No way of being certain but it's reasonable to think the straw is not contaminated just by the nature of intended use. Those bales are used on holiday dining tables and other places that demand clean material. Most are wheat stubble left after the wheat grain has been seperated.

Many grain straws are questionable because of the herbicides that could have been used. If you don't know, it should be suspect.

It also depends on how much straw vs. the size of your compost. If it's a small portion of your compost, I wouldn't worry about it.

Until the last year I would have said yes.

But I am becoming very careful about my ingredients. Now I only use grass and leaves from my own yard.

For twelve years I have made compost. My main interest is tomatoes. Each year I added compost to my clay-ish soil.

The first five years I had great tomatoes (when I knew nothing), but the last five years the tomato crop has been very poor. Why?

This year I found out one neighbor, who's leaves I used for two years, has been bring workplace herbicides home because the retail products 'weren't strong enough'.