Quantitative epidemiology of ram attrition in commercial sheep flocks

A series of studies of ram attrition, defined as the loss of breeding rams due to mortality and culling, in commercial sheep flocks in California are presented. Analytical methods emphasize Bayesian statistical inferences in the quantification of ram attrition. Results are derived from survival analyses using retrospective data from herd health records at the UC Davis Veterinary Medical Teaching Hospital, producer responses to a mailed questionnaire survey, and a prospective study of rams from seven commercial flocks in the greater Sacramento Delta region. Survival curves were estimated for rams from two commercial sheep operations in the Sacramento Valley, using complete interval- and right-censored failure-time data. Median time to attrition after entry into the ram flock differed by breed, with a median survival time of 1.30 years for Suffolk rams compared to 2.75 years for Rambouillet rams. A mailed questionnaire survey was used to estimate the annual incidence of ram attrition as a function of flock-level factors. Actively decreasing total flock-size during the three years prior to survey administration was found to be associated with increased incidence of ram attrition, whereas purchasing rams from official sales and maintaining rams on the same property as ewes year-round were factors associated with decreased incidence of ram attrition. In a prospective study of ram attrition (490 rams) among seven commercial sheep flocks in California, 32.7% of enrolled rams were observed to die during the study and 19.6% were culled. A direct, approximately linear relationship was identified between ram attrition during the breeding season and age at the start of the breeding season. Poor body condition at the start of the breeding season was associated with increased incidence of ram attrition during both the breeding and non-breeding seasons. Increased incidence of attrition during the non-breeding season was detected among rams with excessive body condition loss during the breeding season, as well as for rams with abscessed peripheral lymph nodes at the end of the breeding season. These studies demonstrate associations and quantify risk of ram attrition with variables including breed, flock population dynamics, purchase history, body condition score, age, and superficial abscesses.