Some chemical company reps and others are stressing the need to get weeds knocked down, and to get a residual product on. Weeds got an early start in many areas that were unseasonably warm in March and early April. A lot of corn was also planted early. Even if a residual herbicide was applied, it may be a stretch to think it will still provide protection several weeks into the season.
One problem this year may be that since planting is so early in some areas, there may be late-season flushes of weeds that will be germinating after the chemical has run its course. Sequential applications or post programs may need to be designed around this reality.
Several studies at universities and other places have shown that weeds , especially thick stands of greases, that compete with corn early may be more damage to final yield than once thought. A Farm Progress/ Precision Planting study in 2011 was plagued with weeds in the second planting. The weed control program was to be all post, but rains delayed the application until the corn reached the V7 to V8 stage. In 2010 a similar thing happened, and the herbicide application was amazingly effective. Bill Johnson, Purdue University weed control specialist, warned that was a fluke, and he was proven right last year. The control using the same recipe wasn't nearly as effective in 2011.
The result last year indicated obvious yield losses in the replicated plot. Although weed control wasn't a factor being evaluated in the test, the range where weeds were the worst yielded the least, sometimes by a significant amount. In some cases the weed pressure was so intense early that even though the corn was planted in early June, which usually produces taller corn, the corn was actually shorter at harvest than normal, apparently due to stunting by early-season weed pressure.
The best advice from specialists now is to avoid early-season competition from weeds. The addition of resistance issues with weeds in many areas to various herbicides, particularly glyphosate, complicates the matter of post control, and makes it even more important to control weeds early while they are small. Keep an eye on early-planted fields to make sure weeds don't start popping up, even if you have already applied a residual herbicide. This may be a year when you must make adjustments on the go.