The rapid adoption of Roundup Ready (RR) soybeans has grabbed the attention of persons involved directly and indirectly in agriculture. Approximately 27 million acres of RR soybeans were planted in 1998, and the acreage committed to this technology is expected to increase as RR corn becomes more widely available. Rapid shifts in herbicide use patterns are not new - Pursuit captured 75% of the Iowa soybean market in less than five years. However, the potential for RR crops to dominate both the corn and soybean herbicide markets does make this a somewhat unique situation.
In terms of weed management, one of the topics of debate has been whether or how quickly weeds will adapt to the RR system. Monsanto has taken a strong position stating that the likelihood of resistance developing to Roundup is very low. However, resistance is not the only mechanism by which plants can escape a herbicide. When evaluating the potential for weed shifts that diminish the effectiveness of a herbicide, one must consider factors other than resistance.
Herbicide resistance is defined as the inherited ability of a weed to survive a herbicide dose that kills the wild type of that species. The two important points of this definition are 1) the ability to survive the herbicide is genetic and passed along to progeny, and 2) the native population is controlled by the herbicide. For the purpose of this paper, I will use the term tolerant to describe a species that inherently has the ability to survive a herbicide. Using Pursuit as an example, common lambsquarter would be considered a Pursuit tolerant species because Pursuit never provided acceptable control of lambsquarter. Waterhemp would be considered resistant to Pursuit since Pursuit initially provided good control of this species.
Weed shifts are an inevitable consequence of weed control. All control strategies, whether manual, mechanical or chemical, select for weeds that are able to survive that tactic. Plants that survive a control tactic produce seed and therefore increase in density, whereas plants that are killed fail to produce seed, resulting in a reduction in their occurrence. This paper will address the issue of weed resistance and tolerance to Roundup, and the impact it may have on weed management systems relying on Roundup.
Shifts to Tolerant Species
It is likely that most growers will have problems with weeds able to tolerate the normal use rate of Roundup before Roundup resistant weeds appear. As with any other herbicide, weed species vary in their tolerance to Roundup. Highly susceptible weeds, such as giant foxtail, can be controlled with lower rates than required to control weeds with a higher level of tolerance, such as velvetleaf. The typical use rates of Roundup Ultra (24 to 32 oz/A) were chosen because they are the lowest rates that will provide consistent control of most major weeds found in corn or soybean fields. However, there are several common weed species found in the northern corn belt for which these rates are at the level where inconsistent performance can be expected. Label rates will provide acceptable control of these weeds under optimum conditions, but there will be situations when, due to environmental conditions, application techniques, weed size, or other factors, unacceptable control will be obtained. Weeds that fit into this category include black nightshade, waterhemp, yellow nutsedge, velvetleaf, common ragweed and others. The consistency of control of these problem weeds can be improved by increasing the rate of Roundup, however, at some point it may be more economical to use other management strategies.
This past year numerous instances of control failures occurred with Roundup. Almost all situations involved species that possess a relatively high tolerance to Roundup. Several factors have been blamed for the poor performance, including abnormally cool temperatures and excessive rainfall. Although the duration and size of area affected by conditions unfavorable for herbicide performance will vary from year to year, they can be expected to occur every year. The best means of dealing with these situations is to delay herbicide application until growing conditions improve. However, as farm size increases, many growers do not have this flexibility, and thus need to spray during less than ideal conditions.
An interesting occurrence in Iowa during the 1998 growing season was reduced performance with Roundup applications made in the evening compared to morning or afternoon applications. The problems with evening applications were common enough that many commercial applicators shut down their sprayers at 6:00 or 7:00 PM. There is little information available to explain this problem, but ongoing research at Minnesota and Missouri may provide some insight in the near future. An article in the journal Soviet Plant Physiology reported the target site for glyphosate, the active ingredient in Roundup, was four to ten times more sensitive to glyphosate in the light than in the dark. This might partially explain the problem, but it is important to keep in mind that evening applications have not caused problems in previous years. It is likely that other environmental factors (temperature, soil moisture, etc.) were involved in these problems. We may not see similar problems with night applications for another 20 years, and at this time Monsanto is not planning to place any restrictions on the label concerning evening applications.
