Expression and detection of oestrus in cattle.

For herds using AI heat detection rate and calving rate are the two major determinants of compactness of calving, of the proportion of cows that fail to conceive in a defined breeding season. Numerous factors affect the expression of heat including, housing arrangement, floor surface, feet and leg problems and status of herd mates. The number of mounts a cow receives increases with the number of cows that are in heat simultaneously up to about 3-4 cows in heat. Generally, cows that are themselves in heat, coming into heat or were recently in heat are most likely to mount a cow that is in heat. Cows that are at the mid-stages of their cycles (day 5 to about day 16) are least likely to mount a cow that is in heat and consequently could be termed "poor heat detectors". Similarly, cows that are pregnant show less interest in mounting other cows that are in heat. In smaller herds and as more cows become pregnant the likelihood of more than one cow being heat on any given day becomes less, consequently, making heat detection more difficult. The single most important factor affecting heat detection efficiency is that those responsible for checking for heat should fully understand the signs of heat and be fully committed to heat detection for as long as it is planned to use Al. Technological aids to improve heat detection include the use of tail paint, oestrous synchronisation, vasectomised bulls, pressure activated heat mount detectors, radio telemetric devices, pressure sensitive mount count devices and pedometers. As herd size increases and labour become more expensive there will be a greater adoption of some of these technological aids.