Tips for mentally stimulating collie cross?

I have an 8 month old border collie x lab, as it's coming into better weather, he's now encountering sheep, cows, chickens etc for the first time and his herding instincts are running wild. I live surrounded by fields with cattle and other animals grazing in them so he's rearing to go.

Last week I accidentally dropped the leash on a walk and he headed straight to a field with cows in it and began herding them, took me a while but I finally got him back, and I am aware how dangerous it is for him to be doing that. But I can just tell he is dying to give into his instincts, so I'm just wondering, is there anything I could do so he could exercise those instincts, but without bothering any farmers cattle?!

During the winter I would walk him without a leash as there was rarely anyone about & there was no animals in the fields but I am too nervous we will encounter a field of cattle & he will bolt it, so he's not getting to run about and expel the energy he already has.

I know keeping him mentally stimulated is important so I give him his meals in kong wobblers, nose work, hide & seek, chasing etc.. but just wondering if there is anything I can do to exercise his herding instincts!

TIA!