Nutritionists formulate feeds based on the genetic potential of the pigs being fed. However, animals may not achieve this potential due to environmental constraints such as: disease challenges, environmental temperatures, feeder space, social stresses and stocking density challenges leading to a reduction in the potential growth rate (Ferguson et al., 2001; Wellock et al., 2006). An experiment was designed in which the potential growth of pigs was reduced to determine whether their nutrient specifications could be reduced concomitantly. Group size and floor-space allowance were used to reduce potential growth rate as it is well established that social stress is one of the most potent factors responsible for the inability of an animal to achieve its inherent growth potential (Ferguson et al., 2001). Here, we assessed the efficiency of lysine utilisation by pigs between 40 and 85 kg live weight, subjected to different degrees of environmental stress.