When you are fighting The Boss at the field of daffodils, throw a grenade anywhere when you get the chance. If you hear the sound of an animal turning into a ration, put your thermal goggles on and look for the ration with a special snake inside. Press Start and look at your food. If you picked it up, you will have a snake named after one of the sons of Big Boss. You can eat it, and he loves it. If you do not find a ration or hear a sound, throw another grenade. Eventually you will get one. They are mostly near the rocks.