Its winter but that doesn't mean training the ponies is less rigorous. Winter or summer, they are taken outside for morning and afternoon session to improve their fitness and stamina. Also because for the majority of their time in character they are kept in isolation, it is good for them to see the outside world. They enjoy the air and the view, it must be better for them to see trees and beautiful woodlands than the stark reality of their stable blocks.
Not that they are allowed to simply enjoy themselves, weight training is a favourite with the handlers. The ponies must not allow the heavy steel ball to touch the ground, if it does, then a caning will be the reward.
1. HDRI photo
2. White ground plane
3. Layer on ponies head for the Slave brand
4. Reality render
This leads to a great image with the forest reflections on the steel balls and the shadow on the ground. Expressions and pose are good for both the mistresses and ponies, to the point where I have nothing to add.
The only thing I could offer as food for thought is the ground and how it interacts with the trees. The ground looks too white and glossy and maybe having a snow texture applied to it would make a difference? For example
With the ground and the trees look at the straight line where they meet, this is a difficult one to offer suggestions as you could just have a photo background and lose the shadow, or have a ground plane which might not join perfectly.
Did you consider using a sun light for this scene? Just a thought it might have added some good shadows? Or looked awful!
Sounds like a fair and well earned reward