Factors affecting the reproductive efficiency of artificial insemination programmes in a seasonal breeding pasture-based dairy system with the aid of milk progesterone.

To determine the factors affecting reproductive efficiency in a seasonal breeding artificial insemination (AI) programme in dairy herds in Uruguay, a field trial was conducted on three dairy farms. Lactating cows (n=328) intended to be bred at the following breeding period were selected. The trial started 1 week before the beginning of the breeding period and lasted 80 days or until first service. Milk samples were obtained biweekly for 12 weeks, starting 1 week prior to the breeding season to determine progesterone concentrations. Data indicated that 12.5% of the cows were anoestrus at the beginning of the season remaining in this condition throughout the trial. Most of the anoestrous animals were first-calf heifers. A varying number of cycling cows (8.5%) at each farm were never reported in heat. Oestrous detection rate for cows cycling, as determined by progesterone patterns, was evaluated in three periods of 21 days and resulted in an overall efficiency of 46.9%. Main factors affecting oestrous detection were farm, with an effect of parity (67.8%, mature cows and 33.2%, first-calf heifers) and no effect of days postpartum. Mean interval from the beginning of the breeding season to first breeding was 27.4 days, which was also affected by farm. Neither parity nor days postpartum affected this parameter.