Commercialisation of animal agriculture changed the phenotype and production characteristics of livestock. The sigmoidal growth curve and sequence of physiological events remained virtually unchanged, but the rate and extent of these processes increased remarkably. Physiological limits to growth are apparent in species selected for accelerated growth and production, like stress sensitivity, PSE and DFD syndromes in livestock, doublemuscled cattle, the callipage gene in sheep, ascites and associated metabolic defects in broilers, leg problems in layers, abortions in Angora goats, wet carcass syndrome in sheep, and other tissue defects as well as reproductive failure due to interactions between the growth hormone cascade, gonadotrophic axis and endocrine factors that regulate metabolism like thyroxin and leptin.
Although, the physiology of animals is generally quite forgiving, there are warning lights on the
horizon. The challenge in livestock production should shift towards synchronising the best
genotypes in a specific environment with the most appropriate and environmentally acceptable technologies available to produce consistently high quality meat. Manipulation of the quality of animal products through feeding, breeding and physiology will become increasingly important, provided that these technologies are practical, economical and do not detract from the intrinsic and extrinsic attributes of animal products, or any other aspect relating to environmentally acceptable or ethical livestock production.