I guess that all depends on how old your pups are.
If this is a reoccurring event that happens every year during this time, then perhaps the grass eating could contribute to their irregularity, but does it come and go?
Are you adding pure pumpkin to their diet? Whatever you do, DO NOT give them pumpkin patch leaves. The leaves act as a laxative and have a very ill-effect on their bowel movements.
In any event, it should be something that you monitor and if their stools continue to appear loose, you may perhaps need to speak to your vet and seek their opinion.
Like I said, my Gryphon grazes like a cow as well but with switching his diet, it truly helped with firming him up--in all aspects.