Factors associated with fertility of nulliparous dairy heifers following a 10‐day fixed‐time artificial insemination program with sex‐sorted and conventional semen

Abstract

To examine factors associated with fertility on dairy farms that used a common fixed-time artificial insemination (FTAI) program in yearling heifers. Records were analysed from 954 yearling heifers on 10 south-west Victorian dairy farms that used a common FTAI program, involving the insertion of a 1.9-g progesterone-releasing device for 10 days; 2 mg oestradiol benzoate at insertion; 500 µg cloprostenol on day 7; and FTAI 48 h after device removal. Weight, age, expression of oestrus, sire, semen type (frozen sex-sorted or frozen conventional) and timing of insemination were examined for their relationship with first-service conception rates. Heifers over 300 kg body weight were 1.18-fold more likely to express oestrus during the FTAI program. For every extra 1 kg, there was a 1.5% increase in the likelihood of expressing oestrus. First-service conception rates were 40.3% and 56.0% for sex-sorted and conventional semen, respectively, and were significantly higher when oestrus was expressed. The difference was greater for sex-sorted semen (3.4-fold) compared with conventional semen (1.5-fold). The interval from device removal to insemination varied between 47 and 51.4 h and had no significant effect on conception rates. However, there was a trend towards a higher conception rate for sex-sorted semen when inseminations were performed >50 h after device removal. Increased fertility was associated with larger heifers and heifers that expressed oestrus, particularly when sexed-sorted semen was used. Variation in the timing of AI with respect to device removal between 47 and 51.4 h did not adversely affect conception rates.