India is having increasing difficulty controlling weeds in its wheat fields. Weed scientists suggest that this may be due to the evolution of herbicide-resistant weeds. Should such weeds become widespread, wheat yields could decrease enough to threaten India's self-sufficiency in wheat production. Despite this danger, few efforts have been made to determine whether herbicide-resistant weeds are evolving in India. If they are, it will not be easy to manage them. The lack of patent protection in India will constrain the importation of other herbicides that might effectively control the weed species found in India's wheat fields as well as the importation of genetically engineered wheat varieties that can detoxify herbicides normally damaging to wheat. Thus India's wheat farmers may have to control herbicide-resistant weeds by changing their tillage patterns or by periodically growing crops other than wheat. The latter strategy must be considered carefully as there are limits on demand for non-wheat crops.
In India, as in many countries, wheat is a staple crop, consumption of which allows people to meet a large portion of their caloric requirements. It has become an increasingly important source of nutrition in India since 1966, when dwarf, high-yielding varieties of the crop were introduced in the country. Cultivation of these varieties has allowed India to meet its domestic demand for wheat and, in recent years, to put a portion of its wheat crops in reserve.
Although India is seeking to diversify its crops, it will be necessary for the country to increase wheat yields in order to keep pace with the food needs of its increasing population. This may be difficult as there are signs that such yields may be declining in some of India's key wheat-producing states. On some of the wheat farms in these states, there is evidence of diminishing returns to inputs of fertilizer and irrigation and of an increasing problem with weed control.
The weed control problem has attracted the attention of weed scientists, who have become concerned with the same problem on wheat farms in developed countries during the last decade. They fear that in India, as in these countries, weeds are becoming resistant to herbicides applied to wheat crops. They also realize that, if left unchecked, the evolution of herbicide-resistant weeds could seriously reduce grain yields in India. It is estimated that if such weeds become widespread, yields would decline by 30 percent or more. Although such a reduction would threaten the country's self-sufficiency in wheat production, few agricultural experts outside the weed science community have paid much attention to the weed control problem in India.
Potential strategies for slowing the evolution of herbicide-resistant weeds are constrained by economic and policy factors, as well as by lack of knowledge about weed biology. It is clear that international agricultural research organizations and agricultural support institutions in India will have to give priority to weed control research if efforts to make India's wheat production system sustainable are to be successful.
Evolution of herbicide-resistant weeds
Herbicides are widely used in crop production to control weeds, which compete with crops for light, space, nutrients, and water. They typically work by disrupting the metabolism of weeds. One way they do this is by inhibiting enzymes, which catalyze biochemical reactions such as photosynthesis, within the plants.
Most herbicides are applied at rates sufficient to kill 85 to 95 percent of a weed population. The remaining weeds normally survive because they are shaded by other plants, are missed during spraying, or germinate after the herbicide has dissipated. Because of genetic variability within a species of weed, however, a small number of individual weeds are able to survive the application of a particular herbicide that is effective in controlling the vast majority of the weeds of the same species. If the herbicide is used on one population of the weed species year after year, the weeds that genetically differ from the rest of the weeds in the species will multiply. As the weeds that are susceptible to the herbicide are removed from the population, the resistant strains have more room to grow and to flourish. Without intervention—such as the use of an herbicide that attacks the weeds at a different point of vulnerability—these strains will become a predominant part of the population.
The incidence of herbicide-resistant weeds in wheat crops is a relatively recent phenomenon, but one that is widespread. Such weeds were first observed in the early 1980s in Australia and were later observed in England, Israel, the United States, and Canada. In these countries, the resistant strains do not appear to have proliferated through the spread of seeds. Instead, they seem to have evolved concurrently in a number of locations as the result of the selection pressure exerted on them by herbicides. In the United States, for example, herbicide-resistant populations of some broadleaf weeds common in wheat have appeared in areas where the herbicides chlorsulfuron and metsulfuron-methyl have been applied over a period of four to five consecutive years.
