Where fostering is necessary the sow should be distracted with a tasty titbit and the young to be fostered gently handled with the foster mothers babies to transfer the smell. If fostering is not possible, guinea babies should at least have the company of an adult sow to teach them all about eating solid food and how to behave as a guinea pig. Guinea pigs cannot vomit and rely on the adults around them to show them what is good and safe to eat.
Hand feeding should only be undertaken as a last resort and must be done with great care as the most common cause of death amongst hand fed youngsters is milk on the lungs. Goat or sheep milk can be used fresh or powdered or one of the veterinary milk substitutes such as Cimicat Esbilac or Lactol. Dip a small clean paintbrush or eyeshadow sponge applicator in the milk substitute and let the baby suck on that. (Tip from Nichola Hadley's article in Cavies Magazine) or use the method recommended by Myra Mahoney in her Really Useful Guinea Pig Guide, and feed a mix of evaporated milk and cooled boiled water (50:50) on a small piece of bread on a spoon.
If the youngsters are orphaned at birth and are being hand reared you will help them greatly if you can obtain some form of colostrum which helps support the immune system. If you live in the country you may be able to obtain fresh or dried/powdered colostrum for lambs, kids or foals. In urban areas you will probably have to use powdered colostrum for kittens. Colostrum is also used as a health supplement for people so you can now buy organic colostrum via mail order. Although it will not provide the same antibodies that the mother's milk would provide, all colostrum is a rich source of lactoferrin which is very important for the immune system.
After feeding the baby must be encouraged to urinate. The mother does this by licking the babies genitals after feeding. Humans can get away with using a cotton ball dipped in warm water!
Guinea pig milk is 3.9% fat, 8.1% protein, 3% lactose 0.82% ash and 15.8% solids.