No matter what trainers and clinicians say, in my opinion, you can never completely desensityze your horses to everything they could possibly encounter. It just ain't gonna happen. I think sometimes, the illusion is there that desensityzing takes care of all the spooky objects and sounds that can be encountered. In the real world, there are too many variables.
I have worked on the desensityzing issue with my two geldings ever since I got them four years ago. They've been through the plastic bag lesson. They've had whips smacked beside them. They've been introduced to leaf blowers, motorcycles, gunshots, lawnmowers, tractors, jumping up and down at them and they always come through those "lessons" with flying colors.
Yesterday, I walked around the side of the barn, while they were standing in their stall/corral area, dragging a large tarp I was putting away for the season. You would have thought I was a mountain lion aiming to pounce on them. Good grief!
The funny part, I thought, was that I had been very noisy as I walked, dragging the big green tarp, which was the noisy part, because I knew they were there. They had seen me come outside. They knew I was in the hay building because they can peek around the other side of their enclosure. I'm pretty sure they would have assumed I was getting them some hay.
At any rate, I thought it would be a good lesson for them. I also thought that the sight of me having possession of the tarp would have made a difference in their flight response. Heck no! They both jumped! Spirit ready to fly out of there but then turned, and snorted as I walked up to the gate with tarp in hand. Bo, the ever lazy and not so skittish one, stood in his spot and snorted. So, I took about 15 minutes to let them see the giant green monster was not going to eat me or them.
Finally, after about 10 minutes, they both put their noses on the monster, sniffed around it, nuzzled it and all was right in their world again. This was not the first time they'd seen a tarp, they'd just never seen or heard one like this, I'm guessing. In the past, I had laid a tarp on the ground and had them walking over it. Different situation this time.
Desensityzing to common noises and various spooky objects is great but you have to remember, once again, the whole sensitivity issue comes down to horse personality. You can never cover everything, and some things just come up without warning. Be aware of how your horse reacts, if he's sensitive or has more of a "so what" attitude. Between my two guys, Bo is a laid back, lazy boy. Not much startles him and he is often slow to respond if it does. Spirit, on the other end of the spectrum, has always been more sensitive to sounds and environment. I am aware of their differences.
Loggers have been taking down trees up in the hills across the road from our property. Yesterday was especially noisy. Between the trees booming with thunderous thuds as they hit the ground and the numerous chain saws blaring all day, the horses were a little edgy anyway. They heard all the commotion, but didn't know exactly where it was coming from because the loggers were up on a ridge. Sound echos and travels down here in the valley so most of the sounds were amplified all day long. I think some of that had an affect on why they were so jumpy when I appeared with the noisy tarp. I'm going to drag that tarp out again today, see how they act towards it.
Desensityzing is something that does need to be done, but you have to remember, it can only be accomplished within reason. You have to account for your horse's personality type.
If you're out on a trail, you have to remember how your horse reacts to external stimuli. What he sees or hears one way, one day, he may see or hear in a completely different way the next time, depends on his personal experiences and his personality.
Sometimes, these issues can be very frustrating but keep in mind, horses are animals, and they have a different way of looking at the world. In their minds, they're life depends on their reactions. If it's something they don't know, or haven't experienced, it may kill them. Of course, when you think about it, not much different than as humans, our choices can make or break us every day too, we just have the capacity for reasoning the outcome. We know about the environment. It's out job to translate for them sometimes.
Great book that I'm going to re-read this winter, Animals in Translation by Temple Grandin and Catherine Johnson. I read it a few years back before I'd had much real life experience with horses. I think I'll get more out of it this time around. Some of the ideas discussed in the book: language is not a requirement for consciousness, animals do have consciousness; the single worst thing you can do to an animal is make it feel afraid, and animals have their own set of skills and animal genius. I've noticed this book is often recommended reading for animal behavior interests.
Horses will surprise you, somewhere, some time, down the road. The better you try to understand them, as individuals, the better off you'll be when coping with the surprises thrown your way.