To prevent the increase of herbicide-resistant weeds in wheat crops, U.S. wheat growers have been advised to alternate herbicides, one year applying a herbicide that targets a particular biochemical site of action, the next year a herbicide that targets a different biochemical site of action, and so on. In some cases, it has been recommended that they use two or more herbicides simultaneously or discontinue use of particular herbicides that may exert too strong a selection pressure on weeds. Such strategies are viewed as sufficient for the short to medium term, or until the problem of herbicide-resistant weeds warrants more drastic action.
To control herbicide-resistant weeds, U.S. wheat growers have been advised to alternate herbicides, use several herbicides simultaneously, or discontinue use of herbicides that exert too strong a selection pressure on weeds.
Such action has been necessitated in Australia and in one area in England, where weeds in wheat crops have acquired tolerance to one or more herbicides not as a result of direct exposure to those herbicides but through exposure to similar herbicides—a phenomenon known as cross-resistance. In Australia, more than 3,000 large wheat farms, comprising nearly one million hectares, have weed biotypes that are resistant to virtually all selective herbicides that can be used on wheat crops. In addition to alternating herbicides or discontinuing their use, Australian wheat farmers have been forced to change cropping systems in order to incorporate integrated weed management strategies, such as using mechanical means to remove weeds from fields or allowing pasture to grow in place of wheat every other growing season. These strategies impose economic hardships on growers, because they are less cost-effective than the use of herbicides.
Herbicide use in India's wheat production
During the early 1960s, agricultural scientists were experimenting with ways to increase the productivity of India's native wheat varieties. Their research indicated that a substantial increase in the use of fertilizer would result in increases in the vegetative growth of the wheat plants, causing them to fall over and wheat grains to become lodged on the ground. Because the amount of fertilizer used on fields sown with native wheat varieties could not be increased, the maximum yield of India's wheat farms was limited to 11 or 12 bushels of wheat per acre. Such a yield was insufficient to meet India's demand for wheat in the mid 1960s, when two years of drought (coupled with an increasing population and a decreasing amount of land that could be converted to grain production) forced the country to import food grains in order to avert widespread hunger.
The crisis sparked interest in some high-yielding dwarf varieties of wheat that had been developed in Mexico. These varieties, which were introduced in India in 1966, were superior to India's native wheat varieties in that they had thicker straw and grew no more than two feet high. Thus farmers could increase inputs of fertilizer in order to increase yields without causing the plants to droop and grains to lodge on the soil. Moreover, the new varieties matured earlier and put out a larger number of tillers than traditional varieties do. (Tillers are stems that form from buds below ground on the main stem and that eventually produce heads and increase the supply of seed.)
The adoption of dwarf varieties allowed India to increase its wheat production fivefold in twenty-five years; however, it also necessitated a dramatic increase in the use of herbicides. When tall varieties of wheat were planted, weeds were not a serious problem. Such varieties grow faster than both grass and broadleaf weeds and thus are able to compete successfully for sunlight. However, grass weeds such as Phalaris minor (canarygrass) and Avena spp (wild oats) are highly competitive with dwarf varieties of wheat. Canarygrass and wild oats grow faster and taller than the dwarf varieties and broadleaf weeds, and thus block sunlight from these plants. They are also more responsive to fertilizers than dwarf wheat varieties and broadleaf weeds. If uncontrolled, grass weeds (particularly canarygrass) can remove much of the nitrogen from fertilizer applied to wheat fields.
Moderate infestations of canarygrass and wild oats—100 to 250 of the weeds per square meter—reduce wheat yields by approximately 34 percent. On most wheat farms where canarygrass is a serious problem, the ratio of the weed to wheat plants is about four to one. At this level of infestation, wheat yields are reduced by approximately 60 percent. In some instances, wheat fields have been plowed under or used as green fodder due to severe infestations of canarygrass.
Neither mechanical nor manual removal of weeds is an effective means for controlling grass weeds in India's wheat fields. Mechanical weeding is not feasible because wheat plants are sown very close together. Manual weeding is not feasible because both wild oats and canarygrass look like wheat in their juvenile stages. By the time these weeds are distinguishable from wheat plants, the damage due to competition has already occurred.
