ORIGINAL RESEARCH Assessment of age at first parturition by accounting censored data: the example of small ruminants in agropastoral herds in Senegal Matthieu Lesnoff & Renaud Lancelot Accepted: 12 February 2010 / Published online: 10 March 2010 # Springer Science+Business Media B.V. 2010 Abstract 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 distribu- tions 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 agroeco- logical 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 recom- mend using survival models (e.g., the KM method) to avoid potential biases due to data censoring. Keywords Age at first parturition . Survival model . Goats . Sheep . Small-holder farming systems . Senegal Introduction Age at first parturition (AFP) is an important reproductive trait when evaluating herd productivity in tropical small- holder farming systems, particularly for small ruminants with high demographic turnover and utilization for meat or cash flow sources (e.g., Legesse 2008). Reliable AFP estimates require individual animal follow-up data where birth and first parturition dates are precisely known. In Africa, this type of data has mostly been collected in experimental stations (e.g., Mukasa-Mugerwa and Lahlou- Kassi 1995; Gbangboche et al. 2006; Wilson and Murayi 1988) since longitudinal surveys in traditional villages are cumbersome and expensive. Although some AFP studies have been derived from on-farm longitudinal surveys (Tuah and Baah 1985; Gatongi et al. 1997; Osaer et al. 1999), they most frequently used cross-sectional retrospective data, based on farmers’ interviews and their recalls of the females’ ages at delivery (e.g., Dumas 1980; Wilson and Durkin 1983; Hassan 2000). The recall errors during the retrospective interviews of the farmers imply many survey M. Lesnoff UMR SELMET, CIRAD, Campus International de Baillarguet, 34398 Montpellier Cedex 5, France M. Lesnoff (*) UMR SELMET, CIRAD, Immeuble Kouma, Quartier Niaréla, BP 1813 Bamako, Mali e-mail: matthieu.lesnoff@cirad.fr R. Lancelot UMR CMAEE, CIRAD, Campus International de Baillarguet, 34398 Montpellier, Cedex 5, France Present Address: M. Lesnoff Programme Bovins, IER, Station de Sotuba, BP 258 Bamako, Mali Trop Anim Health Prod (2010) 42:1155–1159 DOI 10.1007/s11250-010-9540-3