Winter weed update from MSU CAT Alert

2004 comparison of commercial weed control programs in corn
2004 economics of commercial weed control programs in soybean
Selection of glyphosate-resistant weeds

Jim Kells and Kathrin Schirmacher
Crop & Soil Sciences

A field trial was conducted in corn in 2004 on the MSU Research Farm to compare weed control, corn injury, corn yield, and economic return of the dominant herbicide programs marketed in Michigan. Major herbicide companies were asked to each submit four weed control programs for the study based on soil type and weed infestation history. Site characteristics and herbicide application timings are described in Table 1.

Table 2 lists the treatment details for the 24 treatments, sorted by application method and need for Roundup Ready hybrids. Weed competition was very high at the research site. The weed-free corn yield was 218 bu/A and the weedy (untreated) yield was 87 bu/A. Thus uncontrolled weeds at the site reduced corn yield by 131 bu/A (60%).

The site received excessive rainfall shortly after planting. A total of 6.1 inches of rainfall occurred within 2 weeks of preemergence herbicide application.

Table 3 lists the weed control effectiveness, weed control cost, corn yield, and gross margins for each of the herbicide programs in the trial. Table 4 summarizes weed control effectiveness, weed control cost, corn yield, and gross margins for each of the six systems included in the trial.

View Table 2
View Table 3
View Table 4

Observations
Excessive rainfall after herbicide application reduced weed control with preemergence programs, which often resulted in lower yields. The most expensive programs involved two applications. The least expensive programs involved one application. Three of the five most expensive programs were Roundup Ready programs. One of the five least expensive programs was a Roundup Ready program. The programs with the highest occurrence of yield loss were conventional treatments involving a preemergence application. Those treatments that caused significant corn injury had the lowest corn yields and gross margins. Four of the five most costly programs had high gross margins. Similarly, four of the five least costly programs had high gross margins. The major factor affecting gross margins was corn yield.

Christy L. Sprague
Crop & Soil Sciences

A field trial in soybean was conducted at the MSU Research Farm in East Lansing to compare weed control, soybean injury, soybean yield, and economic returns of dominant weed control programs being marketed to Michigan growers. Each major herbicide company was asked to submit up to four weed control programs for the study based on soil type and weed infestation history. Site characteristics and herbicide application timings are described in Table 1.

Table 2 describes the herbicide programs selected by each company for the 26-treatment study. Herbicide programs are sorted by application method and the need for Roundup Ready seed. Yield loss due to weeds was extremely high at this location. The maximum soybean yield from this trial was 67.7 bu/A and the weedy (untreated) yield was 21.3 bu/A. Thus, uncontrolled weeds at this site reduced soybean yield by 46.4 bu/A (68.5%).

Immediately after planting and application of the preemergence herbicides the site received 0.53 inch of rainfall, which may have contributed to the persistent injury that was observed from some of the soil-applied herbicides.

View Table 2
View Table 3
View Table 4

Results
Table 3 lists soybean injury, weed control effectiveness, weed control cost, soybean yield, and gross margin for each of the 26-herbicide programs included in the study. Table 4 summarizes soybean injury, weed control, the five most and five least expensive programs, soybean yield, and gross margins for each of the five weed control systems included in the trial.

General conclusions
The immediate rainfall after application of the preemergence herbicides led to persistent injury from some of the soil-applied herbicides. Herbicide programs that did not provide greater than 90% control of all weeds, in particular common ragweed, did not rank among the highest yielding herbicide programs. The programs that did not control common ragweed or programs that had significant injury that persisted throughout the season did not rank among the highest yielding or the highest gross margins. Not all programs that were among the highest gross margins were among the highest yielding programs. Four out of five of the most expensive programs included programs that involved two applications and all five programs that were among the least expensive involved only one application. However, all but two programs with the highest gross margins were sequential herbicide programs (2-pass). Several programs, conventional and Roundup Ready, provided excellent weed control, high soybean yields, and high gross margins.

This study is the results of one year and one year only. Please take this into consideration when forming your own conclusions. This study will be conducted again in 2005 to help strengthen conclusions.

Christy Sprague and Jim Kells, Michigan State University
Chris Boerboom, University of Wisconsin
Kevin Bradley, University of Missouri
Jeff Gunsolus, University of Minnesota
Bob Hartzler, Iowa State University
Bill Johnson, Purdue University
Mark Loux, Ohio State University
Dawn Nordby, University of Illinois
Micheal D. K. Owen, Iowa State University
Bryan Young, Southern Illinois University

It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere five years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone.

A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates. The comments in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. However, the question that a person should ask about these studies is whether or not they can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test if resistance will develop. For example, the University of Wisconsin has a 7-year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in these plots. Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio, Tennessee, or Delaware is not truly resistant? Obviously not. Small-scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small fraction relative to total glyphosate. The true real test to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year.

Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a “high selection pressure” is maintained. In this case, “high selection pressure” refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices. This potential was confirmed this week at the North Central Weed Science Society Meeting where glyphosate-resistant common ragweed weed was reported. This is the first report of glyphosate-resistance for common ragweed. It was identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. The common ragweed in this field had a high selection pressure for glyphosate with one or more glyphosate applications per year. This is the second example of a glyphosate-resistant weed that has developed in a Roundup Ready cropping system with high selection pressure. We do not know which glyphosate-resistant weed will be the next to develop or when it will occur, but high selection pressure will likely result in additional cases of resistance.

Midwest weed scientists believe in the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance.

2)    Alternating glyphosate use with other herbicide modes of action between years, and

3)    Incorporating appropriate integrated weed management practices such as cultivation.