Unfortunately, our ability to anticipate problems with herbicide performance due to weather or other factors is limited. Given this ability, we could adjust management plans to minimize risks, such as increase herbicide rates or delay applications. We do know that extremes in temperatures, soil moisture, or other factors that stress plants may reduce herbicide activity, but to predict exactly when weed control will be reduced to unacceptable levels is beyond current capabilities. Some people have devised simple guidelines to help make decisions when to shut down spraying operations. Examples of these would be not to spray when the sum of the temperature and relative humidity exceeds a certain level, or to not spray when temperatures are below 50 F, etc. These guidelines may be of some value, however, plant-environment interactions are much more complicated than these simple concepts. Because of their simplicity, these tools may not be much or any better than using basic common sense.
Waterhemp – It won’t go away in RR systems
The recent increase in waterhemp populations is a classic example of a weed shift. Native to the midwest, it was a secondary weed problem throughout the 1970’s and 1980’s. However, changes in herbicide use patterns and tillage created an ideal environment for this weed, and it is now the number one weed problem for many growers. Waterhemp has thrived under systems relying on ALS herbicides due to widespread ALS resistance in this species. The introduction of RR crops has raised hopes that this weed would again fade into the background. However, I believe waterhemp has the ability to create problems in RR systems, and thus serves as a good example of the type of weed shifts we will see with RR technology.
The first factor that favors waterhemp is that it has a fairly high tolerance to Roundup. Numerous fields in Iowa in 1998 needed to be retreated with Roundup Ultra due to waterhemp surviving the initial application. In one situation, waterhemp survived the first two applications of Roundup Ultra (total of 60 oz/A), but was eventually controlled by a third application of 48 oz/A. Waterhemp was collected from this field and rate response studies conducted in the greenhouse. Although waterhemp was controlled with Roundup in these studies, the effective rates were at or above typical field use rates (32 oz). The rate required to control the ‘problem’ biotype was not higher than the rate required to control a biotype from a field with no history of control problems with Roundup. The relatively high tolerance of waterhemp to Roundup will lead to inconsistent control of this weed in the field.
A second characteristic that will allow waterhemp to be a problem in RR systems is its emergence pattern. Waterhemp germinates later in the growing season than most other common agronomic weeds. In a central Iowa study, approximately 70% of total waterhemp emergence in 1996 occurred after June 20. Only 10% of total emergence was observed after this date with giant foxtail, woolly cupgrass and velvetleaf. The lack of residual activity with Roundup allows weeds emerging after application to avoid the herbicide. In many years the majority of waterhemp will not be affected by applications of Roundup timed according to weeds that emerge at the same time as the crop (foxtail, velvetleaf, etc.).
While late emergence of waterhemp will allow it to escape many Roundup applications, it is important to note that late-emerging plants are at a competitive disadvantage with the crop due to the delay in emergence. A study in central Iowa in 1998 followed the growth of waterhemp plants that emerged at different times in relation to soybean (same time as soybeans and at the 2,4 and 6 trifoliate stage). Approximately 90% of the waterhemp that emerged at the same time as soybean or at the 2nd trifoliate stage survived until maturity. Survival of plants emerging at the 4th and 6th trifoliate stage of soybean was 75 and 25%, respectively. The earliest emerging waterhemp grew to a height of 88", whereas plants emerging at the 4th and 6th trifoliate stage reached a height of 65" and 40", respectively. Dry matter accumulation of waterhemp was affected more by emergence date than height, plants emerging at the 4th trifoliate stage accumulated only 19% of the biomass of plants emerging with soybeans. Although delayed emergence reduced the survival and growth of waterhemp in soybeans, plants emerging at the 4th and 6th trifoliate stage could still impact crop production through yield reductions, interfering with harvest, or weed seed production.
Problems with late-emerging weeds will vary depending upon environmental conditions. Late-emerging waterhemp will be a greater problem in years with abundant moisture during May and June, as was experienced in much of Iowa during 1998. The same study evaluating the influence of emergence timing on waterhemp competitiveness in soybean was conducted in 1997, a year with limiting moisture early in the season. In the 1997 study, no waterhemp emerging after the 3rd trifoliate stage of soybean survived.
