In Zimbabwe, yield of oyster mushroom ( Pleurotus ostreatus
) grown on sole substrates of maize or sorghum straw is low, <75% biological efficiency translating to less than a kilogramme of fresh mushroom per kg of dry substrate. This study aimed at determining the effect of mixing sorghum or maize straw with various proportions of bean straw, on the yield of grey oyster mushroom. Maize and sorghum straws were singly mixed with 0, 20, 40, 60, 80 and 100% bean straw, followed by a spawning rate of 5%. Eighteen days after incubation in a dark room, bags were hanged in a mushroom growing house with controlled temperatures (25-26o
C). Mixing bean with maize straw had a significantly higher oyster mushroom yield (1, 229 g) than with sorghum straw (1,138 g); leading to biological efficiency of 83 and 74%, for maize straw and sorghum straw, respectively. Yield and biological efficiency decreased from 1,357 to 977 g plot-1
and 97 to 61%, respectively, when the proportion of bean straw was increased from 0-100%; while number of days to second and third harvest increased. There were significant interactions (P<0.05) for the number of pins, days to second and third harvest, between maize or sorghum straw with bean straw. Maize straw gave better performance with high percentages of bean straw than sorghum straw. Results showed that maize straw mixed with 20% bean straw produced better yield of grey oyster mushroom.