Impact of selection for increased daughter fertility on productive life and culling for reproduction
H. D. Norman, J. R. Wright*, R. H. Miller; Animal Improvement Programs Laboratory, ARS, USDA, Beltsville, MD, USA

Selection for increased daughter pregnancy rate (DPR) over 2 generations was examined to determine if such selection had affected cow fertility and productive life (PL). Holstein artificial-insemination bulls with a predicted transmitting ability (PTA) for DPR based on ≥35 daughters were grouped by quintile based on PTA DPR. Then 25 cow groups were formed based on sire and maternal grandsire (MGS) quintiles. Cows had birth dates from 1988 through 1999 and calving dates from 1990 through 2005. Cows that changed herds or had unreported lactations for their first 5 parities were excluded as were herds with <10 cows. Data were available from 4,380,300 cows in 31,759 herds. Mean cow PL was 27.0 mo; time opportunity was a restricting factor. Mean PTA DPR was 2.0 for sires and 2.1% for MGS for the cow group with highest sire and MGS quintiles and –2.1% for both sires and MGS for the group with lowest sire and MGS quintiles. Least squares difference in PL was examined on a within-herd basis with cow birth year in the model. Cows from the highest sire-MGS quintile group had 4.2 mo longer PL than those from the lowest sire-MGS quintile group and were less likely to be culled for reproductive problems (10.0 versus 13.3%) based on reported reason for record termination. Difference in PL between cow groups with highest and lowest sire quintiles for PTA DPR ranged from 2.8 to 3.3 mo; corresponding difference for MGS quintiles ranged from 0.8 to 1.4 mo. Because each month of additional PTA PL is valued at $29 in the current USDA lifetime net merit index, a 200-cow herd from the highest sire-MGS group for PTA DPR would be worth about $7,500 more annually than a 200-cow herd from the lowest sire-MGS group without considering any additional income or expense associated directly with DPR. Selection for increased DPR across generations is expected to produce cows with longer herd life because they are less likely to be culled for reproductive problems.

KEYWORDS
daughter pregnancy rate
productive life
fertility