Effect of heat stress on conception in a dairy herd model under South African conditions.

Three regression models are proposed for predicting reproduction in a model dairy herd under South African conditions. Conception rate (CR%) was related to mean monthly temperature-humidity index (THI) by; CR% = 31.15 THI - 0.25 THI(2) - 890.2, and first service conception rate (FSCR%) to THI by; FSCR% = 173.45 - 1.79 THI. Conception rate was related to numerical month of the year (M) by; CR% = 11.86 M - 0.82 M(2) + 26.36. The relation between mean monthly THI values and the conception rate of dairy cattle is significant. Further investigations to test the proposed regression models under various dairy herd conditions and to improve reproduction in South African dairy herds are needed.