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Estimate Relative Conception Rate of AI Sires

Dairy Pipeline: September 1999

Ray L. Nebel
Extension Dairy Scientist, Reproduction
Virginia Tech
(540) 231-4432

What is the range in fertility of sires available from AI organizations? Estimated relative conception rate, or ERCR, is a measure of conception rate of a service sire relative to service sires of herdmates and are calculated by the DHI processing center DRMS in Raleigh, N.C. These conception rate summaries are available from the DRMS web site at and are updated twice yearly. Only first services are used in an Animal Model program similar to the one used to compute production PTAs. Estimates are adjusted for the effects of each herdmate's early lactation energy-corrected-milk production, lactation number, stage of lactation, month of breeding, herd and year. ERCRs are calculated from the AI services of Holstein and Jersey bulls in DHI herds of all states that report to DRMS with the exception of Puerto Rico. Services from AgSource in Wisconsin and Pennsylvania DHIA are also included. Services reported in the most recent three years are used. ERCRs are expressed as a percent difference of conception rate from the average AI service sire of herdmates. For example, in the results from 1995-97, bulls ranged from -19% to +11%. This means that the average service to the most fertile bull produced a calf 11% more often than a service to the average AI service sire of herdmates. Similarly, a service to the least fertile bull produced a calf 19% less often. The average value was 0 and there were approximately equal numbers of positive and negative values. Although the values in 1995-97 had a range of 30 percentage points, 65% of the bulls were in the range of -3% to +3%. Bulls within this range are of acceptable fertility and most selection emphasis within this group should not be for fertility. Printed with ERCR are estimations of repeatability and the number of services that were used in the calculations. Just as reliability is used as an indication of accuracy for PTA milk, so should repeatability and number of services be used when judging the accuracy of ERCR. Repeatability ranges from 20 to 99 and a repeatability of 99 indicates the most accurate ERCR. ERCRs are available for only the service sires whose ERCR has a minimum of 50 services (repeatability = 20) in the most recent 3 years. Approximately 59% of the active Holstein and 63% of the active Jersey bulls had enough services to be included in the May 1999 ERCR calculations.

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