The steppe tortoise (Russians) prefer the broad-leaf greens and weeds over grass. You can start with a base of Spring Mix, and to that, add whatever edible weeds you can find from outside.
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As mentioned above, the bulk of a steppe tortoise's diet (as in all Testudo species) should consist of a variety of leafy greens. I feed my guys the following:
Romaine lettuce
Prickly lettuce (wild)
Endive
Chicory
Dandelion (wild)
Arugula
Kale
Collard
Mustard greens
Occasionally they also browse on weeds like spotted spurge. Sometimes they eat grass as well. They also like flowers. Once a month they get a treat of fruit, like bell pepper, cucumber, tomato, grape, raspberry, or strawberry. They may also eat invertebrates like snails, slugs, or rolly-pollies from time to time.