Commercial Agriculture Crop Institute Proceedings - 1997

Presented at the 1997 Missouri Commercial Agriculture Crop Institute

On the Horizon for Weed Management

Presented by:
Bill Johnson, State Extension Weed Scientist, University of Missouri, Columbia
Andy Kendig, State Extension Weed Specialist, University of Missouri, Portageville
Reid Smeda, Weed Physiologist, University of Missouri, Columbia
Bob Kremer, Research Agronomist (Weed Science), USDA-ARS, Columbia
Bill Donald, Research Agronomist (Weed Science), USDA-ARS, Columbia
George Smith, IPM Coordinator, University of Missouri, Columbia


Introduction

The purpose of this paper is to contemplate the future of weed management. Our goal is to advise growers of new trends and technology in weed control and anticipate how these tools can be incorporated in their farming system. When considering the future of weed management, it is likely that they one have to consider government programs, erosion control, water quality concerns, profitability, quality of life, rural infrastructure, conflicting time issues and farm goals and limited financial resources. The information in this paper is intended to stimulate thought on these issues and is merely the thoughts and opinions of the authors.

Herbicide-Tolerant Crops

The advent of herbicide-tolerant crops could have a dramatic impact on weed management decisions and weed populations. Herbicide-tolerant crops allow one to apply a herbicide on a crop which has been modified to be tolerant to that herbicide; the non-modified crop will suffer severe injury when treated with that herbicide. The end result is that broad-spectrum herbicides such as Roundup and Liberty in crops such as corn, soybeans, rice, and cotton can be used to manage grass and broadleaf weeds. Herbicide-tolerant crops such as IT/IR (imidazolinone tolerant/imidazolinone resistant) corn and STS (sulfonylurea tolerant) soybeans have a higher tolerance to herbicides which inhibit the target site of these families of herbicides (aceto-hydroxy acid synthase or ALS). This allows us to manage ALS-herbicide (Scepter, Pursuit, Classic) carryover, use soybean herbicides such as Pursuit on corn, and use higher rates of sulfonylurea herbicides on soybean without crop injury.

There are numerous potential liabilities associated with herbicide-tolerant crops. One is outcrossing of herbicide-tolerant crops with weedy relatives. This is not a substantial problem with corn, soybean, or cotton due to lack of weedy relatives. However, grain sorghum (shattercane, johnsongrass), rice (red rice) and wheat (jointed goatgrass) all have the potential to outcross with their respective weedy relative and create another herbicide-resistant weed. For example, if Roundup Ready grain sorghum outcrosses with shattercane, the progeny will be resistant to Roundup and we have lost an effective means of controlling this weed.

Another potential disadvantage of herbicide-tolerant crops is that volunteer crops such as corn could become a weed in soybean. Poast-tolerant (SR) volunteer corn will not be controlled with Poast or Poast Plus in soybean. Volunteer Roundup Ready or Liberty Link corn will not be controlled with Roundup or Liberty, respectively, in Roundup Ready or Liberty Link soybeans. This may force growers to spend additional money to control these "weeds".

Lack of rotating crops resistant to herbicides with the same mechanism of action could result in selection for new herbicide-resistant weeds. When all of the planned herbicide-tolerant crops are commercially available the following corn:soybean rotations will be possible:

These rotations will result in using the same mode of action repeatedly and act to select for weeds resistant to the herbicide used in both crops. Similar rotational concerns currently exist with Pursuit, Broadstrike, Poast Plus and Prowl, since they are labeled in both corn and soybeans. There will be increased opportunity to mismanage herbicides and end up with another problem weed similar to our current situation with waterhemp.

On the positive side, herbicide-tolerant crops will provide new strategies and flexibility for managing weeds if used properly. Herbicides such as Roundup and Liberty control a broad spectrum of grass and broadleaf weeds and will allow one to reduce reliance on the confusing array of tankmixes and premixes. Less emphasis will be required on the higher use rate, soil-applied herbicides which are currently under scrutiny due to detection in surface water. This will facilitate implementation of IPM programs (see Mandated Pesticide Legislation section). Herbicide-tolerant crops may also encourage use of economic thresholds in weed management decisions if reliable thresholds can be developed. Herbicide-tolerant crops may facilitate integration of conservation or no-tillage production systems since herbicides such as Roundup and Liberty control a broad range of weeds and reduce emphasis on complete burndown before planting the crop.

