Can total postemerge programs provide optimal yield?

Encyclopedia Article

Although the concept of total postemergence programs is not new, the availability and demand for Roundup-Ready soybeans has resulted in a large shift towards this weed management strategy. The excitement over the performance of Roundup seems to have caused some people to overlook basic principals of weed competition. Because of this, many people may be unknowingly sacrificing soybean yields when using this technology.

Relying on Roundup Ultra, or any other non-residual postemergence program, requires considerable management in order to maximize yields and profits. While there are situations where both optimum weed control and crop yields can be achieved with a single postemergence application, this management strategy frequently results in a compromise in one of these factors. An understanding of the critical period of weed competition is necessary to design managment programs that will consistently eliminate yield losses associated with weed competition.

Weeds that emerge at or near the time of soybean emergence typically will not begin to compete with the crop until two to four weeks after emergence. The exact time period varies depending upon environment, weed species and population, row spacing and other factors. Thus, postemergence applications made within two to three weeks after emergence usually will prevent significant soybean yield losses due to early-season competition.

The second group of weeds of concern are those that emerge later in the growing season, after the postemergence herbicide has been applied. Late-emerging weeds are at a competitive disadvantage with soybeans due to the head start provided the crop, thus weeds that emerge more than one or two weeks after soybean emergence will not reduce yields as much as those that emerge with the soybeans. Weeds emerging more than four or five weeks after soybean emergence frequently will not impact yields, especially at low to moderate populations.

The difficulty in managing weeds with a single postemergence treatment is that weeds typically emerge throughout the growing season. If treatments are applied within three weeks of soybean emergence to eliminate early-season competition losses, weeds that emerge after the crop may still be competitive and cause yield losses. Delaying application to minimize problems with late-emerging weeds may result in losses from weeds that emerged with the crop. 

The critical period (time period weeds can grow with crop without impacting yields) varies widely depending upon specific conditions in the field.  Figure 1 illustrates how the window of opportunity to protect crop yields varies under three hypothetical situations.  The left portion of the curve represents yield losses from weeds emerging after control strategies are implemented.  Yield losses are greatest if application is made near crop emergence.  The right portion of the curve represents losses due to weeds that emerged with the crop.  In the center of the curve there can be additive losses from both groups of weeds.  The X-axis labeled A represents the most desirable situation - there is about a 4-week window where weeds could be controlled without experiencing yield losses.  This situation would most likely occur in fields with low weed populations and soybeans planted in narrow-rows.  The X-axis labeled B also represents a situation where it is possible obtain optimum yields with a single-pass, however, the application window is only a week long.  If the grower miscalculates when this window occurs, or if he is kept out of the field by weather conditions, significant yield losses will occur.  X-axis C represents a situation where there is no single application timing that will produce optimum yields.  All three of these situations can occur, but unfortunately our ability to predict which situation exists in a specific field is limited.  This puts a grower at risk when using one-pass programs, particularly in fields with high weed populations.

Researchers in North Carolina evaluated critical periods of competition between common ragweed and soybeans. This study supports the theoretical windows of opportunity presented in Figure 1.   Common ragweed that emerged at the same time as the soybeans was allowed to compete for 2, 4, 6 or 8 weeks after soybean emergence (WAE) to determine the impact of early-season competition on soybean yields. A second series of studies evaluated the impact of ragweed that emerged 2, 4, 6 or 8 WAE on soybean yields. The experiment was conducted during two growing seasons at two locations, providing four sets of competition data.

The impact of early and late season competition is shown in Figures 2A and 2B. Averaged over the four experiments, common ragweed competing with soybeans for the first two WAE did not impact yields, however, losses of 3 and 6% occurred if ragweed was left to compete four or six weeks, respectively (Figure 2A). Ragweed that emerged six WAE or later did not reduce soybean yields (Figure 2B). Ragweed emerging two WAE reduced yields 17%, whereas weeds emerging four WAE reduced yields 1%. Ragweed allowed to grow the entire growing season reduced yields 62%.

Figure 2C shows the combined impact of early and late emerging weeds. Under the conditions of these experiments, it was impossible to eliminate yield losses with a single weed control strategy. If weeds were controlled early (less than 4 WAE), late emerging weeds impacted yields. If application was delayed to prevent problems with late emerging weeds, early season competition impacted yields.

As stated earlier, the critical period of competition varies depending upon specific conditions encountered in the field. Evaluating the individual experiments in this research provides some estimate of the risk associated with single pass postemergence strategies (Table 1). The percentage of the four experiments in which the different control timings resulted in significant yield losses and the maximum yield loss associated with the different timings is provided.

Table 1. Risks associated with one-pass weed management program.


% of sites with
yield loss

Maximum yield loss
observed (%)













Coble et al. 1981. Weed Sci. 29:339-342.

Applications made either four or six WAE prevented yield losses 50% of the time, whereas yield losses occurred in three out of four experiments with the earlier or later application dates. Maximum yield losses in individual experiments ranged from 17 to 90%, depending upon application timing. This research indicates that yield losses can be eliminated with a single postemergence application; however, under certain conditions there can be a large penalty for this weed management approach. For example, a single application four weeks after soybean emergence prevented yield losses in two of the four experiments; however, a yield loss of 24% occurred in one of the experiments with this application timing.

So how does one determine the optimum method to use Roundup Ready or other total postemergence programs? It is unlikely that single-pass programs will provide maximum yields in fields with high weed populations or if soybeans are planted in 30" rows. The likelihood of managing weeds with a single-pass is greatly increased if the field has low weed populations and the soybeans are planted in narrow-rows (<15 inches). However, even in these fields there are risks associated with single-pass program. The application window for maximum yields will be relatively narrow, typically one or two weeks. If weather conditions (rain, wind,etc.) keep the sprayer out of the field during this critical period, yield losses can be expected. Also, the large demand for postemergence applications may prevent the sprayer from being available at the optimum time. These factors need to be considered when planning weed management programs.

There are two primary approaches that can be used to minimize the risk of weed competition impacting yields when relying on postemergence herbicides. Preemergence herbicides can be used to control early emerging weeds, therefore allowing the postemergence treatment to be delayed until late emerging weeds are not a concern. The second approach involves a planned sequential application program. The first postemergence treatment would be made typically two to three weeks after emergence and then be followed by a second treatment if late flushes develop following the first application. Interrow cultivation can be substituted for the second herbicide application, therefore reducing herbicide costs.

There is no doubt that Roundup Ready technology has several advantages over most other herbicides. However, the basic principals of biology and weed management remain the same, whether using biotechnology-enhanced or traditional soybeans. Just because Roundup will kill large weeds does not make delaying applications until late in the growing season a desirable practice. Taking into consideration the impact of duration of weed competition on soybean yields will allow growers to use this technology to attain effective weed control and maximum yields.

Prepared by Bob Hartzler, extension weed management specialist

Iowa State Weed Science Online