Interestrous interval profiles of 71 Wisconsin dairy herds and relationship to days to pregnancy

Abstract

Interestrous interval profiles were examined for 71 dairy herds. The median value of the day 21:42 day interval ratio was 1.3, only one herd reached the goal of 6:1 and six herds were better than the action level of 4:1. When the intervals of greater than 48 days were added to 42-day intervals, the median value of this ratio was 0.5. The percentage of interestrous intervals in any category (i.e. 1–3 days, 4–17 days, 18–24 days, 25–35 days, 36–48 days and more than 48 days long) had little correlation with other indices of reproductive performance. The percentage of abnormal intervals (i.e. 4–17 or 25–35 days) had little correlation with herd pregnancy rates. The examination of interestrous intervals had limited value when evaluating reproductive performance of these herds. The low correlations between abnormal intervals and pregnancy rates indicate that evaluations of interestrous intervals are not a good indicator of early embryonic death. However, the low estrus detection rate observed (36%) could obscure increases in early embryonic death. Stepwise multiple regression was used to model the relationship of indices of reproductive performance (days to first service, pregnancy rates, interestrous intervals and Barr's average interestrous interval) to days to pregnancy (DTP). The best model was: DTP = 9.11 + 0.81 × days to first service + 0.60 × Barr's average interestrous interval ( R 2 = 0.97) Unexpectedly, if average pregnancy rate was included in the model, the sign of the coefficient was positive, indicating that days to pregnancy would increase if average pregnancy rate increased. This was the result of the high degree of correlation (Spearman's r+0.83) between average interestrous interval and average pregnancy rates. Dairy herds with low estrus detection efficiency can be expected to have low average pregnancy rates.