Manual weeding is not an effective means for controlling canarygrass in lndia's wheat fields. By the time the weed, which looks like wheat in its juvenile stage, is distinguishable from wheat plants, the damage due to competition has already occurred. Given limitations on manual and mechanical weeding, weed scientists found that the most effective way to rid wheat fields of canarygrass and other grass weeds was to use herbicides
With the introduction of high-yielding dwarf varieties of wheat in India, grass weeds became a problem; because neither mechanical nor manual weeding is effective in controlling these weeds, wheat farmers had to increase their use of herbicides.
Limitations on mechanical and manual weeding led weed scientists to conclude in the early 1980s that the most effective way to rid wheat fields of grass weeds and to maintain the potential yield of these fields was to use herbicides. The primary herbicide used by wheat growers in India to control such weeds is isoproturon. Experiments conducted in the early 1980s indicated that the use of isoproturon could reduce populations of canarygrass by 96 percent. Other experiments of the period showed that wheat yields from plots where isoproturon was used to control canarygrass were approximately 33 percent greater than yields from plots where the herbicide was not used.
In Punjab and Haryana, the two states that together account for 34 percent of India's total wheat production and more than three-quarters of India's procurement of surplus wheat for reserve stocks, approximately 40 percent of the acres planted in wheat are now treated with isoproturon. Farmers who use neither isoproturon nor any other herbicide to control grass weeds in wheat still depend on manual weeding for weed control. These farmers allow weeds to become well established before removing them so the weeds can be used as fodder for cattle. Of course, the delay in the removal of grass weeds means that these farmers are probably losing one-third of their potential wheat yield.
Potential for the evolution of herbicide-resistant weeds
As noted above, the most difficult weeds to control in wheat are grass weeds, which are related to wheat in their taxonomic, phenological, morphological, and biochemical characteristics. Certain grass weeds have evolved morphological and phenological resemblances to wheat during the 8,000 years since its domestication. They may now be responding to the use of herbicides by evolving a biochemical mimicry of wheat's enzyme system that detoxifies herbicides. By producing higher levels of the enzymes in this system, the weeds could detoxify herbicides in the same manner as wheat.
Because herbicide-resistant weeds have been a problem in other parts of the wheat-producing world, it is expected that they will become a problem for wheat farmers in India. Indeed, they may be the cause of the recently observed decline in the effectiveness of isoproturon on some farms where the herbicide has been applied for ten or more consecutive years. On these farms, the use of isoproturon did not achieve satisfactory results in the last two to three of these years, even when it was applied in doses higher than those formerly effective in controlling canarygrass.
In November 1991, an experiment to determine the ability of isoproturon to control the weed was begun by R. K. Malik in India. Canarygrass seeds were collected from six locations in Haryana, where isoproturon had been used continually for ten to fifteen years, and were grown in pot cultures in a laboratory. After the cultures were sprayed with isoproturon, it became clear that the herbicide was not effective in killing one particular canarygrass biotype. An investigation of three of the locations from which the seeds used in the experiment were taken revealed that isoproturon had failed to provide more than 30 percent control of canarygrass at these locations during the winter season of 1991–1992. It also indicated that the poor control could not be attributed to impurities in or the dilution of isoproturon, the method by which it was applied, or the timing of its application.
Some populations of canarygrass may have evolved partial resistance to isoproturon; if so, the selection pressure exerted on weeds by the herbicide, if it continues to be used, might diminish the effectiveness of isoproturon in controlling all populations of canarygrass within a few years.
If some populations of canarygrass in India have evolved partial resistance to isoproturon, wheat farmers must contend with two potential problems. The first is that the selection pressure exerted on weeds by isoproturon, if it continues in use, might quickly diminish the effectiveness of the herbicide in controlling all populations of canarygrass within a few years. The second is that canarygrass might develop cross-resistance to other herbicides that can be used in wheat.
