Comparative growth, body composition, and allometric development of modern and heritage broiler genotypes

Abstract

Efficient growth and nutrient utilization are central goals in poultry production, yet intensive genetic selection has profoundly altered the developmental patterns of modern broilers. Understanding how organ and skeletal growth have adapted to support rapid muscle accretion is essential for improving sustainability and animal welfare. This thesis was based on a single comprehensive broiler experiment conducted under identical management and environmental conditions, from which two integrated studies were derived to characterize the growth potential, body composition, and allometric development of three broiler genotypes differing in selection intensity. Two modern commercial broilers (Ross × Ross 308 and Ross YP × Ross 708) and one heritage meat-type genotype (New Hampshire × Columbian; NHC, unselected since the 1950s were evaluated. In this thesis, all birds (n = 1,760 males) were placed in pens designated for performance (n = 12 pens/genotype) or sampling (n = 8 pens/genotype). Ross genotypes were reared to 56 d, whereas the NHC birds were reared to 63 d to achieve at least 2 kg final BW. The first study compared growth performance: Feed intake (FI), BW gain (BWG), and mortality-corrected feed conversion ratio (FCR) were determined weekly. Carcass parts were measured weekly (1 bird/pen), and body composition was determined biweekly (1 bird/pen). In the second study, the birds on the pens designated as sampling pens (n = 940 male birds) were evaluated. Weekly and biweekly sampling were conducted to assess organ weights, histomorphology of small intestine and tibia traits. Treatments were arranged as a randomized complete block design with pen as the experimental unit and pen location as a random blocking effect. A one-way ANOVA was performed using the MIXED procedure (SAS Institute Inc., Cary, NC) with significance declared at P < 0.05. Both experiments applied nonlinear Gompertz and allometric modeling to quantify genotype-specific growth trajectories and nutrient partitioning patterns. Modern broilers had higher BWG (P < 0.001), and breast yield, as well as lower FCR and reached 2 3 kg of BW in half the time required by the heritage line. Gompertz modeling confirmed greater mature BW (Wₘ ≈ 7.0–7.8 kg) and faster maturation rates (b ≈ 0.047–0.048 d⁻¹) in modern genotypes compared with the NHC line (Wₘ = 3.7 kg; b = 0.032 d⁻¹). Body composition analysis indicated higher protein accretion and lean deposition efficiency in modern broilers, whereas NHC birds displayed proportionally greater fat and ash accumulation at similar physiological maturity. In allometric models, the coefficient α represents the initial proportional size of the organ, while β describes the relative growth rate in relation to BW—Values of β < 1 indicate early-maturing tissues, β ≈ 1 isometric growth, and β > 1 late-maturing traits. The allometric comparisons revealed steeper scaling (higher β exponents) of metabolic and digestive organs (liver, pancreas, intestines) in modern lines, while heritage birds maintained relatively larger crop and gizzard and thicker tibias in proportion to BW. Despite smaller absolute bone mass, the NHC genotype had heavier and wider tibias relative to BW, however with lower mechanical strength. These results demonstrate that decades of genetic selection have shifted broiler growth patterns from balanced organ and skeletal proportionality toward early maturation and enhanced muscular efficiency. The findings provide an updated and integrated understanding of genotype-dependent nutrient partitioning and support future breeding and nutritional strategies that optimize performance and structural integrity in broiler chickens.