Hi Michael-
Currently, there is not an “official” nutrient profile established for senior dogs. AAFCO only has two profiles: Adult or Growth & Reproduction: https://www.dogfoodadvisor.com/frequently-asked-questions/aafco-nutrient-profiles/
Therefore, each company that labels any of their recipes as senior is doing so using their own concept of what a senior dog food should be.
What exactly are you looking/hoping for in a food for your senior dog? Is your collie in good health? They are such beautiful smart dogs!