Why two if one will do?

Encyclopedia Article

Nirvana: a goal hoped for but apparently unattainable1.  Is one-pass weed control an agrarian nirvana, or has technology finally given farmers the ability to consistently control weeds with a single trip across the field? The benefits of one-pass programs are a reduced time commitment and the potential for reduced weed control costs. Anyone involved in weed management can provide numerous examples of situations where one-pass programs have worked. However, research and practical experience have documented inherent risks with this approach to weed management. This paper will focus on factors that limit the success of these programs, and how risks associated with one-pass programs can be reduced.

In order to achieve one-pass control, a herbicide program must be identified that: 1) has excellent activity on the entire spectrum of weeds present in a field, and 2) controls weeds for a sufficient duration to prevent economic losses. Preemergence herbicide programs that provide broad-spectrum control usually will require a combination of two or three herbicides to control all weed species present in a field. Additionally, the herbicides must have sufficient persistence to control weeds until the crop canopy develops sufficiently to shade out late-emerging weeds. 

Rainfall is frequently the downfall of one-pass preemergence herbicide programs,  specifically the lack of rain following application. Most herbicides require at least a half inch of rain to move them from the soil surface to the depth where weed seeds germinate.  Performance will be compromised in years with limited rainfall in the weeks following application and planting.  Many growers manage this risk by applying the herbicide several weeks ahead of planting.  This provides a wider window for rain activation since the herbicide is applied prior to soils reaching temperatures favorable for emergence of most summer annual weeds. However, early application may compromise the ability to achieve full-season weed control due the herbicides being exposed to the environment for a longer period of time. Early preemergence applications frequently allow late-season weed escapes since the herbicide degrades before the crop canopy is fully developed.

The success of one-pass postemergence herbicide programs is limited by the prolonged emergence of weeds. Agronomic fields are infested with a number of weed species. Some species emerge before or shortly after the crop has been planted, whereas other species may not initiate emergence until two to four weeks after planting. The herbicide must be applied soon enough to prevent early-emerging weeds from affecting yields, yet late enough to avoid problems with weeds that emerge after the herbicide application.  

Figure 1 illustrates the risk of yield losses associated with one-pass postemergence program.  The right portion of the curve represents yield losses caused by weeds that emerge and compete with the crop until the postemergence herbicide is applied. Losses from these weeds increase as herbicide application is delayed. The left portion of the curve represents yield losses caused by weeds that emerge after the postemergence herbicide was applied. Under certain situations there is an application window that will protect the crop resulting in optimum yields. Figure1a depicts such a scenario, with no yield loss occurring if the application is made between 3 and 4 weeks after planting. However, if application is delayed until 6 weeks after planting a 5% yield loss would occur.  

Total postemergence herbicide programs are based on the premise that there is an opportunity to achieve optimum crop yields with a single herbicide application (Figure 1a). However, there are situations in which high weed densities, adverse weather, or other factors eliminate this opportunity. In Figure 1b the optimum application timing occurs approximately 3 ½ weeks after planting, but results in a significant yield loss. Predicting when the optimum herbicide application window occurs and having the opportunity (i.e. avoiding rain or wind delays) to complete spraying during this window is an intrinsic risk of one-pass postemergence herbicide programs.

Figure 1.  Importance of application timing

Research has documented the inherent variability associated with one-pass herbicide programs. Results of a regional project evaluating the effectiveness of several weed management strategies in corn are presented in Table 1. Data represent the average of nine experiments and provide information on the consistency of the different strategies. The preemergence program and early post application provided the lowest level of foxtail control.  The preemergence herbicide program provided acceptable weed control at locations with sufficient rainfall, but the average control was reduced due to locations with insufficient rain to activate the herbicides. The early postemergence herbicide applications (1-2” foxtail) provided excellent control of the foxtail emerged at application, but the control ratings were reduced by foxtail that emerged after application. Yields of both treatments suffered due to the inadequate weed control.

The two treatments that provided the highest foxtail control were two-pass programs consisting of an Accent treatment followed by lay-by cultivation. The two late Accent applications provided 90% foxtail control; however, corn yields for these treatments were limited by competition with weeds prior to the Accent application. The only treatment that provided the highest level of weed control and highest yield was the early Accent application followed by a lay-by cultivation. All other treatments represented some compromise between weed control and yield protection.


Table 1. Effect of grass size and Accent rate on giant foxtail control and corn yields. 1
(Source: Tapia et al. 1997. Postemergence herbicide application timing effects on 
annual grass control and corn grain yield. Weed Sci. 45:138-143.


Foxtail Height 
at Post Application

% Foxtail

Corn yield2

Untreated control


0 E

123 E



78 D

167 C


1 – 2”

77 D

171 BC

Accent + cultivation

1 – 2”

93 AB

184 A


2 – 4”

88 C

177 AB

Accent + cultivation

2 – 4”

95 A

171 BC


4 – 6”

90 BC

168 BC


6 – 12”

90 BC

156 D

Data are means of 9 experiments conducted in 5 states during 1992 and 1993.
2 Means within columns followed by the same letter are not different at P<0.05.

Jeff Gunsolus at the University of Minnesota has evaluated the variability in economic returns of one- and two-pass weed control programs. Over several years of research he found relatively small differences between the two strategies in the average economic return per acre.  However, the standard errors of return for the one-pass programs were much greater than those for the two-pass programs. The high standard error indicates greater risk with the one-pass program due to high variability with this approach. In fields where one-pass programs are successful they can provide as high, or higher, returns as a two-pass program. However, one-pass programs are more likely to result in inadequate control than two-pass programs, and therefore have a greater potential to provide low returns. Two-pass programs provide a more stable return per acre than one-pass programs.

The primary advantage of one-pass programs is the elimination of the time and cost associated with additional trips across fields. Farmers must weigh the benefit of these savings against the greater risk of control failures. The risks associated with one-pass programs can be minimized by: 1) carefully selecting fields for this strategy, and 2) regularly scouting fields. One-pass programs are rarely successful in fields with high weed pressure. This type of program should be reserved for fields in which high levels of weed control have been achieved for the past two to three years. Secondly, a systematic scouting program is needed to stay ahead of rapidly changing conditions in a field.  

Many of the risks associated with one-pass programs can be eliminated by scouting fields weekly during the first month after planting. Scouting provides the opportunity to react quickly with remedial strategies (i.e. rotary hoeing in the absence of timely rain; a second application to control weeds escaping the first herbicide treatment) that will prevent or minimize losses. The ability to respond in a timely manner to unforeseen circumstances is essential to reduce economic losses.

In summary, the highly effective herbicides available to Iowa’s farmers make one-pass weed control possible. However, one-pass programs are inevitably more inconsistent than two-pass programs and will result in erratic economic returns if poorly managed. Farmers who adopt a one-pass program and wait until the crop begins to canopy or until harvest to evaluate the success of their program are destined to suffer significant losses in economic returns. However, farmers who select appropriate fields and closely monitor crop and weed development have the opportunity to successfully implement one-pass programs.

1Webster’s Ninth New Collegiate Dictionary

This article was originally developed for the Iowa Soybean Promotion Board.

Iowa State Weed Science Online