Accuracy of recorded birth and calving dates of dairy cattle in the UnitedStates

Frequencies of births that were reported for specific days of the month wer e documented for US dairy cattle born since 1987 by birth year, herd size, and registry status and compared with calving frequencies for those dates. Because birth dates are expected to be random and uniformly distributed thr oughout each month, percentages of births on individual dates were expected to be equal (3.3% for d 1 to d 28, 3.2% for d 29, 3.0% for d 30, and 1.9% for d 31). However, percentages of reported birth dates for d 1, 2, 10, 15, and 20 were higher than expected. The percentage of reported births for d 1 was highest (5.3%) of all days of the month regardless of herd size or re gistry status. The nonuniform distribution of birth dates within month indi cated that a substantial number of birth dates were unknown and that estima ted birth dates had been reported. About one-third of the birth dates recor ded on d 1 appeared to have been estimated, or altered to gain an advantage in cattle shows. The highest frequencies for birth dates on d 1 (5.9 to 7. 4%) were found for registered cows during months that initiated age groupin gs for dairy shows (March, June, September, and December). Birth dates for some registered cows were intentionally misreported as confirmed by compari son of birth dates of individual cows with calving dates of their dams. Rep orted calving dates appeared to be more accurate than reported births; the inflated frequency of recorded calvings on d 1 was only about 30% as large as the inflated frequency of recorded births. Because cow age is determined by birth date, proper reporting of birth dates is important to ensure the accuracy of standardized yield and fitness records and the genetic evaluati ons that are based on those records. When animals' recorded birth dates and their dams' calving dates differ, more credence should be given to the lat ter to improve accuracy.