Horseweed, mulestail, marestail...all common names for a very tough weed in our area currently.  The 2010 season has been a good one for the marestail population in the Doniphan and Brown county area.  Marestail are relatively easy to control in the small rosette stage.  Burndown programs that were sprayed earlier this spring such as the Metro / Prefix or Metro / Authority MTZ combinations with a full rate glyphosate worked rather well overall.  The later sprayed fields after the marestail started to bolt, cut rate glyphosate rates were used, and low gallon water spray rates are where the problems seem to lie.  There is no clear cut solution to controlling these problem weeds.  We are trying several different products currently such as FirstRate, Synchrony, and higher rates of glyphosates.  Understand that none of these programs will acheive 100% control so be realistic on your expectations.  
Problematic weed problems such as the 2010 marestail issues reinforce the importance of a solid preemerge program with residual that is sprayed timley, with the right rates, with the right gallons per acre, and with a growth regulator product such as Metro or 2,4-D.  We have to get these weeds before they bolt otherwise we fight and fight and spend and spend to get them under control.