There is a ton of different pesticides in manure from different sources, particularly manure and even compost with manure as an ingredient. To significant enough levels there is an easily observable effect on crops. Various labs have tested for this.

The sources for this are numerous. Hay is one of them, because commercial hay isn't just grass from the side of the road or wherever, but all different types of grass, wheat, and different types of plants such as legumes, dried, and bundled for sale.

The number of plants that are engineered to resist certain chemicals is massive now, and certain farmers use far too many herbicides on those crops.

The problem is that there is no labeling to know what those animals in any of those products ate, and that glyphosate and other herbicides may simply pass through their system, and don't break down, even at 150 degrees in compost.

You could buy commercial manure and compost and be perfectly fine, because maybe the levels aren't high enough, and you lucked out. But there's just the chance now, that it could kill your garden and make it hard to grow crops for a while after.

It's not that they spray the hay, it's that they spray GMO wheat and soybeans and other crops, and then the animals eat them. This isn't a problem if the farmer they got the crops for isn't psycho and doesn't dose all his crops in glyphosates, but sometimes they are.

Hopefully it will become less of a problem as people realize they cause cancer.