Trends in the reproductive performance of Holstein dairy cows in Iran.

The study was undertaken to determine the trends in the reproductive performance of Holstein dairy cows in Iran during 1994 to 2008. Reproductive performance data for 528,034 lactations of 246,132 cows in 1,822 Holstein dairy herds of Iran were used. The potential effect of calving season, herd, parity, calving year, as well as herd size and 305-day milk production on reproductive performance traits was investigated using multiple regression models. The least squares means of age at first calving decreased by 3.1 (± 0.06) days per year from 806.5 (± 96.3) days in 1994 to 788 (± 89.9) days in 2008. The least squares means of calving interval increased 1.02 (± 0.03) days per year from 394.1 (± 65) days in 1994 to 413.2 (± 81) days in 2008. Greater 305-day milk production was associated with an average increase of 6.55 (± 0.08) days in calving interval per 1,000-kg increase in milk yield. Larger herd size was associated with an average decrease of 0.22 (± 0.02) days in calving interval per 50 cows per herd. The mean number of days dry was 88.6 (± 51.3) days and increased by 0.82 (± 0.02) days per year. In conclusion, reproductive performance in Holstein dairy herds has generally decreased, whereas herd size and milk production have increased over time. Producers may make significant improvements in herd reproduction by reviewing management strategies including the sire selection, reproductive management, inseminator training and techniques, and improved estrous detection. Moreover, it may be advisable for the fertility traits to be included in the genetic selection indices to reduce the rate of reproductive decline.