Weeds can become a major problem in crop production when not controlled prior to planting. This is especially important in no-till systems. Weeds emerging in late summer through fall can overwinter and flower to set seed in the spring and early summer. Identifying weeds before planting corn can be difficult, especially if plants are in the rosette state of development.
Scouting fields before planting can identify what weeds are present, their population, and their growth stage.1 This information helps to determine what management practices and herbicides would be most effective for protecting the upcoming crop prior to planting and after establishment. For more information, read the Q&A article, Making Decisions Based on Weed Scouting in Corn.
Good weed control during the first four to six weeks after planting is critical for maintaining yield potential. A clean start helps to conserve moisture for the crop, promote good seed-to-soil contact, and helps prevent weeds from binding up planters. For more information on early and late burndown herbicide applications, read Benefits and Limitations of Early Burndown Applications or Benefits and Limitations of Late Burndown Applications.
Winter annual weeds have become a larger problem in no-till fields due to the limited use of soil residual herbicides and the reliance on postemergence weed control associated with planting herbicide-resistant crops. Marestail has become a difficult to control weed for no-till fields because there are many populations that are glyphosate-resistant.2 It is important to control marestail when it is small, which can require control as soon as equipment can enter the field.3
Plant debris in no-till fields can provide overwinter protection for germinated weeds, increasing weed populations.4 Fields that have been in no-till production for a few years may have a few scattered patches of weeds, which may not seem economically important to control. However, these patches may have enough time to seed out before a burndown application is made, creating problems in subsequent years.3
Common weeds that may be present in fields prior to planting corn in the South include marestail, henbit, Italian ryegrass, wild mustard, red sorrel, Carolina geranium, smartweed, lambsquarters, and pigweed.
Marestail, or horseweed, forms a basal rosette after germination and seedlings are covered with coarse hairs. Seedling leaf margins are toothed. Plants develop an erect, columnar shape and flowers are white to pink with yellow centers. Marestail is more susceptible to herbicide application when in the rosette stage. It is important to control marestail before it is more than 4 inches tall.
Henbit have square stems, purple flowers, and leaves attached with no petiole (Figure 2).
Italian ryegrass is an annual grass with flat, shiny leaves that can grow in singles or in clumps. This weed can be particularly difficult to control due to an increase in glyphosate- and ALS-resistant populations. Ryegrass is more susceptible to early control prior to tillering, and the bigger the plants, the more applications may be necessary.5
Wild mustard leaves are rough and lower leaves have irregularly-lobed margins and petioles. Upper leaves are smaller and do not have lobes or petioles. Flowers are bright yellow and have four petals (Figure 4).6
Red sorrel is a perennial weed that grows in acidic soils. Plans form as a rosette with flowering stalks that are reddish in color and has prolific rhizome roots (Figure 4).7
Carolina geranium has palmate leaves attached to petioles that are pinkish in color (Figure 6). Leaves and stems have hairs and flowers can be light pink to purple.5
Ladysthumb is a smartweed species, which are native annual weeds that germinate prior to planting corn. Nodes on the stem feel swollen and are surrounded by a thin sheath. Leaves are elongated and alternate and flowers are pink. Smartweed species may be found in wetter areas of a field.
Lambsquarters germinates before corn planting as very small plants with leaves that can be covered with a powdery coating. Leaves form in an opposite pattern and plants can grow almost 6 feet tall. Some lambsquarters may be resistant to ALS-inhibitor and Photosystem II inhibitor herbicides. There may also be reduced sensitivity to glyphosate.4
There are many species of pigweed that are difficult to control including, smooth pigweed, Palmer amaranth, and common waterhemp. Plants continuously germinate and should be controlled before reaching 4 inches tall. Species are difficult to distinguish at the seeding stage, and plants can grow upright or prostrate. Pigweed can have both male and female flowers or individual male and female plants. They are prolific and hardy seed producers and can regrow after tillage and some herbicide application. PRE and POST herbicide applications with residual activity are important for continuous control.8
For more information about managing winter annual weeds, read the Spotlight: Tillage Effects on Weed Management .
1 Pittman, K., Flessner, M., and Ackroyd, V. Start the season out right: plant into weed-free fields. https://integratedweedmanagement.org/index.php/iwm-toolbox/cultural-practices/plant-into-weed-free-soil/
2 Werle, R. and Sandell, L. 2013. Managing winter annual weeds starts this fall. University of Nebraska. https://cropwatch.unl.edu/managing-winter-annual-weeds-starts-fall.
3 Hartzler, B. 2009. Managing winter annual weeds in no-till fields. Iowa State University. https://crops.extension.iastate.edu/cropnews/2009/04/managing-winter-annual-weeds-no-till-fields.
4 Clay, S.A. 2016. Selected broadleaf weeds in South Dakota corn fields. South Dakota State University. iGrow Corn: Best Management Practices. https://extension.sdstate.edu/sites/default/files/2019-09/S-0003-39-Corn.pdf.
5 Barber, T. 2020. Winter weed questions already?. University of Arkansas. http://www.arkansas-crops.com/2020/01/09/winter-questions-already/.
6 Fishel, F., Johnson, B., Peterson, D., Loux, M., and Sprague, C. 2000. University of Missouri Extension. NCR 614. http://weeds.cropsci.illinois.edu/extension/Other/NCR614.pdf.
7 Rumex acetosella. NC State Extension. https://plants.ces.ncsu.edu/plants/rumex-acetosella/.
8 Steckel, L. Pigweed description, history and management. The University of Tennessee. http://www.utcrops.com/weeds/pigweed.htm.