HORT 250 - Biotechnology in Agriculture

Lecture 13 - Herbicides and herbicide resistance in transgenic plants (continued)

In the last lecture I discussed various strategies that are being used to develop herbicide tolerant crops (HTCs). The genes that have been and will continue to be developed utilize one of two strategies to provide herbicide resistance in transgenic plants. Either they encode an enzyme that in some way inactivates the herbicide (metabolism) or they encode a version of the target for the herbicide that is not susceptible to inhibition by the herbicide (target site). While Roundup Ready (glyphosate resitant) soybeans have received a great deal of attention, there are several other similar products under development by other companies. Listed below are the herbicides and crops that have been modified or selected to produce resistant varieties.

Herbicide	Commercial Name	Herbicide Tolerant Crops
Bromoxynil	Buctril	cotton, potato, tobacco
Glyphosate	Roundup	sugar beet, corn, cotton, lettuce, canola, soybean, tobacco, tomato, wheat
Glufosinate	Liberty, Ignite, Basta	alfalfa, sugar beet, corn, barley, melon, peanut, rice, canola, soybean, tomato, wheat
Sulfonylureas		corn, cotton, tobacco, tomato, flax, soybean, sugar beet
Imidazolinones		corn, canola, wheat
2,4-D		potato, sweetgum, cotton
Sethoxydim		corn
Triazines		canola

Many of these are in the research and development phase, while some are unlikely to ever be commercialised, such as resistance to 2,4-D or triazines.

A number of other new developments in the area of HTCs are listed below:

• Approximately 100 seed companies are in the process of developing, and in some cases already have marketed, Liberty-Link corn, which is resistant to glufosinate. This is in collaboration with AgrEvo, the manufcturer of Liberty.
• Liberty-Link soybeans will be available in 1998, providing some competition for Roundup Ready beans.
• Garst is planning to introduce corn with multiple herbicide resistances, to imidazolinones and glufosinate, in the near future.
• Roundup Ready corn will be available in 1998. This will be available only through Dekalb in 1998, but the technology will be licensed to other seed companies for release the following year.
  • it is my understanding that Roundup Ready corn will contain 2 genes for tolerance to the herbicide. The first is for the modified Agrobacterium CP4 EPSP synthase that is not inhibited by glyphosate. The second is also derived from a bacterium and encodes an enzyme that metabolises glyphosate. I do not know if both genes are required for corn to express "field tolerance" to Roundup.
• Roundup Ready cotton was introduced in 1997.
• Canola varieties with resistance to Roundup, Liberty or imidazolinones were introduced in Canada in 1997.
• Asgrow will introduce soybeans with resistance to Roundup and sulfonylureas in 1998.

It is clear that HTCs are going to have a major impact on weed control practices in the next few years. In only 2 years, Roundup Ready soybeans now account for approximately 15% of the soybean acreage planted in the US, and it is projected that they may account for as much as 40% next year. At the same time, competing products, such as Liberty-Link beans will begin to come on the market.

What are the economics of using Roundup Ready beans compared to "conventional" beans?

• Roundup Ready beans carry a premium price for seed of $5 per bag of seed. It takes about 1.5 bags to plant an acre, so Roundup Ready beans cost an extra $7.50 to plant per acre.
• The cost of treating an acre of beans with Roundup at a typical rate is approximately $15 per acre. This is based on only a single application of herbicide in a season.
• By contrast, "conventional beans" do not carry the premium price for seed, however the cost of conventional herbicides for weed control is approximately $7 per acre more than using Roundup.

It would appear that there is little difference in cost between the two methods - pay extra for Roundup Ready soybean seed but spend less for herbicide treatments. It is likely that the manufacturers of herbicides used in conventional beans will reduce the price of these compounds to eliminate the difference in cost of herbicide application, removing one of the advantages currently enjoyed by Roundup.

Why then have Roundup Ready beans been adopted so quickly by growers?

1. Roundup is an effective herbicide, and it is better than most other products at controlling perennial weeds.
2. The combination of Roundup Ready beans and Roundup means that very little thought must be given as to how to control weeds. There is no need to identify the weed and select specific herbicides, so this simplifies weed control.

