The influence of dietary nutrient concentration on performance and the growth of fat depots, breast meat and leg muscles was examined in five groups of male broiler chickens fed ad libitum with combinations of high (H), commercial (C) or low (L) energy and nutrient concentration in starter and finisher diets. Diets were changed from starter to finisher at 3 weeks of age giving 200 birds on each of five treatments: HH, HL, CC, LH and LL. Ten birds per treatment were slaughtered at weekly intervals from 0 to 70 days. Five of these were dissected into component lean tissues of breast muscles (white meat) and thigh+leg muscles (dark meat) and fat depots, and the other five carcasses were minced for chemical analyses. Data were analysed by fitting Gompertz functions to each component.

Analysis of body weight, carcass components and feed intake revealed that at 70 days birds on LH approached similar liveweights to those on HH with lower overall food intakes, comparable feed conversion ratios but slightly greater fat depots. The HL birds had less fat but achieved specific weights over longer periods of time with greater feed intakes and poorer feed conversion ratios. Content of white and dark meat was always greater in HH birds, with differences between treatments being greater for white meat.

Decisions on which conditions are most appropriate will be influenced by time taken to reach specific liveweights if whole birds are marketed, or the rate of growth of individual portions if further processing is considered, together with feed conversion efficiency and the relative costs of diets varying in energy and nutrient concentration.