RESUMEN
This study was carried out to investigate the relationship between early lactation curve parameters and calving interval in Holstein cows of Iran. In order to describe the lactation curve, the incomplete gamma function was fitted to 5,754,428 test-day milk records corresponding to 766,108 lactations on 315,634 cows in 2,448 herds. Cows with higher milk yield during early lactation had shorter calving intervals; however, longer calving intervals increased the 305-d milk production [P<0.01]. Cows with higher yield at the beginning of lactation, steeper ascending, and steeper descending slope had shorter calving intervals [P<0.01]. Calving interval was increased by 2.73 [+/- 0.12] d for every extra kg of milk at peak lactation [P<0.01]. The calving interval was directly impacted by the persistency of milk yield, but milk yield persistency was reduced in cows with shorter calving intervals [P<0.01]