Genotype X environment interaction for growth-related traits in meat-type chickens
The aim of this work was to investigate the growth performance of chicken genotypes subjected to diverse environmental conditions from potentially good (intensive) to harsh and stressful (feed-restricted and extensive) environment for the purpose of selecting genotypes that are best suited for performance under range conditions.
In an attempt to address this problem, three experiments were conducted culminating into the 4 papers outlined in this thesis. Paper I presents part of the results from the first experiment where three chicken genotypes were reared under intensive and feed-restricted extensive conditions. The emphasis of this work was directed at comparing the growth performance and slaughter traits. Genotype x environment interaction for body weights and slaughter traits that did not involve differential ranking but rather differences in spacing between the three genotypes when chickens were expressed to the two rearing system.
Paper II is an extension of the work from the first experiment with emphasis on feed consumption among the three chicken genotypes under choice feeding using commercial starter ration and whole grain wheat. Choice feeding was thought to be easily comparable to bird’s feeding under natural conditions and so genotypes well suited for performance under this system would be more adaptable to foraging conditions. It was revealed that, fast growing genotypes consumed more of each ration (starter and wheat) than the smaller genotypes with maintenance requirements and feed intake relative to body weight indicating a major genetic role and a possible connection with body size.
Paper III is based on the second experiment where two chicken genotypes were raised under range rearing with feed offers adjusted according to the respective adult body weights. It aims at examining the influence of body size on growth and efficiency of food utilisation when genotypes are kept under controlled feed restriction. It was concluded that, much of the differences observed among genotypes with respect to the efficiency of food utilisation reflect the inherent differences in genetic size acting more so early in life when growth is still increasing.
Paper IV uses birds from the second experiment but was specifically designed as an independent experiment aimed at comparing the foraging behaviour of chickens of two genotypes under range rearing in relation to feeding regimen and patch density. The results from this experiment showed that, birds selected for higher appetite levels under ad libitum feeding are as well adapted to extensive foraging conditions and may adapt by increasing their foraging activities in search for more food when necessary.
Key words: chickens, foraging efficiency, genetic size, genotype by environment interaction, growth efficiency, range rearing
