Abstract: This study aimed to analyze the effects of age at first pregnancy (AFP) on milk yield and reproductive performance of cows. In this study, production data of 13 927 Holstein dairy cows from two large-scale dairy cow herds (A herd 8 091,B herd 5 836) in northern China were collected. The age at first pregnancy (AFP), milk production, post-partum first estrus time during the first 2 lactations, first 2 mating time and first conception time during the first 2 lactations were detected. The cows were divided into 8 groups according to the AFP(12-19-month-old).The milk yield and main reproductive performance during the first 2 lactation of cows in different groups were compared and analyzed. The results showed that:1) The AFP of Holstein heifers in two large-scale farms were mainly in 13- and 14-month-old (accounting for 70.1% of the total); 2) AFP could significantly affect the 305 d milk production of Holstein dairy cows in the first 2 lactations (P<0.05), the highest 305 d milk production for the first 2 lactation (15 102 kg vs 15 534 kg) was obtained on 14-month-old for AFP in A herd; 3) AFP could significantly affect the post-partum first estrus time and first conception time of Holstein dairy cows in the first 2 lactation (P<0.05), the optimal post-partum first estrus time and first conception time for the first lactation was obtained on 14-month-old for AFP; 4)The post-partum first mating time of cows with AFP at 16-month-old in A farm was significantly higher than that of cows with AFP at 17-month-old for the first lactation (P<0.05), there was no significant difference among the other groups (P ≥ 0.05), and the difference in mating time among the groups was less than 5 days; 5) Based on the analysis of the AFP records of the cattle in the farm (2 703 heads) and the culling cattle (660 heads) in A farm, it was found that the AFP of the cattle in the farm was 14.52-month-old, which is significantly higher than the AFP of the culling cattle (P<0.05). In the northern region, under the current production management level, 14-month-old may be the most suitable age at first pregnancy for Holstein heifers, which has certain reference value for the age at first pregnancy for Holstein heifers in large-scale dairy farms in China.

Key words: Holstein heifers, age at first pregnancy, milk production, reproductive performance