Iams Large Breed is pretty low in protein and fat for a hard working dog. I'd suggest you switch to a Performance (Eukanuba Performance, Eagle Pack Power etc)food with 30% Protein and 20% Fat.

It takes a dog 12 to 16 hours to fully digest a meal, so the feeding the night before a hunt is the important one. Give the dog 2/3 of it's food at night and 1/3 in the morning.

If your dog is otherwise healthy, and you are feeding a Performance food, you shouldn't need to supplement.