Impact of age at first calving on performance traits in Irish beef herds.

Reducing age at first calving (AFC) has been a challenge in beef herds. There is anecdotal evidence that herd owners choose to calve heifers older because of the perceived consequences of calving heifers at 24 mo of age compared to 36 mo on performance traits in beef herds. The objective of this study was to estimate the association of calving heifers at younger ages on subsequent performance traits, calving interval, longevity, cow weight, dystocia, and progeny weaning weight for parities 1 to 5. Available to the study after data edits were 219,818 calving interval records, 219,818 longevity records, 118,504 cow live-weight records, 230,998 dystocia records, and 230,998 weaning weight records. Linear mixed models were used to quantify performance of each trait in AFC groups for each parity. As parity increased, there was a favorable reduction in calving interval and dystocia (P < 0.001), while the likelihood of cows surviving reduced (P < 0.001). Both cow live weight and progeny weaning weight increased as parity increased. Age at first calving only had a significant association with dystocia within parity 1 (P < 0.001), where older heifers at first calving subsequently had lower risk of calving. Calving interval for parity 1 cows was observed to be longer by 6 d in cows that calved for the first time at 33 to 36 mo compared to cows calved for the first time at 22 to 24 mo (P < 0.001). No statistical difference was observed for longevity between cows with an AFC of 22 to 24 mo compared to cows with an AFC of 33 to 36 mo (P > 0.05). Cows that calved at a younger age did wean lighter calves for their first three lactations (P < 0.01) but had no association with weaning weight for parity 4 and 5 cows (P > 0.05). Cows with a lower AFC were lighter for parity 1 to 4 (P < 0.001); at parity 5, AFC had no association with cow live weight (P > 0.05). The performance of mature cows for calving interval, longevity, calving difficulty, cow live weight, and weaning weight was not impacted by AFC. In conclusion, calving cows for the first time at younger ages do pose risks and associated performance loss but this risk and loss should be minimized by good management.