Early Season: According to research from the University of Minnesota, three to four inch weeds have the capability to take three bushels per acre every day off the high-end yield potential of your corn crop. By letting those weeds go for two weeks at the beginning of the growing season, we’ve lost 27 to 29 bushels/acre in yield potential! That’s not a good way to start the year. The cost of a common pre-emergence corn chemical program runs about $18-$24 per acre. This shows us that the economics favor using a residual pre-emergence program to effectively manage weeds!
Mid-Season, Plant Development: As we move later into the growing season, the importance of a clean field continues to be significant. At the V3 stage, leaf orientation is being determined. So, if there is competition in the row, the corn plant will turn it’s leaves and will not try to intercept the light between the rows. Through the process of photosynthesis, the plant continues to grow through the amount of sunlight it is able to take in. This is an important developmental stage, because ultimately sunlight makes corn. The diameter of the corn ear is being determined at the V5 stage, so if there is any competition at this time it can be devastating to the final yield. These two-time periods are crucial to the corn plant development for protecting yield potential.
Mid-Season, Nutrient Competition: The second direct effect is the yield loss from the competition for nutrients and water. If the weeds are left uncontrolled through the entire growing season we can have over 50% yield loss, but even at 6-8 inches tall we can see a 10-20% reduction. Of course, the yield loss can be higher if fertility is low or it’s a dry year.
In times of low commodity prices, it might be tempting to cut costs, but if we cut chemical costs we can create long term problems – the residual weed seed can haunt you for decades! For example, lamb’s quarter seeds are viable for over 20 years. What was saved in one year, will become a cost for years to come.
Here are a few opportunities for managing weed problems effectively:
Combination of pre- and post-application herbicides: Two applications obviously equals two passes and two costs, so how does this become cost effective for you? The primary reason that we see increased yields with two applications is because not all weeds emerge at the same time. For example, lamb’s quarter will start growing when the soil temp is about 40 degrees and will be done by late June. However, water hemp will keep germinating all season long. Obviously, trying to target killing all of the weeds with one application with no residual is impossible.
Switching modes of action year after year: Switching modes of action will greatly extend the life expectancy of a given chemistry. Research has shown that if the mode of action is changed or rotated each year the life span of a chemical is about tripled. Furthermore, recent research by the University of Minnesota has even shown that the best practice is to use at least two modes of action. If this approach is used you not only get a better weed kill but also extend the life of a chemistry by up to 4 to 5 times.
Consider narrow row spaces: Using narrow rows is another great way to get the rows to canopy faster, and once the row has canopied the germination of weeds almost completely stops. As a general rule:
In conclusion, controlling weeds all season long may seem impossible, but the one thing that saves us each growing season is that eventually the corn and soybeans grow tall enough to close the row and then the majority of weed germination and growth stops. One thing that can be done to speed up that process is to switch to narrower rows. It will allow for faster and quicker canopy of the crop.