The composition and in vitro digestibility of the leaf and stem tissues of guinea grass (Panicum maximum Jacq.) were analysed at several stages of regrowth in two controlled environments differing in irradiance. The high irradiance environment was a growth room with a 25/15 °C day/night temperature, a 16 h day length and an irradiance of 300 ?E m-2 s-1. The low irradiance environment was a growth cabinet with similar conditions but with an irradiance of 175 ?E m-2 s-1. Plants grown under low irradiance yielded slightly more dry matter than plants under high irradiance after 70 days of regrowth. This was associated with morphological changes induced by low irradiance; these plants were taller and had larger and wider leaves than those plants grown in the high irradiance environment. Leaf senescence occurred earlier under low irradiance conditions but the number of fully expanded leaves per tiller as well as days from leaf emergence to full leaf expansion were similar for both environments. Nitrogen uptake into the shoot tissues was stimulated in the low irradiance environment. In vitro digestibility (IVD) of leaves and stems declined with age as lignin content increased in both environments. Cellulose, protein, total available carbohydrate (TAC) and alcohol-soluble carbohydrate (ASC) contents were not consistently related to IVD across ages in the two environments. Plants in the low irradiance environment had significantly higher lignin and cellulose, but lower IVD, TAC and ASC.