Crossbreeding parameter estimation for functional longevity in rabbits using survival analysis methodology 1 M. Piles,* 2 J. P. Sa ´ nchez,† J. Orengo,* 3 O. Rafel,* J. Ramon,* and M. Baselga† *Unitat de Cunicultura, IRTA, Torre Marimo ´n s/n., 08140 Caldes de Montbuı ´, Barcelona, Spain; and †Departamento de Ciencia Animal, Universidad Polite ´cnica de Valencia, Camino de Vera, 14, 46071 Valencia, Spain ABSTRACT: A complete diallel cross involving 3 ma- ternal lines of rabbit was performed to estimate cross- breeding parameters for functional longevity. This trait was defined as the ability to delay involuntary culling. The lines considered, A, V, and Prat, had all been se- lected by litter size at weaning for a long period. Data were related to a total of 653 does belonging to the 9 genetic types from the diallel cross; does were reared and bred on the same commercial farm. Survival analy- sis was performed using a Cox proportional hazard model. The model incorporated time-dependent factors, such as year-season, litter size, and the interaction be- tween cycle and physiological status of the female; time- independent factors, such as the genetic type of the doe; and sire and dam random factors. Crossbreeding Key words: crossbreeding parameter, longevity, rabbit, survival analysis 2006 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2006. 84:58–62 INTRODUCTION Does used on commercial farms in intensive rabbit production usually come from crosses between mater- nal lines selected for prolificacy (Baselga and Blasco, 1989). Crossbred does are expected to show better re- productive performance than purebred does because of positive heterosis in reproductive traits, complementar- ity of lines, and dissipation of accumulated inbreeding within lines (Baselga et al., 2003). Crossbreeding pa- rameters have been estimated for reproductive traits in prolific species (Bidanel et al., 1989; Nofal et al., 1996); however, parameters have not been estimated for longevity, defined as the time from the start of pro- 1 This research was supported by INIA SC96-024. The authors acknowledge E. A. Go ´mez for his contribution to the design of the experiment and J. Terrades and the staff at La Balma farm for their contribution to the experimental work. 2 Corresponding author: miriam.piles@irta.es 3 Current address: Departamento de Produccio ´n Animal, Facultad de Veterinaria, Universidad de Murcia, Spain. Received April 25, 2005. Accepted August 24, 2005. 58 parameters were estimated from the solutions obtained for the type of doe and its estimated variance-covari- ance matrix, using a generalized least squares proce- dure. The estimated parameters were the differences between lines in direct genetic effects and maternal genetic effects and individual heterosis. Relevant differ- ences were observed in direct genetic effects between lines A and Prat but not in any maternal effects. Hetero- sis was found to be significant and favorable between lines A and Prat, and between the lines V and Prat. The magnitude of this effect was variable but important, especially in the first cross. Results stress the impor- tance of using crosses between specialized lines to pro- duce does for intensive meat rabbit production. ductive life until death or culling. In recent years, this trait in rabbits has received increasing interest because of the problems associated with high doe replacement rates (currently approximately 120%/yr; Rafel et al., 2000), and some investigations have been carried out (Garreau et al., 2001; Sa ´ nchez et al., 2004). A distinctive characteristic of longevity is the pres- ence of censored data, which are encountered when a study finishes before the event, in this case death or culling, occurs. Censored data provide partial informa- tion in the sense that we only know that the event had not occurred when the records were obtained (Klein and Moeschberger, 1997). Censored data should be in- cluded in the analysis because removing them or treat- ing them as uncensored records could lead to biased estimates (Guo et al., 2001). Survival analysis method- ology (Kalbfleisch and Prentice, 1980) allows the treat- ment of censored data, permits the inclusion of time- dependent covariates into the model, and takes into account the nonnormality of the trait. The aim of this work was to estimate crossbreeding parameters of functional longevity, defined as the abil- ity to delay involuntary culling (Ducrocq, 1994), in the cross between 3 maternal lines of rabbit. Published December 8, 2014