Weed research in western Nebraska

By Dr. Robert G. Wilson, Professor of Agronomy and Horticulture

Panhandle Research and Extension Center

Weed science research was initiated at the Panhandle Research and Extension Center (PREC) in the spring of 1975, when Robert G. Wilson joined the faculty at the center. This new position was created to meet the demands of producers in western Nebraska for weed control information for irrigated crops and to explore methods for suppressing Canada thistle. The need for new and improved thistle management techniques caused producers across Nebraska to lobby the legislators in 1974 for the creation of two new weed science positions, one at Scottsbluff and one at Clay Center.

Over the past 35 years weed control research has been conducted for sugarbeets, dry beans, alfalfa, corn, chicory, sunflowers, potatoes, onions, garbanzo beans, wheat, pasture, and rangeland.

Research Highlights

Irrigation canal.

Weed Seeds: The system of dams, canals, and laterals that distribute water throughout the North Platte River Valley also provide a conduit for the dissemination of weed seed. Seed of 77 different plant species were collected from surface irrigation water over a 2-year period in the Farmers and Enterprise canals. Pigweed species were the most common seed found in irrigation water, but barnyardgrass and lambsquarters followed. During a typical irrigation season surface irrigation water had the potential to introduce over 20,000 seeds per acre into irrigated fields.

Canada thistle roots

Canada Thistle: Canada thistle is a perennial weed that develops an extensive underground root system that allows the weed to spread and overwinter. Root carbohydrate reserves consist of free sugars glucose, fructose and sucrose, and fructans with varying degrees of polymerization. The concentration of root carbohydrates undergo seasonal changes which allow the plant to tolerate environmental stresses. For Canada thistle to successfully survive freezing soils, the carbohydrate composition of roots changes as winter approaches. Long-chain length fructans break down to form kestose, nystose, and sucrose which allow the roots to survive in frozen soils. Fall application of herbicides at the occurrence of the first frost interrupts carbohydrate metabolism and results in increased herbicide efficacy.

Chicory roots.

Chicory: In the fall of 1994 researchers at the PREC were approached by Western Sugar Company to explore the possibilities of growing chicory in western Nebraska. This project evolved over the next 16 years as a major research and extension effort. Chicory is a root crop like sugarbeets, but is a member of a different plant family and hence is not affected by the same diseases that attack sugarbeets. Chicory seed is planted in mid April and can be harvested from mid September through November. Root development increases very rapidly in the fall. On Sept. 1, root yield averaged 15 tons per acre and by Nov. 15 had increased to 27 tons per acre.

Chicory roots contain 20 percent storage carbohydrates as fructans, which can be used as functional food ingredients. Early-season weed suppression is one of the major obstacles in growing the crop. Weed control research identified two herbicides, imazamox (Raptor) and rimsulfuron plus thifenosulfuron (Basis), that could be applied early in the growing season for selective weed control in the crop. Results from research indicated that chicory can be successfully grown in western Nebraska.

Roundup Ready vs. conventional sugarbeets.

Weed Control in Sugarbeets: Weed control in sugarbeets has been the nemesis of the crop ever since the crop was introduced in Nebraska in the early 1900s. Migrant labor toiled in the fields to remove weeds for the first 80 years of production. Beginning in 1990, the concept of using multiple postemergence herbicide applications nicknamed the half-rate was developed at the PREC. This weed-control program became very popular and reduced the need for hand labor. The major drawback to the program was early season sugarbeet injury. Beginning in 1998, research was initiated on glyphosate (Roundup) tolerant sugarbeets. Glyphosate applied two or three times at 2-week intervals, beginning when weeds were 4 inches tall, provided excellent weed control, yield, and net economic return. Compared to conventional sugarbeets treated with conventional herbicides, glyphosate-tolerant sugarbeets treated with glyphosate had less early season crop injury and a 19-percent increase in sucrose yield per acre. During the 2009 cropping season, glyphosate-tolerant sugarbeets were planted on 99 percent of the sugarbeet acreage in Nebraska and sugarbeet root yields for the 2009 growing season reached an all-time high of 25 tons per acre.

 Research plots in long-term glyphosate resistance study.

Glyphosate-Tolerant Weeds: With the advent of glyphosate-resistant crops in the mid 1990s and their rapid adoption by U.S. farmers, concerns have arisen about weed resistance to glyphosate. In the spring of 1998 a long-term experiment was initiated at the PREC to measure weed populations following multiple applications of glyphosate or alternating glyphosate with nonglyphosate in continuous corn or in a crop rotation of corn, sugarbeet, spring wheat, and soybean with all crops resistant to glyphosate.

After six years, the composition of the weed population shifted from kochia and wild proso millet to predominately common lambsquarters. Using a reduced rate of glyphosate caused the population of common lambsquarters to increase to the extent that corn grain yield was reduced 43 percent compared to corn yield where the recommended rate of glyphosate was utilized. Common lambsquarters control was achieved by applying a herbicide other than glyphosate at planting. This strategy has suppressed the weed population and when combined with glyphosate provided excellent corn grain yields.

In the nonglyphosate treatment, after eight years of use kochia density increased to the extent that additional herbicide was required for weed suppression. After repeated use most herbicide modes of action will select for tolerant weeds, the key to successful weed management is to scout for weed shifts and counter them by adding herbicide with a different mode of action.