I was looking into amaranth myself a few years ago. I can't remember what I found out about it specifically related to feeding it to chickens, but that it can be problematic and does need to be cooked, first. I have grown amaranth, and am growing loads again this year (only because I spilled my seeds

), and I can tell you that harvesting the seeds is very time-consuming. I do go through and pick leaves every day or two and feed those raw to the birds.
The best way that I have come up with to reduce commercial feed intake (thus reducing corn and soy) is choosing breeds that thrive on foraging and just letting them run wild. Right now I have three groups of youths that barely touch their feeders because they're able to find most of what they need in the yard.