The Effect of Time and Other Factors on the Non-Return to Service Estimate of Fertility Level in Artificial Insemination of Cattle

Abstract

For the successful operation of an artificial breeding unit, reliable estimates of fertility are essential. The dairyman is primarily interested in the number of services per live calf born. As a practical estimate Of the efficiency of the technique of artificial insemination, this measure of fertility is impossible to obtain for a large population. Therefore, quicker and easier methods are re- quired. Manual pregnancy diagnosis and the non-return to service estimate are the two most logical methods available at the present time. While pregnancy diagnosis might appear to be the more useful estimate of fertility, in reality it contains most of the errors inherent in any estimate of fertility made before calving and is more difficult to obtain routinely. Most artificial breeding organizations have come to rely on a calculation from the insemination records to arrive at estimates of the efficiency of their opera- tions. The method, universally known as the non-returns to service estimate of fertility, has been described earlier (8). Most research workers have been forced by circumstances to adopt the same method for determining the effect of varying semen treatments or other factors on fertility in artificial insemination. Among the obvious drawbacks of the method are that some cows fail to con- ceive from insemination and may be disposed of before the next estrus, some may be bred naturally at the next estrus, while in others estrus may not be detected. All would be considered as pregnant and would make the estimate of the concep- tion rate higher than the true figure. Casida et al. (3) and Barrett et al. (1) found that 4 mo. after service the non-return to service calculation overestimated the level of conception actually found by pregnancy diagnoses made from 35 to 49 days after insemination. Elliott (5) checked 219 cows directly on farms and found that 68 per cent had not returned to estrus in 1 full month following the month in which the cows were inseminated and 53 per cent had not returned within 5 mo. Of these 219 cows, only 49 per cent were diagnosed as pregnant or produced a living calf, which is 19 percentage units less than the 1-mo. non-returns and 4 percentage units less than the 5-mo. non-returns. However, all biases in non-return calcu- lations do not result in overestimation of conception. Studies (2, 4) indicate that about 3.5 per cent of pregnant cows exhibit typical signs of estrus. Such cows might be reinseminated and thus might not be recorded as pregnant from