Monday, July 15, 2024
General Agriculture

Herbicide Tolerance and Advantages of Herbicide Resistant Cultivars

Herbicide resistant or herbicide resistance is the ability of a plant to survive and reproduce following exposure to a dose of herbicide that would normally be lethal to the wild type. Resistance may occur naturally due to random and infrequent mutations, or may be induced through conventional or recombinant technologies (Prather et al., 2000).

In natural populations, herbicide-resistant plants are rare. Resistant plants likely will persist in infested fields for many years, even in the absence of any additional selection with the herbicide.

There is no evidence that herbicides cause the genetic mutations that lead to herbicide resistance. Producing plants that are tolerant to specific herbicides is actually one of the largest uses of plant genetic engineering.

Herbicide tolerant crops “will allow non-persistent herbicides (e.g. glyphosphate) to be more widely used and will permit postemergence spraying of herbicide-resistant crops.” (Snow et. al, 1997).

It is also pointed out that genetically engineered crops reduced the use of pesticides and are thus environmentally beneficial (Wauchope etal., 2002; Hin etal., 2001; Fernandez- Cornejo et al., 2001).

The first decline in pesticide sales was observed in 1999 (James, 2001). The major factor in this decline is the increased area of transgenic crops: Round Up Ready and Liberty Link transgenic soybean, cotton, maize and oilseed rape (James, 2001).

Table 1 Some of the herbicide resistant crops under development, grouped according to the techniques of development (Connor and Field, 1995).

Techniques of developmentHerbicide resistant crops
Traditional Selectiontriazine-resistant canola

Seed Mutagenesis
terbutryn-resistant wheat sulfonyl urea-resistant soybean imidazolinone-resistant wheat
Cell SelectionSulfonyl urea-resistant canola atrazine-resistance in soybean



Genetic Engineering
Sulfonyl urea-resistance – cotton Phosphinotricin (basta) resistance (rice, canola) glyphosate (Roundup) resistance (cotton, soybeans, maize, wheat) bromoxynil-resistant (cotton, subclover) 2,4-D resistant cotton

Most classes of herbicides exert their effect on a single enzyme which catalyses a key metabolic reaction in the plant.

Herbicides work by affecting a single enzyme, which causes a metabolic change in the plant. There are three methods by which a plant can convey herbicide resistance (OCDE, 1999):

Producing an enzyme which detoxified the enzyme.

Producing an altered target enzyme which is not affected by the herbicide.

Producing physical or physiological barriers to the uptake of the herbicide.

Plants have been genetically engineered to be tolerant of a wide variety of herbicides.

Read Also : Weed Control Methods for Plantain Plantations

A variety of herbicide tolerant plants exists or is currently being developed for herbicide resistance or for use as selectable markers to identify transformed plants. Herbicide tolerance plants that has reached field-testing stages in the U.S. are listed in Table 2.

Table2: Herbicides and herbicide-tolerant cultivars (adapted from Snowet.al,1997)

HerbicideHerbicide-tolerant plant
ButricilCotton, potato, tobacco


Phosphoinothiricin
Alfalfa, Arabidopsis, barley, beet, corn, creeping bentgrass, melon, peanut, poplar, rapeseed, rice, soybean, sugar cane, sweet potato, tobacco, tomato, wheat
SulfonylureaCorn, cotton, grape, rapeseed, tobacco, tomato

Introgression of herbicide resistance from conventionally bread crops into weeds was first reported in 1970 and since has been increasing at an exponential.

Herbicide resistance to 17 classes of herbicide chemicals has been exhibited by biotypes of 172 weed species. Some major classes of herbicides to which weed resistance has been documented are shown in table 3:

Table3: Herbicide resistance (adapted from Pratheretal.,2000)

Type of resistanceNumber of resistant weed species(in the world)
ACCase inhibitors26
ALS inhibitors63
Triazines64
Ureas, amides20
bipyridiliums25
glycines3
dinitroanilines9
synthetic auxins20

As a result, a number of herbicides have lost their agronomic usefulness. Herbicide resistance can be prevented or delayed by

Herbicide rotation;

Monitoring after herbicide application;

Non-chemical control techniques;

Short residual herbicides;

Certified seeds;

Clean equipment.

