Winter annual weeds have become more prevalent in crop production fields over the last five to ten years. These species germinate anytime between late summer and early spring, but typically emerge in the fall, over-winter as small seedlings, and complete their life cycle in late spring. Some of this newfound abundance of winter annual weeds resulted from the adoption of conservation tillage practices (Wicks et al. 1994). Seedlings that were easily removed by pre-plant tillage in conventional systems are commonly allowed to become established and produce seed under no-till conditions where the soil is not disturbed. Winter annuals can also be problematic in conventional tillage systems if they are allowed to emerge after an early fall tillage. Another factor has been the development and use of glyphosate-resistant crops. In 2007, glyphosate-resistant soybeans were planted on approximately 91% of the soybean production acres in the United States (Anonymous 2007). Increased use of glyphosate POST in soybean has led to a reduction in the use of soil-applied herbicides (Anonymous 2003b) that provide suppression of winter annual weeds in the fall (W.G. Johnson, unpublished data). Because glyphosate can be applied to established resistant soybeans, many growers will delay treatment until after crop planting which allows the winter annuals to mature and produce seed. Finally, relatively mild winters in recent years have favored winter annual weed growth and survival (Krausz et al. 2003).

Impacts of winter annual weeds

Winter annual weeds can have a number of negative impacts on sustainable cropping practices. Dense populations of winter annual weeds can slow drying and warming of soil in the spring , the combination of which may lead to delayed planting dates and decreased yields. In conventionally tilled fields, the presence of winter annuals can increase tillage, labor, and fuel costs required for spring seedbed preparation. These weeds are also difficult to control in no-till production systems with late spring herbicide applications because of their advanced growth stage. Similarly, winter annual weeds can interfere with crop seeding depth and crop establishment in high residue areas. Winter annual weeds may also host various crop pests. For example, common chickweed is a good host for black cutworm (Sherrod et al. 1979), a common insect pest of corn. In addition, certain winter annual weeds have been identified as alternative hosts for soybean cyst nematode (SCN) (Venkatesh et al. 2000).

Implications of winter annual weed management

Winter annual weeds can be managed with herbicides, tillage, or cover crops. Although the presence of winter annuals in production fields may cause a problem, operations necessary to remove these plants can also have negative impacts. Tillage can be effective for control of winter annual weeds but it has also been linked to stimulating SCN population growth by aerating the soil (Koenning et al. 1995; Young 1987) and increased risk of soil erosion. Removal of winter annual weeds with a fall applied herbicide creates bare soil which theoretically warms and dries faster in the spring. These conditions have been observed to promote earlier emergence and subsequent management problems of giant foxtail, common lambsquarters, and common waterhemp (Young et al. 2001; W.G. Johnson, unpublished data).

Literature Cited

Anonymous. 2007. Adoption of Genetically Engineered Crops in the U.S. USDA - Economic Research Service Report. Internet URL: www.ers.usda.gov/data/biotechcrops/. Accessed November 2007.
Anonymous. 2003b. Indiana Agricultural Statistics - Annual Summary. Internet URL: www.nass.usda.gov/in/ind_agstat.html. Accessed October 2003.
Koenning, S. R., D. P. Schmitt, K. R. Barker, and M. L. Gumpertz. 1995. Impact of crop rotation and tillage system on Heterodera glycines population density and soybean yield. Plant Dis. 79:282-286.
Krausz, R. F., B. G. Young, and J. L. Matthews. 2003. Winter annual weed control with fall-applied corn (Zea mays) herbicides. Weed Technol. 17:516-520.
Sherrod, D. W., J. T. Shaw, and W. H. Luckmann. 1979. Concepts on black cutworm field biology in Illinois . Environmental Entomology. 8:191-195.
Venkatesh, R., S. K. Harrison, and R. M. Riedel. 2000. Weed hosts of soybean cyst nematode ( Heterodera glycines ) in Ohio . Weed Technol. 14:156-160.
Wicks, G. A., O. C. Burnside, and W. L. Felton. 1994. Weed control in conservation tillage systems. In P. W. Unger, ed. Managing agricultural residues. Boca Raton , FL : Lewis Publishers. pp. 211-244.
Young, B., R. Krausz, J. Young, and J. Matthews. 2001. Comparison of fall applied corn herbicides. Weed Research Annual Report. Southern Illinois University . pp. 107-114.
Young, L. D. 1987. Effects of soil disturbance on reproduction of Heterodera glycines . J. Nematol. 19:141-142.