Known as fat hen, melde, pigweed and goosefoot, lambs quarters is found across most of Europe, but is also found in Oceania, North America, Australasia and Africa.
It’s leaves and young shoots are eaten as a leaf vegetable, either steamed or cooked like spinach
So can guinea pigs eat lambs quarters and if so how much of them can be eaten.

Lets take a look at their nutritional data and find out more.
food and image source info wikpedia
Lambsquarters, raw
Nutritional value per 100 g (3.5 oz)
Energyt180 kJ (43 kcal)
Carbohydratest7.3 g
– Dietary fibert4 g
Fatt0.8 g
Proteint4.2 g
Vitamin A equiv.t580 μg (73%)
Thiamine (vit. B1)t0.16 mg (14%)
Riboflavin (vit. B2)t0.44 mg (37%)
Niacin (vit. B3)t1.2 mg (8%)
Pantothenic acid (B5)t0.092 mg (2%)
Vitamin B6t0.274 mg (21%)
Folate (vit. B9)t30 μg (8%)
Vitamin Ct80 mg (96%)
Calciumt309 mg (31%)
Iront1.2 mg (9%)
Magnesiumt34 mg (10%)
Manganeset0.782 mg (37%)
Phosphorust72 mg (10%)
Potassiumt452 mg (10%)
Sodiumt43 mg (3%)
Zinct0.44 mg (5%)
Link to USDA Database entry
Percentages are roughly approximated
using US recommendations for adults.
Source: USDA Nutrient Database
As you can see lambs quarters contain a very high amount of calcium and are very acidic and have a hint of fat.
Even though they have a fantastic amount of vitamin c in them, the fact that they have such a high calcium and acidic content means that they are not a food that guinea pigs should be eating.