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Dairy Pipeline: April 2003
Bennet Cassell
Extension, Dairy Scientist
Genetics and Management
(540) 231-4762
email: bcassell@vt.edu
Dairy farmers can use two different measures of fertility to begin to offset a trend towards lower fertility seen in many dairy herds. The first of these, Estimated Relative Conception Rate or ERCR, is an evaluation of the fertility of semen from a bull compared to other bulls that breed substantial numbers of cows. ERCR values for the 257 Holstein bulls evaluated in February 2003 ranged from -8 to +5%. These numbers are based on 70-day non-return to service rates from first service, and bulls are required to have at least 300 services before evaluations are published. ERCR's do indicate the value of semen to a dairy herd. A Virginia Tech study by Drs. Pecsok and McGilliard published in the Journal of Dairy Science in 1994 showed that a premium of $2/unit of semen could be paid per one unit difference in ERCR score. For example, two bulls nearly equal for production, SCS, PL, etc. with ERCR's of +3 and 0 would differ by $6 per unit in value of the semen to a producer. Since bulls are very rarely "nearly equal" for anything, ERCR's could also be used to flag bulls with a history of conception problems under field conditions. ERCR's of -4 and below are most likely to cause problems, as they are in the bottom 2.5% of the 257 Holsteins on current active sire lists. The second fertility evaluation is brand new, and applies to daughters of bulls. It is called "daughter pregnancy rate" or DPR, and is based on the probability of a pregnancy in a 21-day cycle in daughters of a bull. Differences between bulls for DPR are smaller than differences in ERCR, as 21-day pregnancy rates are much lower than 70-day non-return rates. The average DPR on the 547 active Holstein bulls with evaluations available in February 2003 was -0.24. DPR's ranged from -4.1 to 2.6. Extremely good bulls (top 2.5% of Holsteins) for DPR would have evaluations above 1.8, while the poorest 2.5% would have evaluations below -2.3. DPR evaluations are based on days open, often (but not always) measured by difference in calving dates minus a standard gestation length. Heritability of DPR is 4%, so accuracy of evaluations will be low, especially for bulls with only first crop progeny available. DPR are useful, nonetheless. Dairy producers should begin to discriminate against bulls with low DPR values as reproductive performance of their future daughters is expected to be poorer than daughters of bulls with better DPR ratings. So - DPR is a sire selection tool, ERCR is a semen use or semen purchase tool, and both are valuable to dairy farmers. Producers may hear some criticism of both of these fertility evaluations, as fertility traits are lowly heritable and both ERCR and DPR will probably change for a bull as new information becomes available. The evaluation procedures will improve through more research, just as they have for production traits. However, as Dr. Phil Senger, former author of the AI column in Hoards Dairyman reminded me recently, "Sooner is a lot better than perfect." The bottom line, though, is that we can change fertility through selection if we are patient and persistent.