Chapter 1 – Introduction: Herbicide Resistance

Genetically modified crops are the most rapidly adopted technology in agricultural history due to the social and economic benefits these crops may offer. Crops that are genetically altered to be tolerant to herbicide, followed by crops resistant to insects, were the first agricultural biotechnology inventions successfully commercially exploited worldwide.

Weeds are one of the major problems encountered in crop management. Weeds compete with crops for water and nutrients and, as a result, decrease farming yields and productivity. Given the harmful economic implications of poor weed management, it is hardly surprising that herbicide production is a main driver of the agrichemical industry.

Until the emergence of genetically modified crops, selective herbicides (herbicides that only kill a specific weed, so they may safely come into contact with a planted crop of a different species) dominated the market. The development of selective herbicides is not an easy task and for this reason only a few common weed species could be contained. Given that each weed requires a different herbicide, herbicide application was frequent, in large volumes and very costly.

The advent of herbicide resistant crops caused a major shake-up in the agrichemical industry. Demand for selective herbicides fell significantly.   In certain countries, for the crops that have herbicide resistance available, herbicide-resistant crops are widely planted and otherwise non-selective (broad spectrum) herbicides are primarily used for weed management. Provided that the field crops are genetically modified to carry gene(s) for herbicide resistance, these broad-spectrum herbicides will not harm the crop. Broad-spectrum herbicides, such as glufosinate and glyphosate, are comparably biodegradable, display low levels of toxicity, and to date, weeds have shown minimal resistance to repeated applications.

Resistance to these broad-spectrum herbicides depends upon the genes that have been inserted into the crop plant. Following an introduction to herbicide resistance genetic modification practices, in Chapter 1, the rest of this technology landscape focuses primarily on the patent landscape surrounding the bar gene, which confers resistance to the broad-spectrum herbicide glufosinate.

The table below shows the most commonly used broad-spectrum herbicides and the genes that are inserted in crop plants in order to confer resistance.

Herbicide Resistance Gene Gene Source
Glufosinate, phosphinothricin, bialaphos bar, PAT (phosphinothricin acetyl transferase) Streptomyces sp

Alcaligenes sp.

Glyphosate aroA, EPSPS (3-enoyl pyruvyl shikimate 5-phosphate synthase) gene Agrobacterium sp
Bromoxynil BXN (Bromoxynil nitrilase) Klebsiella pneumoniae
Sulfonamides DHPS (dihydropteroate synthase), sul Broad host range plasmid
Sulfonylurea ALS (acetolactate synthase) Nicotiana tabacum

According to statistics released by the International Service for the Acquisition of Agri-biotech Applications (ISAAA), the total global area of transgenic crops in 2003 was 67.7 million hectares, a 40-fold increase from 1996 and a 15% increase within a single year.

Approximately six million farmers in 18 countries grew transgenic crops in 2003. The majority of the change between 2002 and 2003 occurred in the United States (63%), Argentina (20.5%), Canada (6.5%) and Brazil (4.4%).  In 2003, Brazil and the Philippines approved the planting of specific varieties of transgenic crops for the first time. Almost one third (30%) of the global acreage was grown in developing countries.

While the number of countries growing transgenic crops has been increasing, so too is the number of varieties of transgenic crops being approved.  In 2003, herbicide resistant crops made up 73% of the total genetically modified (GM) crop growing area, while insect resistant crops constituted 18%. GM crops containing genes for both herbicide resistance and insect resistance comprised 8% of the total GM crop growing area.

The table below compares the total area farmed worldwide using the dominant herbicide resistant crops in 2003.

Crop Planted Crop Area (mha)* Herbicide Resistant Crop Area (mha)* Herbicide Resistant Crop Area as % of Total Crop Area
















*mha = million hectares

While the statistics for 2004 had not been released at the time of writing, it is expected that the overall global area of transgenic crops and the number of countries growing transgenic crops was continuing to increase.

Currently, the agricultural GM market is dominated by a single company, Monsanto, which in 2003 produced approximately 90% of genetically engineered crops worldwide. This most likely reflects Monsanto control of patents conferring herbicide resistance and various Bt toxin genes for insect resistance, and access to enabling technologies that facilitate the development and commercialization of transgenic crops. Another four companies (Syngenta, Bayer Crop Science, Dow and DuPont) produce most other transgenic crops.

The table below shows the major manufacturers of the most common commercially grown herbicide resistant crops.

Crop Glyphosate Resistance Glufosinate Resistance Sulfonylurea Resistance Bromoxynil Resistance
Sugar beet 
(Beta vulagris)
Argentine canola
(Brassica napus)
(Brassica napus)
(Dianthus caryophyllus)
(Glycine max)
(Gossypum hirsutum)
Mon DuPont Calgene
Flax, linseed
(Linum ussitatum)
(Nicotiana tabacum)
(Oryza sativa)
(Triticum aestivum)
(Zea mays)
Mon PHB, DK, Syn, BCS, Myc PHBsc, Syn
(Chicorium intybus)

BCS, Bayer Crop Science; BZ Bejo Zaden; DK, DeKalb Genetics; Mon, Monsanto; Myc, Mycogen (Dow AgroSciences); PHB, Pioneer Hi-Bred (DuPont); SNETA, Societe Nationale d’Exploitation des Tabacs et Allumettes; Syn, Syngenta; US, University of Saskatchewan Research Center; *, obtained through chemical mutagenesis; SC, obtained through somaclonal variation (data obtained from