Theoretical Reproductive Performance in a 120 Cow Dairy Herd With 50% Estrus Detection

 AI service 1 2 3 4 5 6 Days postpartum 1 55 - 76 77 - 98 99 - 120 121 - 142 143 - 164 > 164 Total open cows 120 84 58 41 28 20 Normal cows (P = 0.9)2 108 75 52 36 25 18 Detected-mated (P = 0.50)3 54 38 26 18 13 9 Calving, this AI (P = 0.675)4 36 25 18 12 9 6 Cumulative pregnant cows 36 62 79 92 100 106 Days Open 5 2387 2221 1935 1618 1315 1044 Cumulative Days Open 2387 4608 6543 8161 9476 10519

1 Cows mated after a voluntary waiting period of 54 days.
2 Assume 10% of cows are abnormal due to reproductive pathology/acyclicity.
3 Assume that 50% of the normal, cycling cows are detected and mated at each successive estrus.
4 Cows that successfully conceive, complete gestation and calve normally from each insemination. The probability is a combination of 90% chance of conception and 75% chance of successful gestation/parturition. (P = 0.9 x 0.75 = 0.675)
5 Number of cows calving x median days postpartum for this insemination.

Mean Days Open (pregnant cows) = 10519/106 = 99.2

But six cows were still open after six inseminations and should be culled for reproductive failure. If these animals remained in the herd for a full year, this would add 14 x 365 = 5110 days open to the total.

Mean Days Open (all cows) = 10519 + 5110/120 = 130.2

Services per Pregnancy (106 cows) = 2.54