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Soybeans are frequently injured by postemergence broadleaf herbicide applications. But it is often unclear whether this injury results in soybean yield reduction. A three-year field study was conducted from 1993 to 1995 to identify methods to predict soybean yield loss from herbicide injury. Herbicides were sprayed at both normal and double rates to insure that some injury occurred. It was assumed that injury from double herbicide rates in a normal year would have similar effects on soybean yield as normal use rates under adverse environmental conditions which increased soybean susceptibility. Basagran (bentazon), Classic (chlorimuron), Cobra (lactofen), Galaxy (actifluorfen + bentazon), Pinnacle (thifensulfuron), Pursuit (imazethapyr), and Reflex (fomesafen) were included in the study. While soybean injury frequently differed between treatments, yield losses caused by the different herbicides fit the same statistical models. The best predictor of yield loss was soybean canopy reduction 22 days after herbicide treatment (DAT). The second best indicator was visual injury rated 8 DAT. To accurately rate or measure injury, each field must include a representative nontreated area for comparison.

Figure 1. Nontreated soybean plants. (click to enlarge) Figure 2. Soybeans with 15% injury. (click to enlarge) Figure 3. Soybeans with 30% injury. (click to enlarge) Figure 4. Soybeans with 50% injury. (click to enlarge)

Visual injury rating

Visual injury rating is estimated by determining the average percentage of stunting, chlorosis, and/or necrosis (burn) occurring to treated soybean plants compared to nontreated plants. Examples are shown in the accompanying photographs which compare nontreated soybean plants (Figure 1), and soybean plants with visual injury ratings of 15% (Figure 2), 30% (Figure 3) and 50% (Figure 4). Visual injury most accurately predicted yield loss when rated 8 DAT. Prior to that time, maximum injury symptoms had not been expressed. Later than 8 DAT, soybean plants had begun to recover and injury symptoms were not as severe.

Soybean canopy reduction

Soybean canopy reduction was determined by measuring height and width of both treated and nontreated soybean plants. Height and width were multiplied together, and percent canopy reduction calculated by dividing the value of treated plants by the value of nontreated plants. Canopy reduction calculated 22 DAT most accurately predicted soybean yield loss from herbicide injury.

Estimating soybean yield loss from herbicide injury

Figures 5 and 6 plot visual injury ratings and canopy reduction values against percent soybean yield loss, respectively. Note that the overall level of yield loss differed between the three years of the study. The greatest yield losses occurred in 1993. Yield loss was very low in 1994, probably because nearly ideal growing conditions enabled soybeans to recover more fully prior to harvest. But, results clearly demonstrate that in two of the three years, injury from postemergence broadleaf herbicides caused soybean yield losses. In each year, a straight line relationship occurred between percent visual herbicide injury or canopy reduction and percent soybean yield loss. Unlike frequent claims of herbicide manufacturers, soybean yields were reduced by even the lowest levels of herbicide injury in 1993 and 1995. Regression analysis (the percent yield loss lines and equations shown in Figures 5 and 6) only estimate percent yield loss because of variation among year and within each year. Yet these yield loss predictions may still help soybean growers assess risks and make better management decisions. To estimate percent soybean yield loss caused by herbicide injury, multiply percent visual injury 8 DAT by 0.25 or percent canopy reduction 22 DAT by 0.37. The actual yield loss may be more or less than the estimate depending on the subsequent weather conditions.

Using predictions of soybean yield losses caused by postemergence herbicide injury

It is important to remember that herbicide injury is frequently a necessary consequence of weed control. Uncontrolled weeds can cause greater soybean yield losses than moderate levels of herbicide injury. But, when two herbicides are equally effective in controlling weeds and are similarly priced, growers can maximize profits by using the product least likely to injure soybeans. Growing herbicide tolerant or resistant soybean varieties also can prevent soybean yield losses from herbicide injury, but profitability is also affected by factors such as variety yield potential, agronomic characteristics, disease resistance, and seed price (including technology fees).

Gordon Harvey is professor of agronomy, College of Agricultural and Life Sciences, University of Wisconsin – Madison.