Heavy weed infestation could have resulted from heavy rains that neutralized some herbicides or hurt canopy development. Weeds that emerge after the V3 stage in corn are at a disadvantage, and their impact may be minimal unless they completely cover the soil. Iowa State Extension weed specialist, Bob Hartzler, says 3-4 in. weeds in 30 in. corn do not warrant a spray.
This may be one year when you have weedy fields at harvest time, but Hartzler says that may be something you just have to ignore for now. He says late emerging weeds will have minimal impact, but may produce a lot of seed, and create a dense weed mass next year. If you are aware of such a potential problem, create a 2009 weed management plan.
If you had no chance to apply pre-emergent weed control, short corn should be treated as soon as possible to avoid further yield loss from weed competition. Your weeds may be 5-6 in. tall, and a 5-10% yield loss on 150 bu. corn is worth $90 on $6 corn, and that cost estimate is increasing says Illinois Extension weed specialist Aaron Hager.
Hager warns about potential problems with applications of post emergent herbicides to corn that may be older than the product label allows. Its height may be short, but it may be physiologically older, and he says do not apply the product if the corn is too old. Hager says if any tank-mixed products are used; follow the most restrictive product label.>