Monthly model for genetic evaluation of laying hens - 1. Fixed regression

1. This paper addresses the possibility of using a monthly model for the ge netic evaluation of laying hens, based on the definition of a test day mode l with fixed regression as used in dairy cattle, in which monthly records w ere treated as repeated measurements of the same trait. 2. Production records of 6450 hens, daughters of 180 sires and 1335 dams we re analysed using an animal model with restricted maximum likelihood (REML) . The traits considered were individual monthly egg production and cumulati ve egg production in 11 months. Four different models were fitted to variou s combinations of monthly and cumulative records. The covariates were deriv ed from the regression of Ali and Schaeffer (1987). 3. Spearman rank correlations were computed to compare breeding values from different models. Two types of correlations were computed: between individ ual breeding values and between sire breeding values based on subsets of fu ll-sib records. 4. The results indicated that a monthly model with nested covariates produc ed higher heritability and permanent environmental variance than the models with non-nested or without covariates. The estimates of heritability obtai ned from monthly model were lower than the estimates from the cumulative mo del. The monthly model resulted in higher correlations of sire breeding val ues between two subsets of full-sib records than those from cumulative mode ls. 5. In conclusion, the monthly model with nested covariates appears to be be tter than the model with non-nested covariates or without covariate. Althou gh the heritability estimates obtained from the monthly model were lower, t he monthly model with nested covariates could be better than the cumulative model for genetic evaluation of laying hens in the 1st cycle of laying per iod when using either full or part records. The use of information from odd months of production could be of interest for the evaluation of full recor ds.