If you are seeing weeds from the road in your cornfield you are losing yield.
Numerous research studies have proven that once weeds get to be two inches tall they can cause a yield loss. Weeds can grow one inch a day when they are small and there are good growing conditions. Often it is a day or two between when the decision is made to spray and the spraying occurs. If there is rain the delay can be longer. Some growers feel that since they are using glyphosate to kill weeds in their Roundup Ready crops they want to let most of the weeds emerge. The problem with this strategy, especially in corn, is if you wait that long you are sacrificing yield. An option with corn is to apply a residual herbicide with glyphosate to kill the next flush of weeds rather than waiting for them to emerge.
With soybeans there are fewer residual herbicides to tank mix with glyphosate. But the principle still holds that if you can see the weeds you are losing yield. You can spray early and if there is another flush respray with a low rate of glyphosate on its own. If you are spraying non-Roundup Ready beans do not let the weeds get too big. It is better to go too early and have to respray the next flush than go too late. Too often growers delay their post-emergent soybean spray and are dissatisfied with the control. The weeds are set back but not killed. When that happens they are even harder to kill with a respray.
And when spraying post-emergent herbicides on corn or beans, be concerned about crop safety. The corn crop has come through a stressful spring. This is not the year to “push the envelope” with non-registered tank mixes. BF