Herbicide resistant weeds can be managed by:

Herbicide rotation;

Fallow tillage;

Close cultivation.

Prevention of weed seed spread through the use of clean equipment,

Monitoring the initial evolution of resistance by recognizing patterns of weed escapes typical of resistant plants,

Control of weeds suspected of herbicide resistance before they can produce seeds.

Advantages of Herbicide Resistant Cultivars

Herbicide Resistant

Several advantages can be identified for the utilization of these new cultivars and these include:

Increased options – the availability of an extra herbicide option is extremely valuable. In some cases it simply allows the crop to be grown and in others it provides an alternate mode of action for the management of herbicide resistance development in the weed population;

More flexibility – the increased options provide greater flexibility in terms of crop rotations and the ability to respond quickly to market opportunities;

Increased safety – where safer chemicals are able to be used, the risks to personal safety are clearly reduced. There may also be advantages where the chemical involved is environmentally benign or is used in very low concentrations;

Reduced crop/pasture damage – in some cases herbicides do inflict damage to the crop or pasture in addition to the weeds. By having the resistance character in the productive plant the chances of yield-depressing damage may be averted.

This is exemplified by the use of bromoxynil in subterranean clover where damage to the pasture can be significant, depending on the timing of the application;

Simplicity – in many cases the production system can be simplified – one application can replace two applications or two herbicides.

We need to be aware, however, that in nature, simple means unstable and we need to think of the system in holistic and not marginal terms.

Read Also : Mulching of Plantains and the Advantages

Concerns

Whilst the emergence of these cultivars is attractive in many cases, there is a number of aspects that should not be ignored:

Further development of resistance – The risk is that transgenic crops increase the reliance on a few herbicides and accelerate the selection pressure on weeds to evolve resistant biotypes. Herbicide resistant weeds can also appear via gene transfer from crop to weedy relatives.

Another problem of herbicide resistance now and for the future will involve weeds which exhibit multiple herbicide resistance.

Unknown in plants until recently the phenomenon of multiple resistance is defined as the expression (within individuals or populations) of more than one resistance mechanism.

Multiple resistant plants may possess from two to many distinct resistance mechanisms and may exhibit resistance to a few or many herbicides.

Double and triple herbicide resistance plants have been found in canola fields in Canada in 1997 (Thomas, 2001; Westwood, 2001; Hall et al., 2000).

Self-sown crop seed – these Herbicide Resistant crops will often germinate at the following season break where seed has been carried over from the previous harvest.

Where they have resistance to herbicides, then clearly they will not be controlled by the application of that herbicide.

Thus, for example in the case of glyphosate resistance, application of Roundup to provide a weed-free seedbed will be somewhat less successful where self-sown crop plants are present;

Resistant escapes – in some cases, the opportunity exists for the resistant genes to escape. In the case of canola, for example, the ability exists through cross pollination to transfer the resistance character to related species (for example, wild radish, wild turnip) ultimately defeating the purpose for which the HR cultivar was developed;

Abandonment of integrated weed management as previously described, there will always be the temptation to forget the application of the principles of integrated weed management because of the increased simplicity of the system.

Such temptation needs to be avoided in order that we preserve the lives of both the cultivars and the herbicide;

Impact on conservation farming – this is particularly important when considering the impact of glyphosate-resistant crops. Roundup is an essential part of the conservation farming system and we need to preserve its role.

Resistance buildup by weeds to this herbicide threatens our modern, soil-conserving practices of reduced tillage and we do not want to revert to the traditional ways of intensive cultivation. Resistance to glyphosate is now with us and we need to proceed with caution.

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Agric4Profits

Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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