Consider Herbicide Carryover Potential before Planting Wheat or Forage Grasses this Fall

Published: August 6, 2012

With the extreme drought we have experienced throughout the state 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.

In fields where soybeans were the previous crop, the likelihood of carryover injury to wheat is lower, but still possible in a year with as little rainfall as we have experienced. There are generally fewer residual herbicides applied in soybean, but that trend is changing. Also, as a result of our glyphosate-resistant waterhemp problem throughout the state, the herbicide fomesafen , which is the active ingredient in Flexstar, Flexstar GT, Rhythm, and Prefix, has now become a very common post-emergence herbicide of choice in soybean. Fomesafen-containing products have a 4-month wheat replant interval and in areas that have received little to no rainfall following application, fomesafen carryover injury to wheat or other forage grasses can be a concern this year.

The rate of herbicide applied and the timing of the herbicide application are other factors that influence the likelihood of herbicide carryover injury to wheat or other rotational crops. Simply put, the higher the rate of herbicide applied and the later the herbicide application was made, the greater the chance that some of the herbicide will remain to cause carryover injury to wheat. For example, if Flexstar or Flexstar GT applications were made later in the season to control glyphosate-resistant waterhemp in soybean, these sites are more at risk for herbicide carryover this fall.

The soil pH can also influence the likelihood of herbicide carryover injury. This is most often a concern with ALS-inhibiting herbicides, especially those containing chlorimuron or Classic (Valor XLT, Envive, etc.). With soils that have a high pH, some ALS-inhibiting herbicides as well as atrazine are more likely to persist and carryover.

In fields where there is a high degree of concern and/or uncertainty about herbicide carryover, one way to obtain more information is to conduct a soil bioassay. This is a simple test that can be done to ensure you don’t waste a lot of money by planting an entire field and then observe that the entire stand is injured as a result of herbicide carryover. To conduct a soil bioassay, gather several soil samples from across the field in question several weeks before you intend to plant your fall-seeded crop at that location, then take those soil samples and mix them together and place the soil in some kind of greenhouse flats or pots. Plant your wheat or forage grass seed into these pots and wait for the seedlings to germinate in order to observe any signs of herbicide carryover injury that may be present. In order to have a comparison, it will be important to follow this same procedure at the same time with soil from a location where you know there are no concerns with herbicide carryover.

Ultimately the best practice is always to follow the label of the herbicide product(s) that you have applied, but this year with the drought we have experienced it will also be important to consider each of the factors discussed above in relation to the field(s) in question. If several of these factors indicate a high probability of herbicide carryover, then it is a good idea to abandon the field until next spring and rotate to another location where the probability of herbicide carryover is not as high.