There is one final point to make in this discussion on the economics of using Roundup Ready beans. The calculations given above assume that both Roundup Ready beans and "conventional" beans have the same yield. However, there is some indication that Roundup Ready beans actually suffer from a yield penalty, they have lower yield than the best non-transgenic beans, perhaps by as much as 3 bushels per acre (a good yield for soybeans would be around 50 bushels per acre). Why would the Roundup Ready beans have a lower yield? One explanation for this observation is the time it takes to produce the transgenic plants. When I discussed the methods used for transformation of plants, I mentioned the time it takes to evaluate transgenic lines, study the inheritance of transgenes, evaluate performance, etc. It likely takes 5 years from performing the initial transformation before the product is marketed. During this period of time, plant breeders are still working to improve the conventional soybean varieties, and are making small but significant gains. By the time the Roundup Ready beans are marketed, the best conventional bean varieties have been further improved by the breeders so that they have a higher yield than the beans that have been modified through transformation. As these genes for herbicide tolerance become incorporated into more of the elite breeding lines used for variety improvement, this yield penalty suffered by Roundup Ready beans is likely to diminish and disappear in the coming years.

We have spent most of our time discussing the manipulation of herbicide resistance in agronomic crops. However, there is also significant interest in applying this technology to other plants and crops, as outlined below.

• horticulture crops (vegetables, fruits, etc.) where the choice of herbicides and other pesticides is in some cases extremely limited.
• forest tree species where competition from weeds is a major problem, especially during seedling establishment. Herbicide tolerant forestry tree stocks might allow weed control with broad spectrum herbicides.
• landscape plants including grasses. Golf courses have serious weed control problems that could be alleviated using this technology. There may be other applications in the landscape area for development of herbicide tolerant varieties.

In each of these examples, it is important to consider the cost and time involved in developing these novel plants. Among the expenses are:

1. development of a transformation system for that species.
2. design and construction of gene(s) for herbicide tolerance. As the scientific understanding of these problems increases, it may be quite feasible to utilise essentially the same gene(s) in many different species.
3. testing and evaluation of transgenic lines to ientify those with ideal characteristics.
4. propagation and large scale production of selected material

The investment required for a single species or crop is quite substantial and may not be justified on economic grounds by the sale of seed or increased use of a particular herbicide. For example, the development of herbicide tolerant melons might not result in significantly higher prices for melon seed, and the acreage of melons that would be prayed with the herbicide might not produce a significant increase in herbicide sales.

To end our discussion of these aspects of HTCs, let us consider the impact of these technological developments on various groups that are involved.

Chemical companies

• We will discuss next time whether or not this will lead to increased use of herbicides (I think it will not increase their use). However, it will likely affect what herbicides are used. Currently, Roundup is being used on all of the Roundup Ready beans at the expense of herbicides that used to be used on "conventional" beans. As more HTCs are released, this will continue to change.

Farmers

• They have been using herbicides for many years and will see little difference. It may be more complicated to work out the economics of which variety and herbicide to combine, but these are problems that farmers have faced before. HTCs will open up some new possibilities for weed control, but they will also change some of the management decisions about crop rotations. For example, it would not be wise to plant Roundup Ready beans in the same field after Roundup Ready corn - how do you control volunteer corn in the soybeans?

Consumers

• I see little impact on consumers. The price of soybeans are unlikely to be affected. Unless the consumer wishes to avoid items that contain products from genetically modified organisms (which will become increasingly difficult to do) the vast majority of the population will be unaware of any change.

Seed companies

• The providers of seed will have to contend with developing varieties with resistance to different herbicides while at the same time providing high yield, disease resistance, tolerance of environmental stresses, etc. So this will add another layer of complexity to plant breeding and the production, distribution and marketing of seed. Therefore for seed companies I see this development as adding complexity to their task without offering much in the way of increased return, i.e. higher prices for seed.

The entry of HTCs into the agricultural marketplace over the next few years will likely cause significant changes in the market for herbicides and seeds, as well as changes in the weed control practices that are used. Stay tuned!

Next time we will discuss the potential risks and benefits of using new herbicide tolerant crops (HTCs).

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