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Dairy Pipeline: December 2003
Bennet Cassell
Extension, Dairy Scientist
Genetics and Management
(540) 231-4762
email: bcassell@vt.edu
This past August, USDA added three traits, daughter pregnancy rate, service sire calving difficulty, and daughter calving difficulty, to Net Merit. Only service sire calving difficulty had been available to producers before the August 2003 proofs. Here is some basic information about the new traits.
Daughter pregnancy rate or DPR is calculated from days open, a trait estimated by subtracting 280 days from any calving interval in the USDA files. Cows need two calving dates to have a calving interval, so cows all the way back to the 1960 origins of USDA files contribute to DPR evaluations. Proofs for days open are expressed as pregnancy rates, which measure the probability that a cow will become pregnant in a 21-day heat cycle. Bull proofs for DPR are not greatly different from bull to bull, as the standard deviation of true transmitting abilities for DPR is about 1.4%. DPR averages -0.2% for active AI bulls in the November 2003 proofs. Bulls with higher proofs, like +1.5 or +2.0 for DPR, are expected to sire daughters with greater fertility (higher pregnancy rates) than bulls with lower proofs. DPR gets a relative weight of +7% in Net Merit, and is expected to improve very slowly, about 1 % per decade, through selection on Net Merit. This is the first trait published by USDA to improve fertility through selection.
Service sire calving ease (SCE) and daughter calving ease (DCE) measure the ability of a first calf heifer to deliver the offspring of a bull (SCE) or the ability of a daughter of a to bull to give birth to her first calf (DCE). SCE has been used for years to choose "easy calving" bulls as mates for heifers. Daughters of "easy calving" bulls may not be the best at delivery of their own calves, however. DCE identifies bulls whose daughters give birth easily. Genetic evaluations for SCE or DCE are based on producer scores of 1 to 5 for increasingly difficult births, and the data are usually reported through DHI. Both traits get a negative emphasis of -2% in Net Merit, with expected declines in percent difficult births in heifers of -1.3% for SCE and -1.6% for DCE per decade through selection on Net Merit. DCE is the first trait available to producers from USDA to actually select against difficult births as a trait of the female.