Effects of Selection for Body Weight, Egg Weight and Heterozygosis on Laying House Performance

INTRODUCTION SELECTION for extremes in metric traits usually lowers reproductive fitness. The reduction is correlated with both the metric trait itself and with the increase in inbreeding inevitable in populations of finite size. For example, Nordskog and Hassan (1971) found that hatchability was maximum in Leghorns when egg size was about 50 grams. A 10-gram increase above the optimum reduced hatchability more than 10% while a 10 gram decrease lowered it 4%. In addition, hatchability declined 2.6% for each 10% increase in inbreeding. The objectives of this study were: (1) To measure effects of finite population size and metric trait selection on laying house performance and (2) to examine the question as to whether a strain of chickens, subjected to metric trait selection over several generations, lose genes for high egg production as measured in a subsequent cross line test. The metric traits considered were body size and egg size…