Every substrate, the material a mushroom feeds on, has a carbon to nitrogen ratio, written C:N. Straw is mostly carbon; nitrogen is the scarce part. DMR states that this mushroom needs a C:N ratio of 40 to 60, far higher than other cultivated mushrooms. In plain words, it is happy on poor, woody material that button mushroom could not use without weeks of composting.
That is why plain paddy straw, soaked and drained, is enough. Indoor growers use cotton waste, which DMR calls the preferred material for that method. Either way the wet substrate should hold 60 to 65 percent moisture: squeezed hard, it should not drip.