What are your favorite exercises for suppling a horse? Are there any supling exercises that you can do out in the field or hacking or do you only work on suppling in the ring?
start with long and low- trot poles or cavallettis help here. Another method (as mentioned) would be lateral movements: shoulder-in, shoulder-out, ronvers, travers, leg yeild, ect. Above all- Transitions, baby! Incorporate them into EVERYTHING!
It is easier for the horse if the dressage never ends. Think of the trail as a long narrow dressage ring and do all your lateral exercises on the trail. Hills are great for developing gaits. Uphills are for lenthening strides which progresses to medium and then extended gaits. Downhills are for shortening strides which progresses to collection and eventually piaffe.
Walk Trot Transitions, making sure you don't just let them putter down and fall into a walk, and that you prepare by collecting the walk and keeping them up before asking for the trot. Make them push off, not just start slow and then get active.
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Are you gonna get back on or just lie there and bleed?