MANAGEMENT Fly spray savvy

Q:After I ride, I often hose off my horse, particularly on hot days. Then I apply fly spray before turning him out. But I always do my best to dry him off before spraying on the fly repellent. Is this necessary?

If I simply apply fly spray to my horse when his coat is wet, what would happen to the active ingredient as his coat dries? Would it dissipate so much it would be less effective? Or would the chemical become more concentrated, possibly becoming harmful? I wonder about this every summer.

Name withheld by request

A:You need not worry about applying fly spray when your horse’s coat is wet or damp, as long as it’s not dripping wet. The recommended amount of pyrethrin to be applied is determined by the concentration of the product being sprayed. Therefore, the amount of active ingredient applied to a horse will be the same regardless of whether his coat is wet or dry during application. Once his coat has dried, the same amount of the active ingredient is spread over the area sprayed. A wet or dry coat will have no impact on the safety profile of the product when applied according to label directions.

To affect the efficacy of a product, dilution would need to occur during the mixing of the solution before it is applied. The amount of active ingredient per area is ultimately what determines efficacy. Once you are applying the product to the horse using the sprayer, the amount of active ingredient per unit area will not change even if a horse’s coat is wet. The only potential for re

duced efficacy is if the animal is dripping wet and product is carried off with the excess water. Brice Besser Chemistry Director, MGK Minneapolis, Minnesota