May 26, 2011

With almost all of the state’s corn planted and more than 70% of the soybeans in the ground, these cool, wet days may be an ideal time to scout your planted fields. Early season scouting will help you identify weed pressure in the field so you can minimize potential yield loss.

Guide to Identifying Common Broadleaf Weeds Photos are courtesy of Craig Langemeier (UNL), Evan Sonderegger (UNL), Phil Westra (Colorado State University), and Weeds of Nebraska and the Great Plains published by the Nebraska Department of Agriculture.
	Giant Ragweed Seedling: Young leaves are opposite with rough hairs. Leaves have three lobes. The first pair of leaves are unlobed. General: Will reach about four feet in height. Leaves have three lobes, occasionally there will be five. It can be unbranched to heavily branched. Emerges: Late March to early June
	Russian Thistle Seedling: Leaves are very fine and needle like. They will be about one inch in length. General: A bushy summer annual that is heavily branched. It can grow up to four feet tall. Emerges: Early April to May
	Marestail/Horseweed Seedling: Young leaves are egg-shaped and have toothed margins. Young plants will form a basal rosette. Leaves are covered with short hairs. General: A winter or summer annual. Seedlings develop a basal rosette. Mature plants form a central stem that is covered in small white flowers. Emerges: Late March to June, also from late August to November
	Kochia Seedling: Young leaves are gray and covered in hair. They will form a basal rosette. General: An erect, heavily branched annual with small leaves. It can grow up to four feet tall. The plant color is blue green. Emerges: Early April to July
	Common Lambsquarters Seedling: Does not form a basal rosette. Leaves are toothed along the margin. Young leaves are covered in a gray, mealy coating. General: An erect summer annual that will grow to about three feet with profuse branching. Young leaves are gray. Emerges: Early April to June
	Velvetleaf Seedling: Young leaves are heart-shaped and covered with hair. When crushed, the stems emit an unpleasant odor. General: An erect summer annual that can grow to four feet in height. The leaves and stems are covered in soft hairs and it is smooth to the touch. Emerges: Early April to late May
	Common Sunflower Seedling: First leaves are opposite and elliptic and covered in short, stiff hairs. General: An erect summer annual that will reach about six feet in height, culminating in a large yellow flower. The flower is smaller than in crop sunflower. Emerges: Late March to June

Broadleaf weeds cause yield loss throughout the growing season by competing for sunlight, water, and nutrients. When scouting, you will see that many winter annuals have flowered and gone to seed. The focus at this time should be shifted to managing summer annual broadleaf weeds.

Broadleaf weed species such as kochia, marestail, Russian thistle, giant ragweed, velvetleaf, common sunflower, and common lambsquarters can reduce yields in both corn and soybean if not properly managed early in the growing season. Scouting for weeds will give you the upper hand for better control.

As weeds grow they not only compete for light and space, but below ground they will compete for water and nutrients. Early scouting will help you determine the best time to apply herbicide to be most effective.

Treatment

With early season POST applications, consider tank-mixing glyphosate with a residual product to suppress or control later germinating weeds. If weed pressure is severe early in the growing season, you may want to apply a herbicide to minimize yield loss and preserve water and nutrients for crop growth and development. This time frame represents the critical period for weed control. In corn, it ranges from the six-leaf stage (V6) to the nine-leaf stage (V9). In soybean, it ranges from the first to second trifoliate leaf stage (V1 to V2) until the first reproductive stage (R1) where flowers are developing at the first node.

Most broadleaf species can be controlled with a 32 oz rate (3 lb ae/gal product) of glyphosate when they are less than six inches tall. After weeds grow beyond this height, a higher rate may be necessary for complete weed control. However, reducing dependence on any single herbicide mode-of-action can help delay the evolution of resistance to that mode-of-action.

Glyphosate resistance has been documented in kochia, marestail, and giant ragweed in the Midwest. These species are found in many Nebraska corn and soybean fields.

The weed species highlighted in this article have already emerged and are growing. Using effective tank mix partners with glyphosate (POST to the weeds) is a good management approach to effectively control the emerged weed populations and appropriately manage for glyphosate resistance.

Craig Langemeier, M.S. Graduate Student,
Evan Sonderegger, M.S. Graduate Student,
Greg Kruger, Cropping Systems Specialist, West Central REC, North Platte
Lowell Sandell, Weed Science Extension Educator, Lincoln

Note

For more information on the critical period of weed control referenced in this article, see a 2001 article published in Weed Technology: Critical Period of Weed Control in No-Till Soybean (Glycine max) and Corn (Zea mays) by C. Halford, A.S. Hamill, J. Zhang, and C. Doucet.