I would approach things from the ground up. Hand walk him to the barrels, make him check them out. If he starts to tense, stop and let him look. Praise any calm behavior regarding the barrels. Put treats on them and let him find them, make the barrels a calm influence. When he's nosing around them and actually touching him, praise him as well. And then end on a good note. Patience is really a key factor because each time he gets a frustrated reaction from you he's reinforcing that the barrels caused you to be tense, which is a reason to be scared of them.
The same with the cows. Hand walk him toward them and make him stand and look at them. Don't allow the rear/run reaction. Once he starts to get tense, stop and just let him look and if the wind is in the right direction, let him smell the cows. When he starts to relax, approach closer and when he tenses, repeat. Continue until he's as near to the cows as you need him to be. If he's relaxed enough to graze, perfect. Patience, again, will win out. He takes his cues from you so if you get angry, all he reads is tense which makes him think that his " herd " should be scared of whatever is causing the reaction. This will also go a long way towards teaching him how to spook without his out of line reactions like rearing or running off. Make him face the object and be still until he calms down.
That's what I would do anyway.