fly, remember.)
Loafing areas, in the shade in the summer or sheltered from wind in the winter, often have more manure present than other parts of paddocks or fields. In warm weather, this attracts more flies to the places in which the horses are trying to find shelter from uncomfortable conditions. In any weather, standing in manure for days at a time encourages foot infections.
Grazing itself is affected by manure left in the field. Horses, unlike most other types of livestock, often choose specific areas of the field as manure "repositories." Usually, soiled areas are then rejected as grazing sites. The grass around the manure then grows to a stage most horses consider unpalatable, while cleaner sections of the field become overgrazed. Meanwhile, the overgrazed portions become home to more and more weeds, making them less desirable for grazing too. In addition, in the heat of summer a hefty "dropping" can turn the grass under it brown in 24-48 hours, reducing grazing even further. Depending on stocking rates, these pastures can have as much as half of the area standing unused due to manure build-up.
So, while some folks in the neighborhood smile to themselves when they see us obsessively shoveling manure every day, we smile too, knowing our residents are the better for it.
More information on manure management:
Manure Management by Cherry Hill
NOTE: Material presented by Traveller's Rest Equine Elders Sanctuary on equineelders.org or in any other manner is for information purposes only. It is in no way intended to replace the services or advice of your veterinarian.