The effects of 1.35, 1.80, or 2.25 kg dry matter (DM) of cottonseed cake, with (1.85 kg DM) or without molasses, on straw intake, diet digestibility, live-weight change, excretion of purine derivatives (PD), and milk production of Boran (Bos indicus) cows were studied. There were no significant (P > 0.05) interactions. Straw intake was 0.47 kg DM lower (P < 0.001) showing a substitution effect of 254 g kg-1 with molasses. Straw intake tended also to decrease from 5.18 to 4.94 kg DM (P < 0.15) with a daily substitution of 107 g kg-1 with increasing levels of cottonseed cake. Molasses increased DM and organic matter (OM; P < 0.01) but decreased crude protein (CP; P < 0.001) and neutral-detergent fibre (NDF; P < 0.01) digestibilities. Molasses (P < 0.05) and increasing levels of cottonseed cake (P < 0.01) increased daily milk yield from 3.06 to 3.26 and from 2.96 to 3.32 kg, respectively. Molasses increased live-weight by 300 g day-1 (P < 0.05). Increasing levels of cottonseed cake caused a live-weight change of 69 g to 467 g day-1 (P < 0.05). A supply of microbial-N increased (P < 0.001) in molasses diets but microbial-N production kg-1 digestible OM was not affected by treatments (P > 0.05).