April 21, 2011

From the beginning of the dry bean industry in Nebraska, preplant tillage has been an integral part of the planting process. Discing, moldboard plowing, packing, and planting all encompassed the conventional tillage program. From the 1940s to the 1980s herbicides were developed for use in conjunction with preplant tillage. By combining shallow tillage with herbicides such as Eptam, Prowl, Sonalan, and Treflan. it became possible to incorporate, position, and trap herbicides near the soil surface where they could kill weed seedlings but not the crop as they both emerged from the soil. By mechanically incorporating herbicides, even if it didn’t rain after planting, the herbicide was still in position to provide weed suppression.

Weed Adaptation Enhances Survival

Because conventional tillage has been practiced for many years, weeds have undergone selection to adapt to tillage. They have become opportunistic and germinate when they have the best chance to survive. Instead of kochia seed germinating when the soil temperature reaches 40°F, as it does in rangeland, in conventionally tilled fields the seed remains dormant until the soil temperature reaches about 60°F, thus avoiding the killing effects of early season preplant tillage. In addition, after being exposed to preplant herbicides for the last 30 years, a third kochia population has evolved that germinates several weeks after planting when the soil temperature 85°F, thus avoiding preplant herbicides.

Other weeds have adapted to tillage by developing a photo-receptor that allows the seed to germinate after being exposed to light created during tillage. If tillage does not occur or the seed remains buried, dormancy continues.

Tillage Effects on Weed Survival

The degree of soil disturbance created by tillage favors some weeds over others. Common sunflower density increased with moldboard plowing, longspine sandbar and redroot pigweed density increased with tandem discing, and kochia density increased with ridge till.

In an effort to reduce soil erosion and production costs, some dry bean growers in western Nebraska have adopted conservation tillage measures that increase the quantity of crop residue on the soil surface. These growers have replace tillage with herbicides and adapted planters to plant through crop residue.

With reduced or no tillage weed seeds are positioned near the soil surface and become mixed with crop residue. Crop residues provide a micro-environment near the soil surface which is moist and supports ideal conditions for weed seed germination. In theory if weeds are effectively suppressed during the first few years of no-till production, the weed seedbank can decline and without further tillage weed seeds are not moved to the soil surface.

In practice, studies have shown that winter annual weeds (horseweed), small-seeded summer annual grasses, and perennials like Canada thistle become more difficult to control as tillage is reduced or eliminated while large seeded broadleaf weeds become easier to control. Crop residue on the soil surface also can intercept soil-applied herbicides and prevent them from reaching the soil surface. Rainfall or irrigation following herbicide application can wash a portion of the herbicide off residues and move it into the soil but, if it remains dry, herbicide is lost and weed control is reduced.

UNL Research

In an attempt to further examine the ramifications of tillage on dry bean weed control, an experiment was initiated in 2010 near Mitchell to study the influence of different levels of corn residue on weed control. In early May corn stalks were shredded and in late May Roundup was applied to kill emerged weeds. During the first week of June the field was divided into three sections.

• In one section there was no tillage and there was approximately 80% corn residue coverage at planting.
• In the second section corn stalks were disced once. which left 56% residue cover.
• In the third section stalks were disced twice which left 39% residue cover at planting.

Dry beans were planted and five soil-applied herbicides were sprayed. After spraying there was a 1.3-inch rainfall from June 11 to June 13. As a result of the rainfall and tillage, a soil crust formed as the soil dried. An attempt was made to reduce crusting by rotary-hoeing; this was only partially successful.

By mid-June crop and weeds had emerged and it became apparent that preplant discing and corn residue had influenced herbicide efficacy and dry bean and weed density. Bean emergence was greatest, crop injury from herbicides least, and weed density was reduced 50% in areas receiving no tillage compared to areas that were disced (Table 1).

No-till Herbicide Options

Herbicide options are reduced when growers move from preplant tillage to no tillage. Only five herbicides are labeled for use after planting and applied to the soil surface before dry bean and weed emergence: Dual Magnum, Outlook, Permit, Prowl H2O, and Valor. All of these herbicides must rely on rainfall or irrigation for movement into the soil.

During 2010 weed control from soil-applied herbicides ranged from 60% to 75%, suggesting that additional weed control measures would have been necessary to achieve optimum bean production (Table 2). Raptor plus Basagran was applied after crop and weed emergence but herbicide crop response was still influenced by preplant tillage. Dry bean plants were more vigorous in no tillage and when treated with Raptor plus Basagran, crop injury was only 4%. When the same treatment was applied to areas that had been double-disced and beans were under stress from crusting, Raptor plus Basagran caused 10% crop response. Weed control from Raptor plus Basagran averaged 86% and was superior to that achieved from soil-applied herbicides (Table 2).

These results are from only one year of study and soil crusting may have complicated the experiment. More study is needed before conclusions can be drawn, but preliminary results suggest that no-tillage production may have advantages in weed control and crop vigor.

Robert Wilson
Extension Weed Specialist, Panhandle REC, Scottsbluff