a1 Departamento de Zootecnia, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 7712, C.P. 776, Porto Alegre 91501–970, Brazil
a2 Department of Poultry Science, Auburn University, Auburn, Alabama 36849–5416, USA
The weight of hatching eggs can influence broiler live performance regardless of hen age. Egg composition is altered with egg weight, but such alterations do not seem to have major effects on broiler growth and processing yields. The chick hatches with a yolk sac which provides nutrients for the transition to independent feeding. Alterations in egg weight and composition do not affect the proportion of yolk sac to body weight as much as its composition, particularly with eggs from very young hens. The contents of the yolk sac are high in fat and protein but very low in carbohydrate, which could lead to ketosis with prolonged fasting. Enhancing the first feed with either carbohydrate or gluconeogenics such as propionic acid may alleviate this ketosis and help early development. The digestive system of the chick is physically complete at hatching but is not fully competent at nutrient retrieval as many enterocytes are orientated to immunoglobulin uptake. Villi length and enzymatic activity increases with feeding, reaching maturity within a few weeks. Access to food and water after hatching varies, and long delays until placement are common. These delays cause losses in live performance. Loss in body weight due to late placement or undernutrition may also affect early muscle development. These adverse effects extend to marketing age and reduced meat yield. Factors that affect early chick development are gaining interest as the length of time to market progressively decreases and the chick's first days represent an increasing proportion of the total time for production.