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Analysis of Seasonal Fluctuations of Reproductive Performance in Dairy Cows¹

Department of Animal Science, North Carolina State College, Raleigh

ABSTRACT

An analysis of the effect of month of calving on length of subsequent calving interval was conducted, using 2,514 records for a 10-yr period from six Holstein herds of the North Carolina Institutional Breeding Program. The average calving interval ranged from 397 to 422 days for calvings in October to May, respectively. Differences in this interval were highly significant (P < 0.01) for both among months in the same year and herd, and among herds, but not different among years for each herd. Monthly fluctuations in calving interval and air temperatures prevailing during 60 to 120 days post-partum were significantly correlated on a within-herd basis (0.30).

A more detailed study was conducted with 391 records from three herds. Results showed that days open are associated with variations in three factors: interval from 70 days postpartum to subsequent estrus, length of estrous cycle preceding successful mating, and services per successful mating. Days open was correlated with each of these independent variables (0.18, 0.81, and 0.26, respectively). On a within herd-year-month and among herd-year-month basis, 76 and 90%, respectively, of the total variation in days open was attributable to the multiple regression of days open on the above independent variables. Following adjustment for all three of these variables, by analysis of multiple covariance, among year-month differences within herds in days open were not significant (P > 0.05). It is suggested that part of the variation is caused by nonphysiological factors (e.g., management), as well as season-herd influences. The large percentages of estrous cycles ending in successful mating, which were of multiple integrals of normal length (17–24 days), also suggest the existence of an estrus-detection problem which varied in intensity among the three herds studied.

¹ Published with the approval of the Director of Research, North Carolina Agricultural Experiment Station, Raleigh, as Paper No. 1377 of the Journal Series.

² Present address: U. S. Department of Interior, Cortland, New York.