Analysis of litter size and days to lambing in the Ripollesa ewe. I. Comparison of models with linear and threshold approaches

¹ Grup de Recerca en Remugants, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
² Departament de Ciència Animal i dels Aliments, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

^* To whom correspondence should be addressed. E-mail: joaquim.casellas{at}uab.es.

	Abstract

The analysis focused on model fitting of 2 ewe reproductive traits, litter size and days to lambing (interval between the introduction of the ram into the flock and the subsequent parturition of ewes). The experimental data set of the Universitat Autònoma of Barcelona flock was used, including 1,598 records of litter size and 1,699 records of days to lambing coming from 376 Ripollesa ewes between years 1986 and 2005. Univariate and bivariate models were considered as starting points with linear or threshold approximation for litter size. Model fitting was evaluated in terms of goodness of fit and predictive ability, using the mean square error (MSE) and the correlation between phenotypic and predicted records (_y,) as reference parameters. The bivariate model was preferable for both variables, minimizing MSE and maximizing _y,. A threshold approximation for litter size was preferable over a linear approximation. Models were also compared with a simulation study, comparing the correlation coefficient between simulated and predicted breeding values (_a,â). The bivariate threshold model was favored, with a _y, of 0.677 and 0.834 for litter size and days to lambing, respectively. Correlation coefficients between simulated and predicted breeding values in the bivariate linear model were reduced slightly to 0.651 and 0.831, respectively, and they were lowest with linear univariate models (0.642 and 0.802). Although the bivariate models for ewe litter size and days to lambing were more accurate than univariate models, the threshold approaches showed a greater advantage under the bivariate model. For the purpose of genetic evaluation of litter size in sheep, use of the threshold-linear model seems justified. In the Ripollesa breed, the evaluation of litter size can benefit from recording birth weight.

Key Words: days to lambing, goodness of fit, litter size, predictive ability, Ripollesa breed