Growth characteristics of selected broiler muscles as affected by age and experimental pen design.

Broilers were grown in conventional pens and in pens modified with barriers and ramps extending across their width. The purpose of these modifications was to increase the use (exercise) of specific muscle groups and measure changes in muscle growth. The breast (pectoralis, supracoracoideus, and gastrocnemius pars externa muscles) from birds reared in conventional pens grew at a more rapid rate than the total body weight of the live bird. Increased growth rates of the breast, pectoralis, supracoracoideus, gastrocnemius pars interna, and femorotibialis muscles were found in birds raised in pens with single ramps. The double-ramp treatment resulted in increased growth in peroneus longus and femorotibialis. Barriers in pens resulted in slower growth rates in gastrocnemius pars externa and iliotibialis, double ramps resulted in slower growth rates in gastrocnemius pars externa, and single ramps resulted in slower growth rates in gastrocnemius pars externa and semitendinosus. Modification of conventional pens caused changes in weight and measurements of many of the muscles studied. Pectoralis muscles from birds reared in pens with barriers were 12.6% heavier after 3 wk exposure than those from birds in conventional pens. The pectoralis muscle of birds after 4 wk exposure to single ramps was 7.1% heavier and the femorotibialis muscle was 19.8% heavier than respective muscles from birds grown in conventional pens. Changes in muscle dimensions showed that conformation of the muscle could be altered by pen modification.