The Power of Residual
Extension Specialist Weed Science IPM
Department of Crop Sciences
The usage of Roundup Ready crops on almost every acre has led us to heavy dependence on POST only herbicide programs with no residual activity. This herbicide management strategy boasts of less time, less cost, and ease of use; however, these benefits are becoming not so appealing considering the consequences of early season yield loss, increased glyphosate costs, glyphosate-resistant weeds, and the associated limited number of herbicides left for weed control due to resistance.
One thing that has been available throughout the Roundup Ready era has been soil-applied residual herbicides. These residual herbicides are the key to maximizing yield potential in both corn and soybeans by preventing early season weed competition (Figure 1 and 2). Early-season yield loss is a hidden danger. We have estimated that a grower can lose 4 bu/A in corn yield for every inch in weed height over 4 inches; while soybean yield may only decrease by 1 bu/A for every inch in weed height over 6 inches. Residual herbicides also reduce weed density and decrease overall weed height allowing greater flexibility for timely POST applications. Many fields are infested with lambsquarters or giant ragweed which emerge early in the season. Usage of a residual herbicide on fields with these weeds will allow your POST herbicide application timing be more effective and control a broader spectrum of weeds. Otherwise, your POST application is going to be based on the heights of your biggest weeds, not the small ones emerging under the canopy of weed leaves. The use of a soil-applied herbicide in corn also has the added benefit of allowing the nitrogen to be available for your crop, and not your weeds. Soil-applied herbicides also provide another site of action for weed control, helping decrease the opportunity for herbicide resistance. If we develop resistance to PPO- and ALS-inhibitors in addition to glyphosate, we are left with only preemergence residual herbicides and tillage for weed control in soybean. The corn residual herbicide options both PRE and POST are greater, and also have the advantage no herbicide resistance in Illinois associated with growth regulator or bleaching herbicides. Of course, there is some hesitation in implementing the usage of a residual herbicide, especially preemergence. These products need moisture or tillage to be activated, they are selective toward certain weed species, and of course they cost money. The differences in weed control among the different soil-applied herbicide make choosing the correct one for your weed spectrum important. In addition, using the full labeled rate of a product is vital in providing the longest duration of weed control and greatest impact on weed height and density. We have not observed a single instance in our studies where the reduced rate of an herbicide has ever provided the same level of control and length of residual activity as a full rate residual herbicide.
The increased price of glyphosate and the high selling prices for corn and soybeans make recovering every bushel of crop reasonable. Therefore, the goal of every herbicide program is to maximize yield, whether through preventing early season weed competition or including a residual herbicide in your POST herbicide application to take care of late season competition and late emerging weeds such as waterhemp which will contribute seeds to the weed seedbank for next year.