Significance of pregnancy rates to successive services to assess the fertility pattern of individual dairy herds

There is considerable inter-farm variation in fertility rates in dairy cattle. The present study was carried out to develop a model to identify factor(s) affecting fertility taking into account intrinsic farm factors. Fifty five yearsʹ data, regarding pregnancy rates of cows conceiving in 11 dairy herds, were subjected to geometric distribution analysis. Based on the overall pregnancy rate for an individual farm, this allowed prediction of the expected pregnancy rate to each successive service on the assumption that there was an equal probability of conceiving to each service. Although the first service conception rate varied from 29 to 61% among herds, pregnancy rates for all the herds for a 50 year period (out of a total of 55 years) fitted the model, thereby suggesting that the herds were under optimum conditions of management for each herd. In five yearsʹ data from four herds not fitting the distribution, the main contributory factor for the poor fit of the model was second service pregnancy rate. To identify the associated risk factors for the non-fit years within herds, variables such as calving to first service interval, inter-service intervals and lactation yield, were compared between fit and non-fit years. Importantly, this made it possible to identify problems pertinent to fertility issues on individual farms, e.g. serving too early after calving, increased embryonic mortality after second service.