Herbicide-tolerant crops such as STS soybeans, SR corn, and IT/IR corn may become less important that Roundup Ready or Liberty Link crops since the herbicides associated with the aforementioned crops control a narrower spectrum of weeds than Roundup or Liberty. Herbicide-tolerant crops may also cause price wars among the chemical companies which would ultimately benefit the grower by lowering the cost of competing, narrower-spectrum herbicides. Herbicide-tolerant crops will give us herbicides with additional modes of action to manage herbicide-resistant weeds. However, one must be cognizant of the consequences of relying on one mode of action to control weeds. The problem weeds in these fields will be those that naturally have a greater degree of tolerance to the specific herbicide. Weeds such as velvetleaf, morningglory, dandelion (Roundup) and barnyardgrass and pigweed/waterhemp (Liberty) may become more prevalent and troublesome. Herbicide-tolerant crops allow us to manage herbicide carryover and drift in certain circumstances since they are tolerant to certain herbicides.

It does not appear from our perspective that the chemical companies are discontinuing the development of new herbicides. So as a summary, herbicide-tolerant crops will probably have a greater impact on the herbicides used than on the way the crops are produced or how much yield is taken from the field.

Biological Control

Biological weed control involves the utilization of natural enemies for the control of specific weeds. The objective of biological control is not eradication of the target species, but reduction of its population and crop competitiveness to an acceptable level under the conditions involved. This may be achieved by direct or indirect action of the biological control agent. Direct action includes boring into the plant and weakening its structure so it collapses and the consumption or destruction of vital plant parts. Indirect action is attributed to the biological control agent canceling the competitive advantage of the weed by reducing its vigor.

Developments in classical biological control will continue to progress for specific weeds in situations where there is enough time to allow released biocontrol agents to build up populations sufficient to control the target weed. An example is the establishment of the flower- and seed-feeding weevils for control of musk thistle infestations in pastures and grasslands. These agents continue to spread and suppress stands of musk thistle over 20 years after their initial release in Missouri. Concerning row crops, very few agents are available that will provide control of the numerous weeds in typical production fields. Short-term progress will occur with development of agents for management of specific weeds, probably different agents for weeds in different environments. Microbial agents especially will likely be developed for specific sites. The agents will be selected for efficacy in specific soil textures, soil moisture and temperature, landscape position, and under different types of tillage. Efficacy of the agents will be increased as new formulations are developed to deliver high concentrations of microbial agents to the field and to enhance attack of the weed target.

Only a few agents for biological control of weeds in cropping systems are forthcoming for commercialization. However, these successes are encouraging for discovery and development of agents on other important weeds. A bacterial pathogen has been successful in controlling several composite weeds (including cocklebur, common ragweed, sunflower) in corn and soybean and is under development as a POST product. A soilborne bacterium for controlling downy brome (cheat) in winter wheat is also under development and may be available within the next two years. Alternatively, efforts are underway to develop genetically-altered agents that are able to control several important weeds in a crop. This includes genetically transferring bioherbicide-producing genes from a soil bacterium into plant pathogenic bacteria capable of infecting target weeds. These are long-term tactics, however, as intense evaluation will be required to meet stringent regulations before approval is granted for release of such organisms in the environment. Agents for control of common milkweed, velvetleaf, hemp sesbania, sicklepod, nutsedges, and annual bluegrass are at various stages of development. The critical factors affecting development of the agents include proper formulation and effective application, sufficient host-range determination, and commercial potential of the product.

Greater progress will be made in development of "natural products" from metabolites of microorganisms and plants. For example, glufosinate, which is commercially available as "Liberty" or "Ignite", is a synthetic analog of a bioherbicidal compound produced by a soil actinomycete that was discovered in Japan. Several other compounds, either broad spectrum or specific in their activity, produced by soil microorganisms have been discovered and await further screening before serious consideration for commercial development occurs.

