Assessment of age at first parturition by accounting censored data: the example of small ruminants in agropastoral herds in Senegal

Age at first parturition (AFP) is an important reproductive trait when evaluating the herd productivity in tropical small-holder farming systems, particularly for small ruminants with high demographic turnover and commercial utilization. The article reports the AFP probability distributions calculated on goats and sheep herds monitored for more than 10 years in three low-input livestock farming systems located from northern to southern Senegal (Louga, Kaymor, and Kolda) and representing different agroecological conditions and animal phenotypes. The AFP distributions were estimated with the Kaplan-Meier (KM) survival analysis accounting for censoring (females lost to follow-up before their first parturition). When measured by the KM estimator of the AFP median, the precocity for both species was almost 4 months later in Louga than in Kaymor and Kolda. In Louga, the precocity was 1.6 months later for sheep than for goats and 2.9 months in Kaymor and Kolda. These estimates were higher than those obtained with the common and crude approach that removes censored data before calculation (relative bias ranged from -12% to -22% for the AFP mean depending on species and site) and those already published in previous studies on the same data sets. In on-farm small ruminant studies focusing on reproductive traits such as AFP distributions, we recommend using survival models (e.g., the KM method) to avoid potential biases due to data censoring.