"I thought that farmers used an agricultural version of Roundup aka glyphosate"

Roundup kills everything. FOREFRONT is an effective selective herbicide which kills docks, thistles and so on in pasture.

However, it remains active in any plant material that it binds to and thus is capable of passing straight through an animal [fed on hay or silage] and into the manure.

It then remains active in the manure and in plants that it comes into contact with. It will break down on contact with the soil, but stacking the manure in a heap is not, therefore, much of a solution, nor is using it on soil which then has crops of susceptible plants.

This is not a problem for farmers spreading the manure on either pasture or cereal crops, as neither are effected.

"If farmers are using something else nowadays, that is a serious problem to the eco-system"

Well, I don't have a comment on that other than that the label on this product clearly explains the issue with manure, and which crops are susceptible, and outlines the farmers responsibility to make any recipient (of either hay/silage or manure/slurry) aware of the issue. (I don't know if the product label has always said this, or if it has been added later).

Clearly the transfer-of-knowledge hasn't been happing though.

The product labels are available at: FOREFRONT*