"...I've been feeding them
most of their greens from what I have in the winter vegetable garden:
collard greens, chard, and parsley..."

watch out with too much of cabbage-like things: guineas get gas from them
and this could have tummy aches as a result. Spinach is not too good for
them either, only occasionally and in very small amounts. Carrot tops are
OK, make sure it's all fresh and green, take out the withered stuff.
I feed my gps beetroot too, occasionally. They love it. But variety is the
key word here. Same as for humans. Check out these sites for more info on
food and nutritional values:
http://www.cavymadness.com/ click 'Care' and then 'Food'
http://www.aracnet.com/~seagull/Guineas/feeding.html
http://www.guinealynx.com/nutrition.html#lists

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Best wishes,

Astrid
(+Sebastiaan, Cleopatra, Kassandra and Cassiopeia)