Ashli,
each Grain farm produces invariably 100 straw in every production cycle (one year on floodplain, 1/2 year on meadow), independent of the fertility and time used for cultivation. Amount of grain produced varies according to fertility, cultivation time, and (on floodplain) can be doubled by "big blessing" from Osiris.
Straw is produced always and you can't get more grain if you decide that you don't want straw... But if you are really producing more straw than grain (I doubt it, it just looks like that cause you are not using the straw), you should consider building farms in better - more fertile - locations, or, if allowed by the game, build irrigation. Irrigation increases production of floodplain farms by 20%, of meadow farms by 40% (but not over 99%!). Pleasing Osiris brings better floods, which means higher fertility and production. Also make sure that for floodplain farms you have the camps nearby; one camp can serve 5 farms without production loss.
You'll need straw for bricks, and later also for meat production in cattle ranches, sometimes you can export it. If you don't need it, just go to trade advisor and switch straw production OFF. Pushers with straw will disappear right after the harvest.
Hope this helps!
Cherub Baltic