Evolutionary hypotheses suggest that higher rates of postembryonic development in birds should either lower the resting metabolic rate (RMR) in a trade-off between the costs of growth and maintenance or increase RMR because of a buildup of metabolic machinery. Furthermore, some suggest that higher rates of postembryonic development in birds should reduce peak metabolic rate (PMR) through delayed tissue maturation and/or an increased energy allocation to organ growth. We studied this by comparing metabolic rates and organ sizes of fast-growing meat-type chickens (broilers) with those of birds from a laying strain, which grow much slower. During the first week of life, despite growing six times faster, the RMR of the broiler chickens was lower than that of birds of the laying strain. The difference between strains in RMR disappeared thereafter, even though broilers continued to grow twice as fast as layers. The differences between strains in growth rate during the first week after hatching were not reflected in similar differences in the relative masses of the heart, liver, and small intestine. However, broilers had heavier intestines once they reached a body mass of 80 g. In contrast, broilers had relatively smaller brains than did layers. There was a positive correlation, over both strains, between RMR and the masses of leg muscles, intestine, and liver. Furthermore, despite delayed maturation of muscle tissue, broilers exhibited significantly higher PMR. We hypothesize that a balance between the larger relative muscle mass but lower muscle maturation level explains this high PMR. Another correlation, between leg muscle mass and PMR, partly explained the positive correlation between RMR and PMR.

The potential of visible and near infrared (NIR) spectroscopy to predict the fat, crude protein (CP) and ash content (g kg−1 DM) in dry ground chicken carcasses was evaluated. In addition, NIR spectroscopy was used to discriminate between ground carcasses from three different chicken genotypes: fast-growing broiler, slow-growing broiler and a layer-type chicken. When corrected for age and body mass (BM), the fast-growing broiler had the highest fat content and the lowest CP and ash content of the three genotypes. In contrast, the layer genotype had the highest CP and ash content and the lowest fat content. The fat, ash and CP content were intermediate in the slow-growing broilers. Spectra could explain a high proportion of the variability in carcass composition with respect to fat ( R2 = 0.93) and CP ( R2 = 0.86) content but less so for the ash content ( R2 = 0.71). Carcasses could be accurately classified according to chicken genotype or dietary treatment using NIR. However discrimination between male and female birds was not so clear, probably because all the birds used in the study were sexually immature.

Intensive genetic selection for fast growth rate in chickens over the last 40 years has halved the time needed for broilers to reach 2 kg in weight (Gyles, 1989), with most of this growth difference occurring within the first 2 weeks of life (Ricklefs, 1985). We investigated the effect of this extensive selection on organ morphology in 3 strains of chicken that have undergone differential rates of selection. The strains include: a modern commercial broiler (Ross 308, FB), a broiler that has not been selected for fast growth since 1972 (Ross 1972, SB) and a layer chicken which has not been selected for fast growth rate (Euribrid HISEX, L).Each strain was grown to the point of maximum growth rate which was at 42, 56 and 112 d for the FB (n= 52), SB (n= 64) and L (n= 120) strains respectively. One-d-old chicks were sexed, wing-banded and reared as separated sexes in pens on a wood-shavings litter. Each strain was grown to standard commercial practice for environmental temperature and lighting regimen. The photoperiods were 23L:1D for broiler strains and 8L:16D for the layer strain. Both strains of broiler were fed standard commercial broiler rations. The layer chicks were divided into 2 dietary groups, 1 group was fed the same rations as the broilers and the 2nd group was fed commercial replacement pullet rations. Water and food were available ad libitum. Individual body weights (BM) were measured daily for the 1st week and weekly thereafter. A random sub-sample of chicks was removed at the same time intervals for serial morphological measurements. Birds were killed by cervical dislocation and weighed. The wet masses of the gizzard, intestine, caeca, heart, liver, brain, lungs, pectoral muscle and leg muscle (surrounding the femur and tibiotarsus) were measured. All tissues were dried to constant mass in an oven at 60°C. Statistical analyses were performed using ANCOVA (Minitab 10·5) and linear regression (Genstat 5), where non-linear data were transformed. There was a very high degree of correlation between tissue masses and body mass, so the mass of the tissue under consideration was subtracted from BM.As expected, both growth rate and BM were significantly different between all 3 strains of chicken and between sexes in each strain. There was no significant difference in BM or organ masses between the 2 dietary groups of L chickens, so these data were pooled for subsequent analysis. All 3 strains of chicken were grown until growth rate peaked which was approximately; 700 g/week by d 35 for FB, 275 g/week by d 49 for SB and was around 175 g/ week by d 63 for the L strain. There was a significant effect of strain (P£ 0·01) on the masses of the following tissues; pectoral muscle, leg muscle, gizzard, intestine, liver, heart, lungs and brain, when the effect of dry body mass was taken into account. However, there was no significant strain effect on the mass of the caeca. There was an interaction between strain and age for all tissues (P£ 0·001). The masses of both pectoral and leg muscle increased with bird age in a ...

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