Crossing over logs and through brush are obstacles that you will most likely encounter while on the trail. Some horses may become worried when underbrush touches their legs and may buck or bolt in response. If you properly prepare your horse first crossing through the woods won’t be a challenge.
A great place to start training is to lunge and/or ground drive the horse over as many obstacles and in as many different places as you can. Many people get into trouble when they try to do these exercises on the trail without teaching them first in an arena. Horses need a foundation. Start with the least scary/smallest obstacles first and work your way up. Do all of your training on the trail alone at first or with one other calm, experienced trail horse. As having your horse follow another experienced horse over the obstacle that he is worried about can help him gain confidence. Once your horse is confident on the trail alone then start adding horses. Pick the people you choose to ride with carefully. You don’t want to go out with other young or inexperienced horses, instead ride with older horses who are experienced and will give you both confidence. Keep your trail rides slow at first. Don’t go out with other riders who want to gallop every chance they get or who will leave you behind without noticing. Don’t train on an obstacle when the other horses are moving on without you. Your horse will only become more anxious and unable to focus on you. Instead have everyone stop and wait while you train before moving on. Your goal as a rider and trainer is to teach the horse that no matter the different obstacles and environments he is to behave at all times.
Start by setting up poles on the ground then lunging and/or ground driving the horse over them at each gait. Progress to sending him over raised poles then small jumps to allow him to practice his foot work. This gives the horse the chance to get used to going over obstacles without your weight on his back affecting his balance, and gives you more control over him on the ground. Once he can go over obstacles in the arena take the horse out on the trail and practice crossing and jumping over different sized logs on the lunge line and while ground driving. Be sure that you ask the horse to cross obstacles slowly too, instead of jumping him over everything. Teach the horse to calmly walk over logs first then speed him up; also transition between gaits often. Ground driving is a great way to get horses used to going out on the trail without riding. You can expose the horse to all kinds of new objects and environments from the safety of the ground first with nearly the same aids you would be using while riding.
When the horse is confident on the ground, start riding over the poles and small jumps in the arena. Once you are both comfortable with that, head out and practice over small logs. Walk him over then bend him in a circle and go the other way. If you aren’t sure how your horse is going to react to an obstacle grab some mane and lean forward a bit in case he goes to jump it. Once you are comfortable, start jumping a few logs from a trot. The horse should stay slow between jumps, if he starts getting quick bend him around in a few circles getting him to listen to you before continuing. You can also stop and back him up every time he moves faster then you want. If the horse keeps jumping or rushing an obstacle make him keep going over it back and forth until slows down then move on. If the horse avoids the obstacle completely by veering off for example to the left, start bumping lightly with both legs on the horse’s sides while at the same time turning him back toward the log to the right. Once the horse is facing the log again stop bumping with your legs and let him go forward. It is important that you turn the horse the opposite way that he wants to go instead of letting him go the left and circling back around. By letting him turn away you are telling the horse that it’s ok if he changes directions on his own you will just go with him. When you turn him the opposite way he wants to go you help balance him out by turning him the opposite way he wants every time, eventually the horse will stop trying to turn on his own and will stay straight and listen to your cues. Keep bumping the horse’s sides with your legs each time he turns away, turn him back towards the obstacle in the opposite direction he wants to go, then stop bumping your legs when he is facing the obstacle. By doing this you are making it hard to avoid the obstacle and making it easy to cross it by using pressure and release.
Desensitize the horse to the sensation of brush on his legs before riding through it on the trail. Do the basic desensitizing exercises first, which teach the horse to be comfortable with the stick, ropes, and a plastic bag around their legs. Then increase your desensitizing by using an empty grain sack to gently slap all over the horse’s body. Start on the withers and work your way down his back and around his legs. Keep slapping the bag against the horse until he stands still, then stop and remove the pressure. Rub him then start again. When he isn’t moving keep desensitizing him with the bag until he shows you a sign of relaxing: such as lowering the head, blinking, relaxing a hind leg, and licking. Only when he does one of those behaviors will you stop desensitizing and let him stand. Slap the bag gently at first then increase the pressure only so the bag makes more noise and he feels pressure like he would if tree limb were to hit him but not too hard.
When the horse is ok with the desensitizing at home, take him out on the trail and have him walk through some brush while you ground drive or lunge him. Try to find some pretty dense brush and have him walk back and forth through it. If the horse gets nervous steer him back toward the brush and have him keep walking through it until he relaxes and realizes it isn’t going to hurt him. Practice on the ground until the horse is fine with the brush hitting his legs. Then start riding him through it just as you did on the ground. If he gets worried, turn him back toward the area he was in and continue to circle until he relaxes then move on. If at any time the horse gets too hard to handle get off and lead him through it until he relaxes. Go back to doing groundwork if you need to until the horse becomes more confident.