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Associations Between Service Interval, Interval from First Service to Conception, Number of Services per Conception, and Level of Butterfat Production

Department of Dairy Science, University of Illinois, Urbana

K. Rottensten and H. Andersen

Institute of Artificial Insemination, Royal College of Agriculture and Veterinary Medicine, Copenhagen, Denmark

ABSTRACT

The interval from first service to conception decreases as the service interval increases from 0 to 127 days, at which it reaches a minimum. After a service interval of 127 days, the interval from first service to conception increases slightly as the service interval increases. The length of the service interval accounts for only 1.4% of the variance of the interval from first service to conception; thus, even though its effect on the interval from first service to conception is significant, its effect is small and of little importance.

Service interval does not significantly affect the number of services per conception and accounts for only 0.3% of the variance of services per conception. In general, the number of services per conception (Y) decreases linearly as the service interval (X) increases, the partial regression of Y on X for a constant butterfat production being –0.0034 ± 0.0023.

The partial regressions of the number of services per conception and the interval from first service to conception on butterfat production for a constant service interval are +0.003 ± 0.001 and +0.091 ± 0.046, respectively, and are significant at the 0.05 level of probability. The partial regression of the number of services per conception on butterfat production for a constant service interval and constant interval from first service to conception is –0.00004 ± 0.0007 and is not significant. It is concluded that there is no real biological relationship between services per conception and level of butterfat production.

The service interval alone accounts for 16.8% of the variance of the interval from calving to conception. The interval from calving to conception increases at an increasing rate as service interval increases to approximately 50 days. After a service interval of 50 days, the interval from calving to conception increases almost linearly and in an approximate 1 to 1 ratio as the service interval increases.

Calving interval has approximately the same relationship to service interval as has the interval from first service to conception. With the present data, the average service interval must be from 36 to 49 days to maintain an average calving interval of approximately 365 days. If the conception rate was increased to 60%, the average service interval allowed could be increased approximately 11.5 days, and a calving interval of approximately 365 days could still be maintained.