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In short, the diet needs to be unlimited timothy hay, one cup of veg per pig per day and one tablespoon of pellets per pig per day. No alfalfa.
Piggies eat hay, veg and a small amount of pellets pretty much from birth and their diet at birth, their diet now at their age and their diet as an adult is exactly the same.
They should not have been kept off of veg, so please do start to introduce veg but you will need to do so slowly - you cannot just give them lots of veg given their digestion has not been built up for it. You need to start right from basics and only give a very small amount of one type at a time. Give that for a week, and then introduce another type of veg, give one for a week and then introduce another etc. All while gradually building up the amounts. The four safe veg which can be fed daily are cilantro, lettuce, cucumber and bell pepper. The guide below shows a sample plate of the sort of amounts they will be able to have once their digestion is used to it. Piggies learn from their mother and from each other so if they have gone this long without being exposed to veg in the presence of other piggies to each learn together, they may be unsure of trying new things so you may need to be patient in trying to get them to eat some
As for the alfalfa, it should not be in the diet. It is not a grass hay and is too high in calcium. A very small baby can be given a very small amount each day but by the time they are weaned at 3 weeks old, it does not need to be in the diet any longer.
You can continue to give the alfalfa pellets until they are four months of age if you wish to do so but make sure you keep it very limited. Most long term owners will never feed alfalfa containing pellets - there is simply no need.
With that said, you should change pellet brands gradually over the course of about two weeks mixing old to new 90:10, 80:20 etc until they are fully on new pellets. However, if you are going to continue feeding oxbow pellets but just switch to the normal pellets then you may not need to be quite as cautious if the ingredients are basically the same (except minus the alfalfa).
The amount of pellets in the bowl in your avatar picture is far too much though - each piggy should have just one tablespoon each day. Never allow them constant access to pellets. Pellets, even ones which do not contain Alfalfa, are still very high in calcium and pellets and unfiltered drinking water contribute most calcium into the diet. Keeping pellets very limited will be of huge benefit to them as it will encourage higher hay intake. Pellets do not make up any significant part of their diet and are in fact the one part of the diet they do not
need to have.
Long Term Balanced General And Special Needs Guinea Pig Diets