EVENT-TIME ANALYSIS OF THE EFFECT OF SEASON OF PARTURITION, PARITY, AND CONCURRENT DISEASE ON PARTURITION-TO-CONCEPTION INTERVAL IN DAIRY-COWS

Objective-To quantify the effect of season of parturition, parity, and various diseases on time to conception. Design-Event-time analysis (s urvival analysis). Sample Population-44,450 cows delivering calves bet ween September 1985 and September 1986, from 6,227 Ayrshire dairy herd s in 80 Finnish communities. Procedure-Primiparous and multiparous cow s were analyzed in separate Cox proportional hazards models for postpa rtum days 56 to 120. Occurrence of 43 diseases or disorders was record ed; codes were condensed into 25 variables offered to each model. Seas on of parturition was categorized into spring, summer, and fall-winter ; for multiparous cows, 3 categorical variables represented parity. Mo dels controlled for 60-day milk and milk fat production, herd milk pro duction, and community. For all observations, parturition-to-conceptio n interval was plotted against calendar day of parturition. Results-Fo r multiparous cows, parturition in the spring or summer and being of p arity 2 or 3-4 (vs older) increased the chance of conceiving; 10 disea ses or disorders decreased this probability. Similarly, in the model f or primiparae, parturition in spring or summer increased the probabili ty of conception, and 6 disorders decreased it. Disorders that were de trimental in both models were anestrus, ovulatory dysfunction, other i nfertility, late metritis, and clinical ketosis. Conclusions-The effec t of season on the parturition-to-conception interval was marked at th is latitude. Parturition during late April, causing confluence of post -partum day 55 with the summer solstice, was associated with the short est parturition-to-conception intervals. Ketosis was found to be an im portant factor in lengthening the parturition-to-conception interval, highlighting the effect of negative energy balance on post-partum rest oration of reproductive function.