A method, system and apparatus for detecting when an animal is in heat and to a method of alerting a responsible individual that the animal is in heat. The method comprises the steps of, using a sensor 5 mounted on a bull, monitoring the activity of the bull in the proximity of the animal. Thereafter, the activity of the bull is analysed before finally it is determined, based on the analysed activity of the bull, whether or not the animal is in heat. The invention shifts the focus from the animal, such as a cow, to the bull and leans on the natural instincts of the bull to determine when the cow is in heat. The sensor preferably forms part of a collar 3. A proximity sensor target 13 is preferably mounted on the target animal 15.
