ISSUE 3 May 28, 2009
PLANTING RESTRICTIONS FOLLOWING 2,4-D APPLICATIONS
Weeds present at planting time in no-till fields are commonly controlled by adding 2,4-D to glyphosate. Its use broadens the spectrum of weeds controlled, provides more consistent control during cool weather, and reduces selection pressure for glyphosate resistance compared to glyphosate only. The primary disadvantages of including 2,4-D are the added cost and the potential for crop injury.
Crop injury risk is minimized by following the planting delays stated on product labels. Ester formulations are recommended over amine formulations for pre-plant applications, due to a slightly shorter half-life and less mobility in the soil profile. Planting seed at the proper depth and ensuring closure of the seed furrow also is important in managing risks.
The restrictions regarding corn planting are based solely on injury risk and vary among manufacturers. For soybean, the restriction is based on both injury risk and residue tolerance, therefore they are uniform on all products.
Planting restrictions following pre-plant applications of 2,4-D. (Rates are based on products containing 4 pound a.i. per gallon. )
2,4-D ester - 7 days following 1 pt; 30 days following 1 to 2 pt
2,4-D amine - 15 days following 1pt; 30 days following 1 to 2 pt
2,4-D ester or amine: 7 days following 1 pt 4 lb/gal; 14 days following 1 to 2 pts.
2,4-D labels do not have preplant/preemergence options for other crops, including small grains. Glyphosate + 2,4-D applied as a preplant burndown in no-till has been used in ND for many years. I doubt growers knew it is not a labeled application. Some growers even consider using the soybean model (allow 7 days before planting with 1 pt/A or less of 2,4-D ester) for using prior to the planting of other broadleaf crops. If 2,4-D is used preplant/preemergence prior to any crop other than corn and soybean the user assumes all liabilty of the application.
RESIDUAL HERBICIDES NO MATCH FOR WET SOIL
Soil-applied herbicides can control many tough weeds but chemical products are no match in wet, soggy fields. A wet spring/summer has made it difficult for crop producers to use soil-applied, or residual, herbicides to their fields ahead of planting. Producers should rush application of herbicide treatments while fields are wet. For best results a good rule is - if it is dry enough to plant, it is dry enough to spray.
As their name suggests, soil-applied herbicides are sprayed on the soil before, during or just after crops are planted. While too much water nullifies their effectiveness, most residual herbicides need some soil moisture in order to work properly. The herbicides are activated by moisture within the soil and are then absorbed by weed seedlings. Weed growth stops or is stunted, leading to plant death shortly after it emerges. A residual herbicide will have activity in the soil anywhere from about one to six weeks after application depending on the product that you use. What you get is herbicide activity that you don't really see because you have fewer weeds coming up. The weeds that do emerge from that residual system are going to be easier to control with POST herbicides.
Other factors to consider are:
Using the appropriate residual herbicide on your worst weed problems and then saving glyphosate for the post-emerge cleanup treatment is a good way to protect crop yields and slow the development of glyphosate-resistant weeds.
Extension Weed Specialist