Divergent Selection for Exponential Growth Rate at Fourteen or Forty-Two Days of Age.: 1. Early Responses

Abstract

Athens-Canadian randombred chickens were divergently selected for exponential growth rate (EGR) to 14 or 42 days of age, resulting in four unique populations: 14L, 14H, 42L, and 42H. Exponential growth rate was calculated as: EGR = [ln b – ln a] ÷ t; where a = hatching weight; b = weight at age of selection; and t = age of selection. In the S4 generation, 14H chicks had significantly greater EGR than 42H chicks at 14 but not 42 days. Chicks from the 42H line had significantly greater EGR than 14H chicks from 14 to 42 days. This meant that 14H chicks were significantly heavier than 42H chicks at 14 but not 42 days. Indeed, 14H chicks were consistently 5 to 10% heavier than the 42H chicks in each generation. Selection for EGR at 14 days was at least as effective in altering 42-day body weight as selection for growth rate at that age during the early generations of selection.Evidence of asymmetry existed for selection for EGR at 14 but not 42 days. Realized heritability of fast EGR14 was .29 ± .08 (SE) whereas that of fast EGR42 was .15 ± .09. Genetic correlation between fast EGR14 and fast EGR42 was 1.45, whereas the converse genetic correlation was .67. This would suggest that all the genes that influence 14-day growth influence 42-day growth, but not vice versa.Growth curve analysis supported the general hypothesis of growth, in that instantaneous growth rate was fastest in the 14H line, as was the decay of growth. Indeed, 14H birds were smaller at and beyond sexual maturity than 42H chickens.