There are many different opinions that exist on the utility of fall herbicide applications and on what effect they may or may not have on soil conditions at the time of corn planting. Some claim that fall herbicide applications can lead to increased soil temperatures at the time of spring planting. Others contend that applying a herbicide in the fall to eliminate winter annual weeds allows the soil to dry out faster in the spring.

In the fall of 2004, we established two field experiments in central and northwest Missouri to investigate the effects of fall or early spring herbicide applications on soil temperature and moisture. In both experiments herbicide applications were made in the fall, 45, 30, and 7 days prior to corn planting. Each timing consisted of the following four treatments: 1.1 lb Princep 90DF plus 1 pt 2,4-D ester per acre, ½ oz Basis plus 1 pt 2,4-D ester per acre, 22 fl ozs Roundup Original Max plus 1 pt 2,4-D ester per acre, and an untreated control. Soil temperature thermometers were placed at a two inch depth to record soil temperature every day from March 1 to May 3. Soil moisture readings were recorded every two weeks at a 4 ½ inch depth beginning in early March and continuing until two weeks after planting.

Based on the first year of soil temperature results from these locations, very few differences in soil temperature were observed between any of the treatments at each of the four application timings. Additionally, the treatment differences that were present were somewhat variable between the four timings. The only consistent difference between treatments spanned over a period of ten days from March 28 to April 6, which is significant as this is the time period when many producers have begun corn planting in Missouri. During this ten-day time frame, the untreated plots ranged from two to three degrees Fahrenheit higher in soil temperature than any of the herbicide treated plots, regardless of the specific application timing. This result was certainly not what we expected, but we will be collecting a second year of data from more trials this year to see if we can draw any specific conclusions pertaining to the effects of fall and early spring herbicide applications on soil temperature at planting.

Similarly, few differences in soil moisture were observed between any of the four treatments or timings. Herbicide applications in the fall did not decrease soil moisture at the time of corn planting. However, at the Northwest location soil moisture readings two weeks after planting revealed significantly less soil moisture in the untreated plots than in plots treated with a herbicide (Figure 1). The untreated plots had approximately 10 percent less volumetric water content than any of the herbicide treated plots. We attribute this response to the presence of dense stands of winter annual weeds in the untreated compared to the herbicide-treated plots, which served to "wick" significant amounts of moisture from these plots. This response has been observed more consistently in our research with fall herbicide applications on soybeans (article coming in next IPCM newsletter), but our results in corn thus far have been quite variable.

One factor that is likely to have a major impact on this response is the amount of rainfall received at these locations throughout the winter and early spring. Last year, both of these locations received significant amounts of rainfall in the winter and early spring months, resulting in relatively high soil volumetric water content throughout the spring. This year, we have been especially dry throughout the month of February and on into the first part of March. If there are differences in soil moisture due to the removal of winter annual weeds with fall or spring herbicide applications, we feel that we are more likely to detect these differences during a dry year such as the one we are currently experiencing.

Overall, after one year of data our field studies have not proven that fall herbicide applications either increase soil temperature or decrease soil moisture at corn planting, as some have speculated. At this time we can only conclude that our data indicate that the removal of these winter annual weeds with fall or spring herbicide applications will have only a minor impact on the soil conditions that exist at corn planting. We will continue to investigate all of these factors in trials that were initiated in the fall of 2005.

Kevin Bradley
573-882-4039

Nick Monnig
573-882-6536