An interesting tip here from Charles Dowding regarding the long lastin effects of herbicides in collected manure or hay.  Try a bit first with a fast growing legume as they are particularly susceptible. Would save a great deal of heart ache.

I'm testing some straw to see if it contains Aminopyralid by planting a tomato seedling in a pot lined with the straw, and irrigating exclusively with water in which the straw has been soaking.  When should I expect to see distortion if the straw is contaminated?

I would think fairly soon, but plant a control seedling at the same time and you will be able to compare growth. The sad thing is it may mess up a whole season if the straw is contaminated. Our hay comes from a wide variety of fields and there is no way of knowing what has been used on it, although it does usually contain large quantities of wild flowers so I guess it's probably OK. I can't  get hold of straw here!

So far the tomato is growing normally.  Just to be safe I will continue to grow it out for the rest of the season.

I'm pretty sure the straw is not contaminated with aminopyralid.  Here's a photo of the Tomato transplant after one month of irrigating exclusively with straw water:

I concur Tyler, no signs of aminopyralid there. If there had been the plant would be stunted with twisting leaves.

Thank you for your expert opinion, Dr!  Now I feel confident that I will be able to use the straw.  It was unlikely to be contaminated, because aminopyralid is mostly used on hay, but I wanted to be sure.  I'm hoping to grow mushrooms on some of the bales.  This will be a cool season project.