April 25, 2008

High commodity prices have rejuvenated optimism in production agriculture. Two years ago it was common to have cash soybean prices in the $5-$6 per bushel range, compared to recent opportunities for selling contract cash soybeans for $14 per bushel. Recent increases in the price of glyphosate products also have been a topic of interest with producers.With the high soybean cash prices and increasing glyphosate prices, it's a good time to take a fresh look at weed control options for soybeans.

In this article we'll focus on herbicide selection for the first application of a planned two-pass weed control program in glyphosate-resistant soybeans. It is assumed that the second pass would consist of only glyphosate-based products applied postemergence.

Given current high commodity prices, it is important to use proven Integrated Pest Management (IPM) principles, such as economic thresholds (ET) to avoid losing yield potential. An economic threshold is defined as the level of weed infestation at which the cost of control equals the increased return on crop yield in the current year. This is a dynamic relationship between commodity price, weed control costs, weed size and density, and crop growth and development. As a commodity price increases, the economic threshold decreases. Table 1 illustrates the difference between economic thresholds at two soybean prices with a fixed herbicide application cost and demonstrates the need for more aggressive weed management strategies when crop prices are high.

Weed Control Timing

Figure 1. Soybean yield loss and beginning of CPWC as influenced by the timing of weed removal and row spacing.

An important consideration of the economic threshold is the timing of weed competition. Yield lost to early season weed growth is often greater than producers realize. In Figure 1, data from Nebraska research indicates that 5% yield loss can occur in soybeans by V3 or sooner, depending upon row spacing. If the soybean cash price is $14/bu, the threshold is only 2.6% yield loss (Table 1). In this scenario, soybeans planted in 30-inch rows are beyond the economic threshold at V1. For soybeans in 15-inch rows, the economic threshold is exceeded by V2, and in 7.5-inch rows by V3. It can be difficult to control weeds by V1 with a postemergence application, so many producers — especially those with a large number of acres to plant — may use a preemergence herbicide to protect yield. A preemergence residual herbicide also can increase timing flexibility for a planned post-emergence application.

In recent years, the low cost and effectiveness of a two pass post-emergence glyphosate herbicide program in soybeans made it unquestionably the preferred herbicide program of most producers. In past years, both generic and brand name glyphosate products were less than most preemergence soybean herbicides; however, with recent increases in glyphosate prices, this dynamic has changed.

Table 2 compares the cost of several common preemergence herbicides and glyphosate-based products. Generic glyphosate products may still be less expensive relative to the listed preemergence products, but brand name glyphosate product prices are similar to most of the listed residual preemergence products labeled for soybeans.

If a soybean grower decides to apply a preemergence herbicide, it is important to select the product that best controls the weed spectrum in a given field. In the end, it can pay dividends by maintaining the highest yield potential for your soybean crop.

Summary

• As commodity prices and input prices change producers should re-evaluate their weed control program in order to maintain high yield potential. With current soybean and herbicide prices, producers may benefit by using pre-emergence herbicides as part of a two pass herbicide program in glyphosate resistant soybeans. This will protect against early season yield loss and allow greater flexibility in post-emergence glyphosate application timing.
• Higher commodity prices reduce the ET and widen the "weed free window," making pre-emergence residual soybean herbicides more economical and useful in maintaining high yield potential.

Lowell Sandell
Extension Weed Science Educator
Stevan Knezevic
Extension Weeds Specialist
Mark Bernards
Extension Weeds Specialist