Abstract: A substance for facilitating the detection of estrus applied to the tailhead and rump of a cow as a patch comprises a mass of microencapsulated dye mixed in a suitable paint. The paint dries as a durable layer the color of which will change when substantial pressure is applied for a predetermined time. Should the cow be mounted by another animal for say five seconds or more a large portion of the microcapsules will break and release the dye which colors the patch. The cows with colored patches can then be found by periodic inspection and removed from the herd for insemination.