Piggies need to be weighed once a week as part of routine care. Please do get a set of kitchen scales so you can carry out a weight check once a week - this helps you pick up any weight loss quickly (you then switch to daily weight checks) as it can be a sign of an illness which requires you to step in.
Weight is not an indicator of what is healthy for their body type as there is a very wide range of healthy weights for adult piggies. Heft is the way to tell what is healthy for the individual piggy. You gauge how much fat you can feel around the ribs - if you cant feel the ribs, then they are overweight. If you can feel every single rib with no fat at all, then they are underweight. You should be able to feel the ribs with a nice thin covering of fat.
Weight - Monitoring and Management
You say you give quarter of a cup of pellets a day - is that quarter of a cup each or quarter for them to share? It is too much though. Quarter of a cup converts to 3.3 tablespoons. If you are giving that amount each, then it is three times more than they should get - each piggy should only have one tablespoon of pellets per pig per day - is each, that 1/16 of a cup each. Pellets are the least healthy and least important part of their diet and overfeeding pellets can contribute to excess calcium (which can lead to bladder problems) and being overweight.
You need to provide a variety of fresh vegetables to then every day, including a vitamin c rich veggie daily to meet their needs. Only giving them veg once a week could mean they are missing out on a balance of correct nutrients.
Long Term Balanced General And Special Needs Guinea Pig Diets
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Yes each guinea pig gets ¼ a cup of pellets a day and I'll start to give them veggies more often now and I will look into on getting a scale thanks