Rascal,I spray pastures in the early Spring to control those thistles and bitterweed. I have neighbors just over the fence that grow cotton ,corn and soybeans so I have to be very careful with drift-- if I wait too late in the season to spray. This kills my effectiveness in many cases because lot of weeds aren't even up by then. I do try to use a spray that has residual but even so it is only effective for so long. Enter the weed wiper.. As you say it is talked of like it is the thing to get and I guess it is, if you have tall weeds or sensitive crops to prohibit spraying. My wiper would be effective on tall plants- I have no doubt of that.... and do have pictures showing obvious missed runs of goat weed and such. Certain weeds absorb better than others. The wide stems of Johnson grass would be a good candidate.... One problem that appeared last year was that I let the weeds get really tall and they laid over as I wiped and prevented optimal coverage and when they laid over, got roundup on the bermuda..
The smut grass was what I was really after though and the crown is short and down with the good grass and the "spikes"above are tough.I was excited by the brown clumps of it that appeared a few weeks later after a 2 way wiping, but they crowns greened back up and "came back from the dead" The only luck I had with the smutgrass was to use 2 1/2 gallons of round up per 10 of water and then heavily wiping in 2 directions......This way, at the end of the season, I had a few more "dusty dead" clumps...
Our county ag office has one available to "borrow" on a reserve it basis. Maybe you could do that before you buy one.
Feel free to ask any questions .. bordercollie
edit: I think a weed wiper is much, much better than another type I have hanging in the barn. The "other type" is a hollow pvc tube with ends and a canvas covering.... I hope I can get $35 for it and I paid a $100- used it once about 10 years ago..... ok for johnson grass but that is pretty much it. I also found directions for making you own rope wick .
The rotating wiper is kept wet with a 12 v sprayer under the hood and just above the roller. I wanted this unit because I had tried a homemade one and it worked good on many things but not the smut grass. . Monsanto use to have plans for a simple wiper under the heading "How to build a pipewick wiper" but again I get better results as much land as I need to cover using this rotowiper. I have a Spiedel wiper that was not what I expected and so I got discouraged with it. ...but with some applications I guess it would have been o.k. just not for me on this uneven terrain. The rotowiper has wheels and goes with the flow 
And I just push the button and dispense the herbicide on the roller when the lather film starts lacking. (Soap is added to the tank and A shaving lather type consistency Keeps it from running off the plant so quickly.) We have had a severe drought or I would have used the wiper more. Our ponds are actually drying up Thanks, Bordercollie .