The evolution of herbicide-resistant weeds is rare in fields where different crops are grown in sequence on the same field; however, it could be occurring in India, where wheat is double-cropped with rice and to a lesser extent with sugarcane and maize. Although wheat and rice are grown in the same field—wheat in the winter and rice in the summer—wheat-rice cropping is essentially a wheat monoculture from the standpoint of grass weed control. This is because rice is grown when fields are flooded with water, and grass weeds such as canarygrass do not germinate in standing water. Thus repeated use of one herbicide could exert the same degree of selection pressure on weeds in a field where wheat alone is grown as it does on weeds in a field where wheat is grown during one season and rice during another.
Strategies for managing herbicide-resistant weeds
Wheat-producing countries in the developed world have demonstrated that the evolution of herbicide-resistant weeds can be slowed by the adoption of strategies that prolong the effectiveness of herbicides. However, these strategies become costly once herbicide-resistant weeds become widespread. Moreover, the kinds of strategies that can be adopted are limited once such weeds make up more than 10 percent of a given weed species. Even when herbicide-resistant weeds make up only a small proportion of a weed population, intensive weed management efforts are required to prevent their increase. These efforts include periodically alternating two or more herbicides, changing tillage systems, and growing different crops.
As noted above, weed scientists in India have begun to conduct both field and laboratory research to determine how tolerant canarygrass is to isoproturon and whether the weed has developed cross-resistance to other herbicides. Their findings will lead to recommendations regarding the future use of herbicides in India's wheat production. These could include changes in the rates at which herbicides are applied and in the timing of application. As in some parts of the wheat-producing developed countries, they could also include the simultaneous use of several herbicides, each of which targets a different biological site of action in weeds; or the abandonment of some or all herbicides used on wheat crops. Recommendations not involving herbicide use could include changes in tillage practices and in the periodic substitution of other crops for wheat, both of which can sometimes prolong the effectiveness of a herbicide.
Recommendations for controlling herbicide-resistant weeds in India could include alterations in the rates and timing of herbicide applications, the simultaneous use of several herbicides, changes in tillage practices, and the periodic substitution of other crops for wheat.
It may be that the above management strategies will fail to prevent canarygrass from evolving resistance to all herbicides normally effective in controlling the weed. Even if the evolution of herbicide-resistant weeds can be delayed in the short run, it has become evident to weed scientists in developed countries that the problem requires long-term monitoring and research. In light of the potential for widespread evolution of herbicide-resistant weeds, either in the short term or the long term, India's agricultural scientists and policymakers may want to consider changes in agricultural research agendas in order to focus on the increasingly complex problem of controlling weeds.
If India is to be successful in dealing with the problem of herbicide-resistant weeds in wheat crops, it needs to establish a committee similar to those that have formed in developed countries to study the problem. A group representing India's herbicide manufacturers, agricultural scientists, agricultural extension workers, and wheat farmers could monitor the spread of such weeds, develop management strategies, and facilitate implementation of those strategies.
Setting research priorities
In 1991, the International Rice Research Institute and the International Maize and Wheat Improvement Center began several research efforts aimed at identifying and correcting problems that limit further increases in the productivity of India's wheat-rice cropping system. In their assessments of India's wheat production, these organizations and the World Bank have noted the increasing difficulty of controlling weeds. However, they have made no mention of the possible link between that difficulty and the evolution of herbicide-resistant weeds, even though such weeds could easily offset any gains in the productivity of India's wheat-rice cropping system. Clearly, management of herbicide-resistant weeds should be considered in any international research efforts to enhance that system.
Several national programs aimed at correcting problems with the system are under way. These programs have focused on the problem of soils becoming saline as a result of extensive irrigation. While improving the operation and management of irrigation systems is important, India's policymakers need to give the same attention to weed management problems that they give to irrigation if India is to realize its goal of self-sufficiency in food production.
To successfully manage herbicide-resistant weeds, knowledge of weed ecology and weed biology is essential. Specifically, there is a need for information about the length of time that weed seeds remain viable in the soil, the ratio of weed seeds that germinate after one year, and the effects of alternative crop production systems on the survival and proliferation of weeds.
