At the beginning of the year the grass may not be growing much, and you will need more trips to the supermarket for vegetable shopping! Once the grass does start to grow it will be very rich, and the guinea pigs will not be used to eating it, so they should initially only spend 1-2hrs on the grass a day, to avoid stomach upsets.
Guinea pigs will love being outside in the spring, but be careful not to let them overdo it!
By April the weeds will be starting to grow (may be weeds to you, but are delicacies for your guinea pigs!) and you can start picking dandelions, milk thistle and plantains.