I have a 10 year old redfoot tortoise. Her natural origins are in South American, so she's not desperately dependent on UV-B radiation to survive like an African species is. She gets plenty of vegetables and has a 5.0 UVA/UVB light that I change about every 6 months (I know that it's supposed to be every quarter, but they are quite expensive and seem to retain some oomph for twice the prescribed time).
Nevertheless, when I take her outside (about once a month in the warm periods), she tends to pick with her beak at the white gravel that we have around the foundation of the house, leading me to believe that she may be calcium deficient. She had a cuttlebone when she was little, but she outgrew the need to chew at it, and I figured she was getting enough calcium through her diet and enough D3 biosynthesis from the UV light.
In pursuit of giving her more calcium, I bought a Ca and D3 sprinkle from the pet store, and I try to put it on her food about once a week. I'm wondering if this is enough, or if I'm fact overdoing it for an adult.