The short answer is a year.
Dawn Hromanik replied and gave me permission to reprint her email (which includes a couple changes for clarity):
Many people feel that a guinea pig´s peak body mass deposition is reached at the reproductive stage of life, but we know that muscle and bone tissue is not complete at that age. The requirement for protein and calcium do decrease at that age, although the animal may not physically mature. Many veterinarians feel that a guinea pig at 8 months of age is as large as they will grow and that is the age of majority. But each breed/line/sex/individual will be different, just like humans, some boys will keep growing at age 19 where in others bone formation/density is complete at age 16. Bone development is usually earlier in females than in males (humans that is). Since our pellets are both high in fiber and low in energy I am more comfortable with a general recommendation of feeding the Cavy Performance until one year of age and then switching to the Cavy Cusine. I am always researching the specific age requirements and when future data is available I will reevaluate our recommendations.
Individualistic decisions need to include your veterinarian in deciding whether switching to a timothy based pellet at an earlier age would be beneficial.
[In a second email she added a few reasons why a vet should be consulted before switching to Cavy Cuisine before a year of age.]
1. Obesity : Some guinea pigs are very prone to obesity at a young age, this would be a case to switch to a timothy based pellet.
2. Skinny pigs: Some guinea pigs have a high metabolism and run on the thin side ... Baldwins or skinny pigs would have a higher energy need due to the fact that they don´t have any hair!...So for the hairless breeds that have trouble keeping on weight I would feed them a blend of the Cavy Performance and the Cavy Cusine.
3. Calcium metabolism: those GP´s that have problems with stones, or a genetic history of stones I would switch over at an earlier age.
Director of Nutrition and Product Development
Oxbow Pet Products
"Specializing in Herbivore Nutrition and Supportive Care"
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