In last month’s newsletter we discussed the effects of fall herbicide applications on soil temperature and moisture at the time of corn planting. In that article, we indicated that some believe that fall herbicide applications can lead to increased soil temperatures at planting and that these applications will also allow the soil to dry out faster by the time of spring planting. However, we found no differences in soil temperature or moisture between fall or spring herbicide applications with regard to corn planting. In this article, we will discuss the first year of results from experiments that were conducted to determine the effects of fall and early spring herbicide applications on soil temperature and moisture at the time of soybean planting.

In the fall of 2004, we established three field experiments in central, northeast, and northwest Missouri. In both experiments, herbicide applications were made in the fall, 60, 30, and 7 days prior to soybean planting. Each timing consisted of the following four treatments: 3.5 oz. Canopy XL plus 1 pt 2,4-D ester per acre, 2.2 oz Canopy EX plus 1 pt 2,4-D ester per acre, 28 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 one-inch depth to record soil temperature every day from March 1 to May 31. 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.

Measurements of soil temperature at planting revealed that the untreated plots ranged from two to four degrees warmer than any of the herbicide-treated plots within each of the four application timings. However, two weeks after planting the untreated plots ranged from seven to 13 degrees higher in soil temperature than the herbicidetreated plots, regardless of the specific application timing. These results are actually opposite of what many might have expected (including us). Plots with dense stands of winter annual weeds left uncontrolled actually resulted in higher soil temperatures at planting and at two weeks after planting than in plots that were free of winter annual weeds due to the fall or early spring herbicide treatments. We believe that these differences might be explained by the differences in soil moisture that were observed in these experiments.

At the time of soybean planting, the untreated ranged from having volumetric water content levels similar to the herbicidetreated plots (central location), to having two percent less volumetric water content at the northern locations. However, the major differences in soil moisture came two weeks after planting, which as discussed previously corresponded to a period where major differences in soil temperature were also observed. Two weeks after planting the untreated plots had a volumetric water content which was seven percent less than the herbicide treated plots at all three locations (Figure 1). We attribute this response to the presence of dense stands of winter annual weeds in the untreated compared to the herbicidetreated plots, which served to "wick" significant amounts of moisture from these areas. Lower soil moisture present in the untreated plots two weeks after planting correlated with higher soil temperatures at this same time. We believe that the untreated plots may have had higher soil temperatures because they had less moisture present in the soil, which enabled them to warm up more rapidly when compared to the herbicidetreated plots.

Another factor which is likely to affect the amount of soil moisture present in untreated versus herbicide-treated fields is the amount of rainfall received throughout the spring. It is likely that lower amounts of rainfall in the spring will lead to larger differences in soil moisture between untreated and herbicide-treated fields. In springs where we have excessive amounts of rainfall, these differences may be minor or nonexistent.

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 soybean planting, as some have speculated. However, our results have shown that the removal of winter annual weeds with fall or early spring herbicide applications can cause lower soil temperatures and higher soil moisture when compared to untreated areas. In our experiments, this response was most noticeable at two weeks after planting. Our first year studies suggest that winter annual weed removal may have a significant impact on soil conditions. Additional experiments were initiated in the fall of 2005 and will continue through 2006 to determine whether or not these results are consistent across years.

By Nick Monnig
and Kevin Bradley
573-882-4039