Steps to recreate:

1. Put two wheat into off-hand.
2. Breed two cows or sheep.

What I expected to happen: the stack in off-hand would run out and disappear.

What happened: the stack in off-hand had a red "0" quantity. I was able to continue feeding this to more cows / sheep and produced a stack with a negative red quantity; see attached screenshots.

I've confirmed that this also happens with seeds when feeding to chickens, and with carrots when feeding to rabbits / pigs.