The only way to know is to watch the heart rate. My horse is far more efficient in a canter than in a trot. Trotting is difficult for her and I have to constantly adjust her to maintain the speed and rhythm. She gets into a canter and could just float for miles without breaking gait. Theyāre all different.
I agree its important to train the trot - and I do. But all else being equal, my horse will always choose the canter over the trot, and her heart rate and recoveries will always be better during and after a long duration of canter than a long duration of trot.
The distance riders Iāve ridden with have always done what the terrain dictates. I have an endurance book that says āNever walk when you can trot and never trot when you can canter.ā I donāt think that 100 mile races arenāt cantered because the trot is more efficient/easier, but because terrain dictates that you cannot. How safe/smart is it to go careening around switchbacks and narrow mountain passes, climbing over rock at a canter?
Look at FEI endurance - the horses canter and gallop 75% of the 100 miles and they can do 100 miles in 9 or 10 hours. Or less! Theyāre essentially flat track races without the intense climbing and rocky terrain that we have in the States.
I think terrain dictates trot or canter much more than anything else.
I am a numbers geek and I study the finishing times of rides. Sometimes a horse will complete a 50 in 5 hours, and sometimes a 50 takes 11 hours. One ride will be flat and sandy, fast, easy terrain. Other rides are hilly, rocky and full of elevation changes. So when the going is good, the horse canters a lot more and makes good time. When the going is rough, the horsehas to walk and trot more and so the finish time is slower.
When I see other endurance riders on trail, most of the time theyāre cantering. But I ride at the front of the pack where the going is faster anyway. Those horses that are out there to top 10 or win are cantering wherever there is good trail to canter. Sure, some horses are just big powerful trotters and they cover a lot of ground. Iāve seen those too. But many of the horses are in a working canter and theyāre moving. Each horse is different.
And FWIW - I agree with letting the horse pick the gait. My horse basically does whatever gait she wants, when she wants. When the trail opens up and thereās a stretch that can be cantered, she just slips into a canter until the terrain becomes such that she has to trot. I donāt mess with her - I let her do her thing. She knows what sheās most comfortable with. I agree that I pick the speed though. She has no idea if weāre only going 1 mile or 100. So that part of the decision is mine, but the gait I almost always leave totally up to her. When she is tired and wants to walk, sheāll walk. She takes care of herself. When sheās cantering and gets tired and wants to trot, sheāll trot. I donāt push her. Iāve always ridden her that way and her recoveries are awesome, heart rates are awesome, and sheās very easy to fit up and stay fit.