Straw could probably be used effectively for lightly loaded structures (slabs, bearing walls in nonseismic areas) the same way that fiber is used currently. It is essentially for crack control and provides nominal flexural capacity which is generally hard to quantify but there are some formulas which are accepted.
Using straw for anything that is loaded in flexure will be a disaster. Reinforced concrete theory relies on the reinforcing to act as a crack stopping mechanism which will yield in a ductile manner. Straw's variable strength and inability to place it at critical sections means that it cannot perform the function of reinforcement as it will be possible to encounter localized weak reinforcement which will not prevent cracking and loss of section leading to progressive and sudden failure.
Using straw for anything that is loaded in flexure will be a disaster. Reinforced concrete theory relies on the reinforcing to act as a crack stopping mechanism which will yield in a ductile manner. Straw's variable strength and inability to place it at critical sections means that it cannot perform the function of reinforcement as it will be possible to encounter localized weak reinforcement which will not prevent cracking and loss of section leading to progressive and sudden failure.