An experiment was conducted using pleurotus ostreatus mushroom growing conditions. Chopped and pressure-pasteurized wheat straw in the form of compost was seeded with mushroom mycelium. For in situ measurements polyester bags with the pore size of 40 µm were incubated in the rumen of 4 permanently ruminal fistulated Holstein steers, for 0, 2, 4, 8, 16, 24, 72 and 96 hrs. According to this equation p = a+ b (1-e –ct), the fraction for dry matter was highest (p < 0.05) for the straw under the mushroom conditions for 12 weeks (0.188 g/g for the week 0 vs. 0.342 for the week 12). In contrast, the b value significantly (p < 0.05) reduced from week 0 to 12 (0.487 vs. 0.336 g/g). The value for crude protein and NDF parameters increased linearly with increasing the growing period. Although, the c rates were significantly different between the samples from week 0 to 12, but this constant was not affected for CP and NDF. It was concluded that generally rumen degradability of different nutrients can be increased by growing the p. ostreatus, although, the c rate was independent. The results showed that straw seeding with p. ostreatus was more effective for longer period.

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