In the UK, markets for ‘welfare friendly’ meat have increased the use of more extensive, deep-bedded straw housing. Straw yard housing may increase physiological development at slaughter (Salmon & Edwards, 2007), with many confounded potential causal factors including increased space allowance, natural lighting/photoperiod, straw availability, increased activity levels and reduced social stress. Physiological development at slaughter is a key issue in the success of heavy pig production utilising entire boars, since greater maturity may increase adverse behaviours, therefore decreasing pig welfare, and increase risk of carcass taint. The objectives of this research were to investigate whether straw supplementation, in the absence of confounded space and lighting effects, influenced physiological development and associated undesirable effects in entire boars and gilts grown from 60kg to 120kg slaughter weight.