Effect of parity, season and FSH treatment on the calving interval of Ayrshire cows in the tropics.

A total of 607 calving intervals on 99 Ayrshire cows with parities from 1 to 10 and belonging to one dairy herd at Iringa in the southern highlands of Tanzania were analysed to study the effect of parity and season or month of calving on the subsequent calving interval. In addition, 22 calving intervals incorporating a superovulatory cycle were compared with those of 52 controls that had calved around the same period. The average (mean +/- SEM) age at first calving was 985 +/- 16 days. The effect of parity on calving interval was highly significant (p < 0.001). Cows that calved in December, at the beginning of the rainy season, exhibited the shortest calving interval (404 +/- 13), while those that calved in September/October, in the dry season, exhibited the longest average calving interval (466 +/- 20 days), although this difference was not significant. Prior treatment with follicle-stimulating hormone to induce superovulation, significantly lengthened the average calving interval (579 +/- 28 vs 457 +/- 15 days; p < 0.001). It was concluded that both parity and FSH treatment affected the calving interval. However, season (rainy vs dry) or month of calving did not influence this interval.