Field trials describe methods to control herbicide-resistant weeds in soybeans

When weeds become resistant to herbicides, the advantage of growing herbicide-resistant crops is lost.

Farming practices that limit the emergence of resistant weeds offer another means of control and thwart weeds' effect on crop production. Finding the right combinations of weed control methods can improve yields and reduce dependence on chemical weed control.

Palmer amaranth is one of the most common-and problematic-weeds in soybean crops across the southern United States. The weed creates a great deal of seed and continues to emerge throughout the growing season.

Because it is difficult to control, particularly now that biotypes display resistance to glyphosate, it is best to combat the weed before it emerges.

The journal Weed Technology offers results of field tests of resistant Palmer amaranth in glyphosate-resistant soybean crops in Arkansas conducted over a 2-year period.

In this study, 250,000 glyphosate-resistant Palmer amaranth seeds were incorporated into the soil, and their emergence was evaluated five times during the growing season. Three farming practices were tested-deep tillage, planting a cover crop of rye, and double-cropping a field with wheat and then soybeans in the same growing season.

Although no-till planting techniques minimize soil erosion and pesticide runoff, a lack of tillage can lead to an increase in weed seed density at shallow depths.

Deep tillage performed once a year in the fall has shown increased crop yields due to better water infiltration and storage. Deep tillage alone produced an 81 percent reduction in Palmer amaranth emergence over the 2 years of this study.

When deep tillage was used in combination with either a cover crop of rye or a first crop of wheat, the results were even better.

When a rye crop was planted to compete with weeds, it produced a 98 percent reduction in Palmer amaranth emergence the first year and a 73 percent reduction the second year of the study. Deep tillage and double-cropping soybeans with wheat offered 97 percent and 82 percent reductions in the first and second years, respectively.

Maximizing the amount of plant residue on the soil surface is the key. The high amounts of rye or wheat plant residue left behind helped control the emergence of Palmer amaranth. Deep tillage and double-cropping wheat and soybeans may offer the best weed control value.

The revenue from an additional crop more than offsets additional expenses in farm machinery, fuel, and labor.