It can be expected that different growth performance between different lines of broilers, or even between individuals, may already be expressed during embryonic life. Considering that 1/3 of broiler's life takes place during the prenatal phase quantitative determinations of energy expenditure (EE) in this phase may be a crucial parameter predicting metabolic rate and consquently, growth performance of post-hatched chickens.

The aim of this investigation was to determine EE in embryos of slow and fast growing lines of chickens. Taking advantage of the indirect calorimetry technique it was also possible to evaluate amount of oxidized fat during embryonic development and to compare daily fat oxidation with changes in the fat content of eggs.

The experiment comprised 48 embryos from a modern, fast growing line, Ross 308 (RO) and 48 from a slow growing line, Labresse (LA) of White Plymouth Rock. The O₂ consumption and CO₂ production were measured in an open-air-circuit respiration unit, and EE from embryos was calculated at an age of 10, 13, 16 and 19 days. Gas exchange was below 10 ml/h for RO and LA by an age of 10-13 days, increasing steeply to a "peak" on day 16 and slowing down between 16 to 19 days. The pattern of curves for gas exchange was identical for RO and LA, but on a lower level for LA. The energy expenditure followed the pattern of curves for gas exchange, with a mean value around 50 J/h on day 10, increasing to 528 (RO) and 402 (LA) J/h on day 19 (Figure 1). The main source of EE was oxidized fat contributing with nearly 100 % to the total EE. Since oxidised fat was the main energy fuel the content of fat in eggs decreased with 2.0 (RO) and 1.6 g (LA) during the incubation period.

It can be concluded that the pattern of gas exchange and thereby the pattern of energy expenditure is independent of genetic origin of embryos. However, the embryos from the slow growing broiler line had a lower metabolic rate and oxidised less fat than the modern, fast growing line. The reduced utilization of yolk fat might be a tool for saving fat reserves for the immediate post-hatching period. It is remarkable that the differences between chickens from fast and slow growing lines were already manifested furing their embryonic development.