I can't be any help on supplements, but since the replies are slow I'll chime in anyhow.
I don't give 'supplements' to my dog. What might work for one dog might give the next dog a case of the runs or worse. Any disruption to the dog's system is bad, but even worse if it ruins a hunting trip.
We all know the drill - a hunting dog is too excited to eat in the morning and too tired to eat at night. Over the course of an extended trip or season the dog loses energy and weight. I cheat the system by spiking my dog's regular food with something proven to be easy on his digestion and also entice him to eat. Mixing some canned dog food into the dog's regular kibble is one such cheat, or mixing in some beef broth, etc.
You mentioned feeding the dog twice a day - morning and night. I would break that into four smaller meals during a hunting day and spike as needed. Dog food is already overloaded with carbohydrates and will provide plenty of energy providing the dog will eat it.
If I'm hunting with pards and their dogs I prefer a spiked dog food that I can hand-feed my dog. Leaving a bowl of spiked dog food around will only cause a battle at the food bowl and risk that my dog will get shorted. In that case I'll often feed canned food from the same manufacturer as the kibble. That's a pretty safe bet for both the dog's digestion and his willingness to eat it. On extended daily hunts away from a vehicle I'll put a chunk of canned food in a zip lock or two and stash it in my vest, and then I'll use the empty zip locks to water the dog.
