When marking treated cows, a very reliable system needs to be used. One of the best places to mark is right on the udder, both sides, or rear. The new FIL Udder Mark is non-irritant and will stay visible for 8 days. It will always be very visible for the duration of the withholding period. We suggest using a red mark when a cow is first treated, then spray over with green when she is clear. No-block nozzle will spray at any angle. Net contents 500ml.