One of the natural ways to reduce and control the population of nuissance flies (e.g. house flies, horn flies) around your horses and stables is through 'parasite flies' or 'predator flies'.
The parasitic fly reproduces by laying its small eggs inside the eggs of larger nuissance flies. The parasitic fly eggs hatch, eat the nuissance fly egg, and then emerge as adult flies. The adult flies then repeat the cycle by searching out more nuissance fly eggs, making a small hole in them and laying their eggs in the eggs of the nuissance flies.
The benefit to horses and people of parasitic flies is that this reproductive cycle destroys the nuissance fly eggs, reducing the number of nuissance flies. As nuissance flies annoy horses (and people), this reduction in flies is beneficial. The predator flies are not themselves a problem as they are very small, do not bite or sting, and remain around the manure piles (looking for host eggs) rather than around horses or people.
Advantages and Disadvantages
The advantage of predator flies is that they reduce the population of nuisance flies. Furthermore, they do this in a completely natural way, without the use of chemicals or anything else which could be a health risk.
Unfortunately, nuissance flies breed very fast during warm weather and it is difficult for the natural population of parasitic flies to keep up. In order to minimise the population of nuissance flies, one needs to buy in additional parasitic flies (they normally arrive in egg format) and distribute them on the manure piles. Although there is a cost to this, in the right circumstances (see below) the cost of this natural solution can be much lower than the cost of alternative solutions (e.g. spraying insect repellent on the horses).
The main disadvantage of this solution is that it's effectiveness depends on the circumstances:
Treat all breeding areas. Nuissance flies can travel long distances, whereas parasite flies tend to remain in one location. If the only muck heap (or other nuissance fly breeding area) near your horses and stables is yours, this is not a problem. However, if you have neighbors with cattle and/or muck heaps near to your horses or stables, then treating your muck heap with parasite flies may be ineffective as you will still be invaded by the nuissance flies from your neighbors, unless they also treat their muck heaps.
Environment. Parasitic flies are living creatures and as such require a suitable environment. Weather extremes (e.g. a long and dry period) can substantially reduce their population, requiring that one purchase replacements, as the reduced population may be unable to keep up with the rapid reproduction rates of nuissance flies. To some extent, one can offset this by maintaining a suitable environment (e.g. lightly water the muck heaps during prolonged dry weather). Read and carefully follow the instructions provided by the supplier of your parasitic flies.
Temperature. Parasitic flies are active only in warm weather (e.g. above 60°F or 10°C). They can be filled by frost, so one should not distribute them when there is still risk of frost.
Timing. The nuissance flies start to breed at the start of warm weather and continue until it turns cold. If one starts distributing parasitic flies at the begining of the season (see above temperature requirements), before the nuissance fly population has had a chance to expand, one does not need to purchase as large a quantity of parasite flies to keep the nuissance fly population under control.
Adults unaffected. The parasitic fly kills the eggs of nuissance flies, but does not affect the adults. Consequently, it does not affect the current population but rather reduces the size of the next generation. One needs to allow a couple of weeks for the current adult population to start to die down before one sees benefits.
Chemicals. Nuissance flies have a greater resistance to pesticides and other chemicals than parasite flies. If your muck heap contains chemicals, even if it is just residual chemicals (e.g. on straw which was heavily sprayed by the farmer), this could kill off the parasite flies without affecting the nuissance flies.
Quantities. Most nuissance fly species have a short breeding cycle, and lay a large quantity of eggs each time. Consequently, as long as they have suitable conditions, they can reproduce very fast. If the population of parasitic flies is too small, one will not see a significant reduction in the nuissance fly population. Some people report that they need to use double the recommended quantity to achieve the desired results.
Not 100%. In nature, no predator will kill 100% of its prey population. Likewise, although parasitic flies may kill of the vast majority of nuissance flies, they will not exterminate them. Consequently, one either needs to live with the remaining population of nuissance flies, or complement this approach with other solutions.
Not all flies. Parasitic flies will lay their eggs in the eggs of nuissance flies, but only certain species, so other species of nuissance flies will be unaffected. Depending on which types of flies you are having problems with, this may or may not be an issue.
Experiences with the use of predator flies to control the populations of nuissance flies are mixed. Some people have had great success with them, while others find them of little or no benefit. It would appear likely that the failures are in most cases due to issues with the above requirements.
Proper management of manure can reduce the ability of nuissance flies to breed and thereby reduce the amount of parasitic flies required. Regular dragging of pasture areas to break up manure piles will reduce the breeding areas for nuissance flies. Likewise, spreading your manure out to dry, or composting it, can likewise reduce the breeding areas for nuissance flies.