Optimum short-term success of biological control of weeds may be demonstrated by either management of naturally-occurring agents or using agents in an integrated approach. Crop environments may be managed to promote predatory insects and pathogens of weeds. Crop residues remaining on the soil surface, for example, promote populations of weed seed-feeding beneficial insects that use the residues as cover. Several examples of combining biocontrol agents with herbicides illustrate that weed control by either or both the agent and herbicide can be controlled or the spectrum of weed control can be broadened. Some agents applied during tillage are more effective than when applied at planting. Crop residues can serve as food sources for applied agents that produce herbicidal by-products. Likewise, cover crops may enhance establishment of microbial and insect agents and potential allelopathic activity of the cover crops might also be enhanced. Highly competitive and/or allelopathic crop varieties may be matched with compatible microbial biocontrol agents in the future to provide early-season weed suppression.

Mandated Pesticide Legislation and IPM

Federal legislation may require 75% of all crop acres to be managed under integrated pest management (IPM) programs by the year 2000. The definition of IPM is "the use of multiple tactics to control pests". IPM practices would include the use of preventative, cultural, mechanical, biological, and chemical techniques to manage pests such as weeds, diseases, insects and nematodes. Genetically transformed crops will give us the opportunity to manage several pests through variety selection. Currently, we have soybeans which are resistant to soybean cyst nematode, phytophthora root rot, and the herbicide Roundup. Future crops may contain genes or properties that make them resistant to insects (such as Bt corn), diseases, and nematodes and also resistant to non-selective herbicides such as Roundup and Liberty.

Realistically, we can probably assume that we are not that far away from having 75% of our crop acres on an IPM program. Typically we do not treat fields for insects or diseases unless they are observed in a field. Soil-applied herbicides are applied based on knowledge of weed history in the field. Some sort of scouting is done before postemergence herbicides are applied. The effect of this legislation on the availability or use of weed management tools (herbicides) is unclear at this time. It may mean that use of residual, soil-applied herbicides will become restricted; especially in environmentally sensitive production areas. This has already happened with atrazine in Wisconsin. Regardless of the legislative affect on soil-applied herbicides, it appears that increased emphasis will be placed on postemergence weed control.

Terms such as IPM are frequently mistaken with IWM (Integrated Weed Management) when discussing weed control. The definition of IWM is the use of multiple techniques used to manage weeds in a given field. IWM would include the use of preventative, cultural, mechanical, biological, and chemical techniques to manage weeds. Preventive methods include using weed-free crop seed, which is regulated by state and federal seed laws. Cultural weed control includes using crop row spacing, appropriate cultivars, plant population, crop rotations, and cover crops to suppress weeds. Mechanical methods include hand pulling, hoeing, mowing, water management, non-living mulches, artificially high temperatures, burning, and tillage to manage weeds. Biological methods are the utilization of natural events for control of specific weed species. Chemical control, the method we are most familiar with, involves using the appropriate herbicide to control weeds.

The benefits of mandatory IPM legislation include increased emphasis on weed identification and scouting to make weed management decisions, possible reductions in pesticide use due to increased reliance on low-rate postemergence herbicides, and increased emphasis of other IWM processes listed above.

Liabilities of mandatory IPM legislation might include loss of effective soil-applied herbicides for weed control, increased weed-seed production and spread, variability of postemergence herbicide performance under drought conditions, increased likelihood of herbicide-resistant weeds due to elimination of alternative modes of action provided by soil-applied herbicides, and increased trips across the field to control escaped weeds if weeds were large and/or drought stressed when the first postemergence application was made.

Custom Application

Due to increasing farm size, distance between farms, complexity of weed control products, and company marketing programs, there is an increased reliance on custom applications of agricultural chemicals. This trend will probably continue and increase in the future as minimum qualifications or licenses are required to buy pesticides and picking the correct herbicide becomes more difficult due to the large number of products available.

