A method of detecting a first cow when mounted by a second cow. One or more beams of light are directed wholly or substantially horizontally at one or more detectors, and at a level above the first cow and corresponding to the additional height of the second cow when mounting the first cow, such that the mounting of the first cow by the second cow breaks one or more of the beams of light to one or more of the detectors and the one or each detected break causes activation of an alarm or a cow identification device or both. Mounting is an indication to the farmer that the cow is ready for insemination, and the invention provides an automatic method of alerting when a cow is in heat, or immediately identifying a cow in heat, which is only activated as and when the cow allows herself to be mounted. Preferably, two or more beams of light and two or more detectors are used, and the detectors work in partnership to coordinate two or more differently located cameras to the location of breakage of the light beams.