The spring growth of winter annual weeds is underway. A winter annual weed is an unwanted plant in your field that germinated last fall, overwintered, resumed growth this spring, and will soon begin flowering and producing seed. There are numerous species of winter annuals weeds common in southeast Nebraska including henbit (figure 1), marestail, field pennycress, shepherd’s purse, and tansy mustard. Nebraska Extension has two practical publications for you to use including EC104 Identification of Winter Annual Weeds and EC130 2022 Guide for Weed, Diseases, and Insect Management in Nebraska.
Winter annuals weeds can negatively impact the growth and yield of corn, soybean, and other cash crops. Potential reasons for this negative impact include soil moisture and nitrogen use, light competition, allelopathy, and more soybean cyst nematodes and cutworms. However, in some cases, winter annual weeds can provide similar benefits as a cover crop in protecting against soil erosion, providing carbon back to the soil, and using excess spring soil moisture and nitrogen.
Local research has shown that delaying the control of winter annuals weeds in the spring can lead to significant yield decreases in both corn and soybean. A 2007-2009 UNL study conducted in Lincoln and Clay Center found that in 5 of the 6 site-years (2 sites per year for 3 years) that not controlling winter annual weeds prior to corn and soybean planting resulted in greater than a 5% yield loss and in 4 of the 6 site-years it exceeded a 10% yield loss. My 2010-2011 research in eastern Kansas determined across 14 site-years and nitrogen fertilizer rates that delaying winter annual weed control until near the time of corn planting reduced corn plant population by 526 plants per acre, early corn nitrogen uptake by 24%, corn ear leaf chlorophyll meter readings at silking by 3.4%, and grain yield by 7.6 bushels per acre, compared to the earliest herbicide application in November and late March.
You can avoid reductions in nitrogen supply and increase corn yields by controlling winter annual weeds as early as possible before planting. On fields with heavy pressure of winter annual weeds, delaying herbicide applications until late April and May could result in an additional 15 to 30 pounds N per acre being needed to achieve similar yields to those obtained when weeds were controlled earlier. If you get in a pinch due to adverse weather conditions and could not control winter annual weeds until planting, some extra nitrogen fertilizer could be applied as starter or side-dressed later to compensate. However, yield limitations from factors other than nitrogen were associated with delayed herbicide applications too.
Given herbicide availability and cost, you could just target those fields with the heaviest winter annual weed pressure now with pre-plant herbicide applications to decrease the probability of reduced corn and soybean yields this growing season. For more information, feel free to contact me at or 402-821-1722. Know your crop, know your tech, know your bottom line at croptechcafe.org.
