Fly Sheets: The One Feature That Decides Whether It Works
Mesh, coverage and UV rating all matter. None of them matter at all if the sheet does not stay where it was put.
Fly sheets are bought on features and abandoned on fit. Almost every unused fly sheet in every tack room was rejected for the same reason, and it is not the mesh.
Fit is the deciding feature
A fly sheet is worn in hot weather, over a horse that is moving, rolling and rubbing, for a large part of the day. Anything that shifts will rub, and a rub in fly season on a horse that cannot stop bothering the spot becomes a sore very quickly.
The failure sequence is predictable. The sheet slips. It rubs the shoulder or the wither. The rub gets worse because the horse is moving all day. The sheet comes off permanently and joins the pile.
Which is why the fit questions come first. Does it sit clear of the wither. Is there enough room in the shoulder for a full stride. Does the drop cover the horse without hanging so low it catches. Are the straps adjustable enough to stop it swinging without being pulled tight.
A sheet that rubs is worse than no sheet, because you end up with the flies and a sore as well.
Then the mesh
Weave matters, and it matters in a specific way. Different biting insects are different sizes, and a mesh that stops one may not stop another. The smaller biting midges that cause the most misery for some horses go through weaves that stop larger flies without difficulty.
The practical implication is that the right weave depends on what is actually biting your horse in your area, which is a local question rather than a specification question.
Heavier weaves are more durable and less breathable. Finer weaves stop more and tear more easily. There is no version that is best at everything.
Coverage, and its trade offs
Belly panels, neck covers and full body designs all extend protection into the areas horses are most often bitten and least able to defend.
They also add places to rub and heat to trap, which brings the argument back to fit. Extra coverage on a sheet that fits well is a genuine improvement. Extra coverage on a sheet that moves is more surface area to cause trouble.
What a fly sheet cannot do
It reduces the problem. It does not solve it, and treating it as a complete answer is where people end up disappointed.
The measures that address the source still matter: managing muck heaps and standing water where insects breed, timing turnout to avoid the worst hours where the horse's routine allows it, and shade the horse can actually use.
A sheet that fits, combined with sensible management of the environment, is a considerably better outcome than the most featured sheet on the shelf worn by a horse that keeps rubbing it off.