When training your dog it's important to make a list of all the things your dog finds rewarding. Whether it's food, toys, belly rubs, access to outdoors, tug, chase, etc. Rank them from "loves the best" to "not a favourite" in each of the categories. Once you have this list, it's easier to figure out what will be rewarding to your dog in a particular situation. If you want to use food for training outdoors, kibble is not going to work as a reward for many dogs. It's not high value enough.The environment is more interesting. You likely have to use one of the foods listed higher in ranking, perhaps cheese or liverwurst. It depends on what your dog loves and on the situation. As a reward for coming when called at home, a cookie may suffice. The same behaviour in a park calls for a better reward such as 
a few pieces of cheese - if that's what your dog finds rewarding! Some dogs don't like cheese. 

 The important thing to keep in mind is that it's what your dog likes, not what you think he should like.