When you equip wheat in your off hand and feed it to a Sheep or Cow it turns into an item with a negative amount. Once you feed it to them don't right click them again or it will disappear, now right click a baby animal to create a larger negative amount. After acquiring -129 a quick re log changes it to a positive 127 stack of wheat. This can also happen with meat and dogs, carrots and pigs, and seeds and chickens. It is shown in action in this video
https://www.youtube.com/watch?v=me7kEW-kVxg