Waterhemp has the capacity to escape weed management systems based on Roundup due to the weed’s high tolerance to Roundup and its late emergence. In some years, waterhemp will be of little consequence due to good conditions for herbicide activity and unfavorable conditions for survival of late-emerging weeds. In other years, waterhemp will cause major problems due to these two characteristics.
Potential for Roundup Resistant Weeds
There has been considerable discussion concerning the risk of weeds developing resistance to Roundup. Some people stated that it was extremely unlikely that Roundup resistance would appear in weeds, but shortly after these statements were made the first case of Roundup resistance appeared in Australia. The potential for resistance to different herbicides varies widely, and evidence supports the view that Roundup is at a lower risk than many other herbicides. Some herbicides are at less risk for resistance because they have multiple sites of action. The likelihood of a biotype having two altered sites of action is much less than that of a biotype having a single altered site of action. It is believed that the amide herbicides (Dual, Harness, Surpass, etc.) have multiple sites of action, thus limiting the development of resistant weeds to these herbicides. Other herbicides may have a low risk of resistance because changes in the site of action that prevent herbicide binding may interfere with the activity of the enzyme. In this situation, most mutations that provide herbicide resistance result in death of the plant since the enzyme is non-functional, even in the absence of the herbicide. This is believed to be the reason for the low risk for resistance with Roundup.
The first reported case of resistance to Roundup occurred in Australia in 1995 and involved rigid ryegrass. Since then, a second resistant rigid ryegrass population has been identified. Rigid ryegrass has a history of herbicide resistance, and resistant biotypes have been identified to all herbicides cleared for control of this weed in wheat. Wheat producers in Australia have had to change their entire production practices due to the lack of herbicides for controlling this weed.
The first Roundup resistant rigid ryegrass biotype was identified in an orchard near Orange, New South Wales, Australia. The site had intensive selection pressure, with two or three applications per year of Roundup (2 to 4 qt/A) for 15 years. The resistant population was found to be between 7 and 11-fold resistant to Roundup compared to the susceptible population. A second resistant rigid ryegrass biotype has been identified from an agronomic site that received less intensive selection pressure.
Rigid ryegrass is the first case of resistance to Roundup occurring in the field. However, the potential for selection of resistant biotypes has been demonstrated under controlled situations. In one study, a five-fold increase in resistance to Roundup was observed in perennial ryegrass when the population was exposed to selection pressure from Roundup over 11 generations. Chris Boerboom, currently an extension weed scientist in Wisconsin, studied Roundup tolerance in birdsfoot trefoil while a graduate student at Minnesota. A three-fold increase in Roundup tolerance in birdsfoot trefoil was achieved through a recurrent selection program.
While these cases are not indicative of an impending threat for widespread resistance to Roundup, they should remind users that resistance management should be a consideration with any herbicide, including Roundup.
Weed shifts occur with all management strategies, including Roundup. The availability of Roundup Ready crops provides farmers a new option for weed management that has advantages over many other products. However, many of the problems, including weed shifts, that have arisen with other herbicides are just as likely to develop with Roundup. It is likely that the initial problems experienced with Roundup will involve weed species such as waterhemp and yellow nutsedge that have sufficient tolerance to Roundup to allow them to occasionally survive commonly used Roundup rates. With continued use of Roundup Ready crops, these tolerant weeds will increase in density such that problems will become more frequent. Farmers will either need to increase the rate of Roundup or use other herbicides that have a higher level of activity on the problem weeds.
The appearance of weeds resistant to Roundup is likely to occur in the cornbelt if current use patterns of Roundup continue. However, it is fruitless to try and predict which species will be the first to develop resistance or when resistance will first appear. The rate that weed shifts occur will be dependent upon how the technology is used. Growers who adopt RR crops in all cycles of the crop rotation will be at greater risk than those who choose to rotate to other weed management strategies. We are fortunate to have a large selection of herbicides available for use in corn and soybeans, thus it is hard to foresee a serious problem such as has occurred in Australia with rigid ryegrass. However, it is still in everyone’s best interest to manage herbicides in a way that reduces the threat of weed resistance. Evaluating long-term weed management programs in terms of selection pressure placed on weeds should be an important component of crop management planning.
Prepared by Bob Hartzler, extension weed management specialist