Perhaps the psychological effect of having the bushes mowed down in front of the advancing troops may have been desired, this in order to gain time for troop placement and gain of a clear field of fire.Exploding canister would have been an appropriate tool for this job. I'll do some more digging in order to get a clearer picture. It's my understanding that Gen. Forrest wanted to flank with his cavalry in this action, partially to avoid charge through undergrowth.