We finally got some decent rain last weekend, anywhere from 1-2 inches, which was truly a "million dollar" rain. It came at a time when the young corn and soybean plants really needed a good shot of moisture to keep them growing. It wasn't all bad to have to wait for rain as dry weather forces the roots down further into the soil to seek moisture and thus ensures a good solid base for the plant for the growing season. The first crop of alfalfa was put up in great shape and this recent rain will help the second crop and beyond. I was hearing reports that the first crop was somewhat short because of the dry month of May.
The next big job on the farm is weed control. The recent rain will result in increased weed seedling growth and weed seed emergence. Weeds compete with the growing crop and need to be controlled in a timely manner in order to maximize yields. It is also important to understand the principles of herbicides, especially those applied as preemergence herbicides. This means that they were applied after the crop was planted but before the crop germinated and emerged from the soil.
The big question is "Will preemergence herbicides still have activity after remaining on the soil surface for several weeks". The answer is generally "yes", and you can expect some activity once rain finally moves the herbicide down into the soil. Herbicide can be lost from degradation, but dry soil conditions will generally limit the rate at which this occurs. Minor losses from decomposition or volatility are possible with certain herbicides, but most are not susceptible to this. The bigger issue here is the emergence of weeds between application and rainfall. Once the weeds are emerged from the soil they can no longer be controlled by the premergent herbicide and a postemergence herbicide treatment will have to be used. Preemergent herbicides control weeds via shoot uptake. When the weeds begin to germinate, the herbicide has formed a barrier in the soil that the weeds reach, uptake the herbicide and then die. If the herbicide is not activated by moisture, such as rain, it does not control the weeds very well if at all. Most grass herbicides fall into this category, including all of the acetamide family. Herbicides that are taken into the weed plant by roots may still provide some control of emerged plants if rainfall occurs before plants have much size.
The general rule of thumb is that a half to one inch of rain must fall within 7 to 10 days after a preemergence herbicide is applied. This rain will move the herbicide an inch or two into the soil surface so that it will be in place to catch the emerging weeds. Weeds generally emerge within two weeks after the crop is planted and then will require another herbicide treatment. Dr. Jeff Gunsolus, Extension Weed Scientist at the University of Minnesota, indicates that weeds that emerge with corn must be controlled within two to five weeks after weed emergence to prevent a yield loss due to weed competition. Soybeans can tolerate 4 to 6 weeks of weed competition before they will start to suffer a potential yield loss. This period of crop tolerance to weed competition is decreased under high weed densities or environmental stresses such as low soil moisture or nitrogen levels. For example, common lambsquarters control by glyphosate (Roundup) herbicide is often poor due to lack of timely application, reduced rates of glyphosate or extended hot/dry weather patterns. So the recent rains not only provide moisture for the growing corn and soybean plants, but also help activate herbicides to control weeds which have a major negative impact on crop yields.