A Comparison of Three Control Populations ,

Three unselected populations of S.C. White Leghorn chickens involving two census sizes and two mating schemes were investigated to determine their suitability as controls. The populations consisted of the Regional Cornell Control (CC) and two (designated by their flock sizes as M250 and M125) sampled from it. M250 and M125 were maintained for 13 generations with random selection and mating. In CC each parent was restricted to one offspring of its sex in the next generation and no full- or half-sib matings were made. Performance of each population was estimated yearly on a random sample of 90 females tested concurrently in the same facility with the other samples. Performances of the lines were quite similar and all experienced significant variation among generation means. Adjustment for regression generally produced small insignificant reductions in generation variance for CC. Although the same reductions in M250 and M125 were significant they were generally small in magnitude. Generation components of variance were derived for each population and differences among the components were attributed to genetic drift and natural selection. Because of its effective size CC was concluded to be suitable as control for long term selection experiments and breeding programs, whereas, M250 and M125 would be suitable for only the short term.