Summary

In the highly developed dairy production management, cows should get pregnant at an optimal interval after calving for efficient milk production. It is one of the top priorities for dairy herd management to maximize reproductive performance in high producing cows. In Japan, average milk yield per cow per lactation has been increased to 147 % from the milk yield 25 years ago to reach 8,600 kg in 2000 (The Ministry of Agriculture, Forestry and Fisheries of Japan). However, average conception rate at 1st AI has been declined from 47 % in 1996 to 43 % in 2000, and the interval from calving to pregnant has been increased to 155 days (Livestock Improvement Association of Japan, 1999). Expression of normal estrous signs, accurate detection of estrus and insemination at optimum time are keys to success in artificial insemination in cattle. Difficulty in detection of estrus and resultant low heat detection rate could be major causes of declining reproductive performance (Opsomer, 2002). It has been reported that duration of estrus in dairy cows has been substantially shortened, the average duration of estrus being less than 10 hours (Dransfield et al., 1998; X