Consider herbicide carryover potential before planting wheat
With the extreme drought we have experienced throughout the Midwest this season, there is no question that the risk of herbicide carryover to fall-seeded crops will be higher than normal this year. Due to the poor corn and soybean crop, many are considering planting more winter wheat this fall, while others are enquiring about the possibility of a fall-seeded forage grass crop as an alternative feed source. While it is difficult to predict exactly when or where herbicide carryover injury might occur, there are several factors that will influence the likelihood of herbicide carryover occurring to these crops. These include the type of herbicide applied, the rate of herbicide applied, the time during the season that the herbicide was applied, the soil pH, and most importantly the amount of rainfall received since the time of the initial herbicide application.
The amount of rainfall received during the course of the growing season is perhaps the most important factor that will influence the likelihood of herbicide carryover injury to wheat or forage grasses planted this fall. Soil moisture is critically important for herbicide degradation, especially in the first few weeks after herbicide application. If adequate rainfall is not received during this time period, then the chemical and microbial processes responsible for herbicide degradation are reduced significantly and the herbicide molecules are more likely to become bound (adsorbed) to soil particles. All of this results in less herbicide degradation and increases the likelihood of herbicide carryover injury. Some herbicides are also degraded chemically in a process called hydrolysis. Hydrolysis is a reaction of the herbicide in question with soil water; therefore when soil water is limited, chemical hydrolysis of the herbicide is also reduced.
Another very important factor that influences the likelihood of herbicide carryover is the type of herbicide applied. As a general rule, corn or soybean herbicides with residual soil activity have the highest potential for causing carryover injury to wheat or forage grasses that may be planted in the fall. This is because residual herbicides are designed to remain in the soil profile for a specified period of time in order to prevent weed seedling germination. Tables 1 and 2 provide a list of some of the most common herbicides applied in corn and soybean production and their corresponding rotational intervals before planting wheat or forage grasses in Missouri.
In fields where corn was the previous crop, triazine herbicides are of the greatest concern in terms of herbicide carryover injury to wheat. These include atrazine or any of the many prepackaged herbicide mixtures that contain atrazine as one of the active ingredients (Bicep II Magnum, Degree Xtra, Guardsman Max, Harness Extra, Lumax, Lexar, etc.). As illustrated in Table 1, it is important to note that atrazine or any of the atrazine-containing products DO NOT allow wheat or forage grasses to be planted in the fall following a spring application, although in some years and in some areas of Missouri certain farmers choose to plant wheat following their corn crop. With the extreme drought we have experienced this year, any wheat planted after a corn crop that has been treated with atrazine this season will be at risk for atrazine carryover injury.