Other research efforts could focus on the genetic engineering of a wheat plant that could detoxify herbicides normally damaging to wheat. Some genes that could be used in the genetic engineering of wheat would make the plant resistant to the herbicides glyphosate and glufosinate, which cannot be used on wheat fields at the present time because they would kill wheat plants along with weeds. The genes that would make the genetically engineered wheat's enzyme system capable of detoxifying glyphosate and glufosinate are introduced from outside the wheat gene pool. In the foreseeable future, therefore, it is unlikely that weeds that are similar to wheat could evolve a biochemical mimicry of the enzyme system.
Policy and economic constraints
India's policy of domestically producing needed goods guides much of its economic development. Self-sufficiency in food grain production is a particularly high priority for India, which does not want another food shortage crisis like the one it experienced in the mid 1960s. Should herbicide-resistant weeds evolve, strategies to check the increase of such weeds fail, and wheat yields decline as a result, India might have to import wheat from the United States and other countries. Doing so would entail a reduction in the amount of hard currency that India spends on petroleum products and other imports vital to its economy.
Rather than import wheat, India might try to deal with the problem of herbicide-resistant weeds by importing other herbicides. If weeds in the country's wheat farms are becoming resistant to isoproturon, which is produced in India, the introduction of imported herbicides may play a key role in an effective management strategy. However, it is uncertain that any of the other herbicides would in fact control herbicide-resistant weeds in India's wheat farms.
Given its policy of domestically producing needed goods and given constraints to the importation of wheat, herbicides, and genetically engineered wheat varieties, India may attempt to manage herbicide-resistant weeds by periodically growing crops other than wheat; however, this strategy may be hampered by limits on demand for non-wheat crops.
Moreover, in the absence of patent protection for pesticides used in India, foreign producers of herbicides have a strong disincentive to introduce new herbicides into the country. Lack of patent protection is also likely to deter foreign biotechnology firms from introducing genetically engineered wheat varieties into India.
Given constraints to the importation of wheat, herbicides, and genetically engineered wheat varieties, India may attempt to manage herbicide-resistant weeds in wheat by making changes in crop rotations and tillage practices. Changes in either will prevent herbicide-resistant weeds from completing their life cycles, but changes in crop rotations must be considered carefully. It would be necessary to assess the economic impacts of growing crops other than wheat in order to determine whether such changes would be prudent. A recent economic analysis of the options for diversifying crops in Punjab revealed that the production of vegetables and fruit were more profitable than wheat-rice cropping. However, increased production of these crops is constrained by limits on demand for the crops and by a lack of facilities for processing the crops before they are brought to market.
It is clear that there are no easy solutions to the problem of herbicide-resistant weeds in wheat. Given that fact, India must act quickly to determine to what extent, if any, herbicide-resistant weeds are evolving in its most productive wheat-growing states. If such weeds are in fact evolving in India, they would likely evolve in several other developing countries. Thus any management strategies that prove effective in India may have to be adopted in Pakistan, Bangladesh, and Nepal, where wheat production practices are similar to those in India, and in China, where the pattern of herbicide use on wheat crops is also similar to that in India. Like India, these countries can ill-afford a decrease in their wheat production due to unexpected weed control problems as they struggle to increase such production in order to feed their growing populations.
The management of weeds in wheat cannot be taken for granted. Weed science research needs to be supported, and policymakers need to be made aware of the prospects for the evolution and spread of herbicide-resistant weeds, as well as the potential impacts of this phenomenon if not anticipated and dealt with in a timely fashion.
Leonard P. Gianessi is a fellow in the Quality of the Environment Division at Resources for the Future. Cynthia A. Puffer is a research associate in the division. This article is based on research conducted by the authors, Jonathan Gressel of the Weizmann Institute of Science, Leonard Saari of E. I. du Pont de Nemours & Company, and Ashok Seth of the World Bank.
A version of this article appeared in print in the May 1993 issue of Resources magazine.
A version of this article appeared in print in the May 1993 issue of Resources magazine.
