Why raw leaves affect digestion differently
Raw collard greens have tougher cell walls than cooked, which means your 15 kg dog's system must work harder to break them down. The few leaves they ate contain more intact fiber that can irritate sensitive stomachs, though the amount consumed here likely won't cause serious trouble.
Unlike some toxic plants, collard greens themselves aren't poisonous - the concern is purely about digestibility. Your dog might pass the leaves nearly whole in their stool, or experience some rumbling as their gut processes the unusual fiber load. Neither outcome harms your other dog who didn't eat any.
The overnight factor with raw vegetables
Since these leaves sat out before being eaten, you might worry about bacterial growth. Fortunately, collard greens aren't prone to rapid spoilage like meats or dairy. At room temperature overnight, they'd dry out more than spoil, reducing any bacterial risk.
Watch for signs beyond typical digestive upset - if your dog develops vomiting paired with lethargy or won't drink water, that warrants a vet call. But simple loose stool from fiber overload doesn't indicate poisoning and requires no intervention for either dog.
Comparing risks between your two dogs
The dog who ate nothing faces zero risk from this incident. Collard greens leave no environmental residue that could affect them later, unlike certain chemicals or toxic plants that linger on fur or in shared spaces.
Your other dog might show mild symptoms simply because their system isn't accustomed to raw greens, not because the food was dangerous. The difference in reaction between dogs would come down to individual tolerance, not dosage, since the amount eaten was small.
When raw veggies become a real problem
While this case isn't concerning, large quantities of raw greens could cause intestinal blockages, especially in smaller dogs. The stems pose more risk than leaves due to their fibrous nature. Your dog's size makes obstruction unlikely from just a few leaves.
Future prevention involves storing greens securely and remembering that while dogs can eat many vegetables, their digestive systems prefer cooked, chopped versions. Neither dog needs treatment now, but adjusting your food storage habits can prevent repeats.
The full verdict, portion guidance and serving notes are collected on the Collard greens food facts page.