Some disadvantages of custom application are less control of application timing by the farmer. The custom applicator may be stretched over several counties and not able to spray a field in a timely manner. Many custom applications use reduced spray volume in order to cover as many acres as possible with 1 tank of spray. Excessive sprayer speed and low carrier volume can result in erratic weed control results, especially if weeds are stressed and the wind is blowing. Some custom applicators choose to buy into company sponsored rebate programs which provide financial incentive to the applicator but may not always include the correct herbicides for the weeds present in the growers field

Advantages to custom application include the possibility of being able to farm more acres since one doesn't have to haul their sprayer to every field. If you have someone do all of your spraying, you don't have to own a sprayer. Many custom applicators have high quality equipment so quality work can be done if the equipment is operated within the limits set forth by the herbicide label. Many custom applicators also have a scouting and record-keeping service which can assist the farmer in making the correct weed control decision and keeping records.

In order to increase profits, custom applicators will continue to increase the services offered to the grower since it is doubtful that there will be a large increase in crop acres sprayed. Software programs are currently being developed to assist management decisions such as pesticide and variety selection, and fertility management; however, be wary of early releases of programs that have not been validated by field research.

Certified Crop Advisor (CCA) Certification

CCA certification is not mandatory at this time, however many agricultural chemical suppliers are requiring their employees to acquire this certification. CCA certification requires the applicant to pass state and national exams on agronomic issues and also have 2 years of experience advising farmers. Ultimately the goal of this program is to enhance the knowledge and ability of crop advisors to give proper management recommendations.

Potential liabilities of this program include the inclusion of farmer certification to purchase agricultural chemicals and taking more management decisions out of their hands if they do not receive CCA certification.

Advantages would include a greater knowledge base or awareness of the farmers achieving certification and thus better decisions on all crop management issues. Increased knowledge might also prevent improper use and reduce off site movement and environmental contamination.

Increased certification and restrictions on pesticide use are likely in the future and will result in fewer decisions being made by farmers and more decisions being made by "experts" such as crop consulting firms, agricultural chemical dealers, seed dealers and farm management firms.

Global Positioning (GPS) and Global Information Systems (GIS)

This is technology that will allow farmers to pinpoint and map variability in a field with sophisticated computer equipment. Possible sources of variation in a field include fertility, pH, soil texture, drainage, compaction, cropping history yield, and weed infestation. One of the biggest benefits to this technology will be to alert the grower to where and what type of variability exits and may prompt them to alter management practices to address low yielding areas.

The effect of GIS/GPS on weed management is unclear. The possibility exists that variable rate applications of soil applied herbicides might be economically feasible in fields with large variations in soil texture and/or organic matter content. However, it is likely that large soil-texture and/or organic matter content variations would need to be present to justify the additional cost of a variable-rate herbicide application. Since there is currently a move away from preemergence herbicide use and towards postemergence herbicide use via herbicide-tolerant crops and impacts from pesticide legislation, variable-rate soil-applied herbicide application may never become widely practiced. However, variable-rate postemergence herbicide applicators may be developed in the future.

GPS/GIS will allow users to make detailed maps of weed infestations in a field and will allow the user to monitor weed patch sizes and specific shifts. This could be important in documenting a spreading herbicide resistant weed problem and appearance and spread of a new weed problem.

GPS/GIS is likely to influence a farmer's practices but the impact will probably be greater in soil pH and fertility management than in weed management.

Herbicide-Resistant Weeds

Since the initial discovery of herbicide-resistant weeds in 1968, more than 80 different species have infested thousands of acres in every major crop and cropping region throughout the world. The appearance of herbicide resistance is fueled by genetic variability which creates resistant individuals, and the selection pressure through continued use of herbicides with the same mechanism of action (molecular target site) which generates a large population.

Although the economic cost of herbicide resistance has been to date, arguably low, the widening incidence of resistance requires attention. Of particular concern are the reports that certain weed populations with evolved resistance to two or more families of herbicides with different target sites. In time, these populations with resistance to one family of herbicides may cross with another population resistant to a different herbicide family, thus creating weeds with resistance to multiple families of herbicides (multiple resistance). This would further limit the ability of growers to manage those weeds chemically. Movement towards total postemergence weed control with herbicide tolerant crops and mandated pesticide legislation would limit the number of herbicide families used for weed control and enhance selection for resistant weeds.

Fortunately, research and education are providing growers with the knowledge to manage herbicide-resistant weeds and minimize selection of additional populations. The take home message for those who manage weeds is to use multiple methods, rotate herbicides not based on trade name but on target site, and isolate plus eradicate resistant populations.