Hi Kelly,
Horsfields are a great species with big personalities (but then I'm a bit biased as I have Horsfields). They are a particularly interesting species where diet is concerned. In the wild, Horsfields live in countries where the winters are long and cold and the summers are very hot, and they hibernate in the winter and aestivate (which is a bit like hibernation but in the heat) in the summer. It has been estimated that in the wild they are only awake for three months of the year and that they are eating for a total of about 20 hours during those three months. So they are sort of programmed 'eat for England' in the short time that is available to them.
Of course in captivity we present them with lovely meals each day and they can easily overeat and grow too quickly, which can give rise to serious shell deformities. Monitoring their weight gain is the best way to control their growth, and it is difficult to say how much and how often, but some of the guidelines that are used are:
-- as much as they can eat in 20 minutes, once a day, or
-- two very small meals a day,or
-- enough flowers/leaves etc. that would make a little blanket to cover their shell once a day, or
-- having one day a week with no food, or
-- having one or two days a week with slightly less food
There are lots of guidelines but in the end it's all up to how much they are gaining. Of course when they are outside in the summer they are getting more exercise and they do tend to eat more.
It's hard to be more precise than that -- I would say just keep good records and try not to panic if they seem to be gaining too much over a short period of time, as it will probably average out in the long run if you control portions. Also, what you feed them can make a difference too. Weeds, flowers, etc. (you can see which ones are good on the plant database on our website) are better than any commercial food, as most of them (not all, but most) are too high in protein, and tortoises need a high fibre and low protein diet to form good muscle, bone and shell. It's also important to give them extra calcium and have a good UVB light so that they can grow strong shells.
You probably already have a good caresheet for Horsfields, but if not here is one:
https://www.tortoise-protection-group.o ... 014New.pdf
I hope I haven't gone on for too long - sorry if I have. Please don't hesitate to ask any questions you might have (I always had loads of questions when I first got mine). It would be lovely to see a photo of them too (we never get tired of looking at photos of tortoises)!
Nina