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While guinea pigs are chewers they don’t actually need sticks to chew on.
Hay and grass are the only things which they need to chew. It is only those which wears down the molar teeth properly and evenly, and properly wearing molars in turn keeps the incisors (which are self sharpening) wearing evenly.
Chewing a stick which is only chewed with the incisors has no effect on the molars.
It would not be a good idea to take sticks from the forest
- In some places there are laws against it
- not all wood is safe. Evergreen trees commonly found in forests, such as pine, are toxic to piggies and contains harmful oils.
- you would also need to know that even if the wood was safe (such as sticks from apple trees, willow trees for example), that it wasn’t covered in anything toxic such as moulds, harbouring eggs of pests etc.