Integrating Herbicide-Resistant Crops with Diverse Rotations

in the Central Great Plains

Randy Anderson

Northern Great Plains Research Laboratory

P.O. Box 459, Mandan, ND

Herbicide-resistant crops offer producers a multitude of benefits for weed management. Producers can use herbicides with different modes of action, thus broadening the spectrum of weeds controlled within a selected crop. Also, this technology can increase flexibility in timing control actions as well as economic returns in some crops.

However, herbicide-tolerant crops may have negative consequences. Crop seed, dispersed during harvest, can establish in the following crop; producers may have to alter their weed control program to manage herbicide-resistant volunteers. A second concern is transfer of genetic resistance among plants. A striking example occurs with canola; volunteer canola was found to be resistant to Roundup (glyphosate), Liberty (glufosinate), and Pursuit (imazethapyr). This resistance was attributed to pollen flow among herbicide-resistant canola cultivars (Hall et al., 2000). A similar situation occurred with winter wheat. Winter wheat cultivars have been developed that are resistant to Raptor (imazamox), with the goal of controlling jointed goatgrass. However, hybrids of jointed goatgrass and winter wheat resistant to Raptor were observed after two years of Raptor use (Seefeldt et al., 1998). Again, transfer of resistance was attributed to pollen flow among species.

A further concern is development of herbicide-resistant weeds due to selection pressure by herbicides (Powles et al., 1997). When a control tactic is imposed on the weed community, weeds adapted to the tactic increase in density over time. A disturbing trend is multiple resistance – weeds resistant to herbicides with one mode of action can be resistant to herbicides with other modes of action, even if those herbicides have never been applied to the species. The cause of multiple resistance is unknown, but selection pressure can result in weeds with different metabolism, thus disrupting activity of other metabolic-based herbicides on that species. An example of multiple resistance occurs with rigid ryegrass in Australia; this species is now resistant to herbicides of most modes of action used in that country (Powles et al., 1998).

The objective of this paper is to review volunteer wheat emergence and cultural strategies that delay development of herbicide resistance, then summarize experiences of producers in the Central Great Plains (eastern Colorado, western Kansas and western Nebraska) with integrating herbicide-resistant crops into their rotations. Our goal is to suggest strategies that minimize the negative consequences of herbicide-resistant crops.

Longevity of volunteer winter wheat seed in soil

Understanding emergence patterns and seed survival of crop volunteers is vital for devising strategies to prevent gene transfer by pollen flow. For example, volunteer winter wheat can survive at least 16 months after harvest and establish seedlings in the next wheat crop of a winter wheat-fallow rotation (Anderson and Nielsen, 1996). In this study, we found that seedling emergence averaged 39 plants/m2 after September 15 (optimal time for winter wheat planting). Seedling numbers ranged from 1 to 126 plants/m2 across four years. Also, pattern of emergence varied; in one year, all volunteer seedlings emerged within two weeks after September 15 whereas in another year, seedling emergence continued for eight weeks. We also found that seedling emergence was two to five times greater in no-till compared to tillage with a sweep plow.

A surprising trait of volunteer wheat emergence was its variability across years. A multiple of factors apparently affects emergence, such as timing of precipitation or tillage; because of emergence variability, it will be difficult to predict levels of volunteer seedlings in the next wheat crop. Our data suggest that organizing rotations to include at least a two-year interval before growing herbicide-resistant wheat again will help in managing volunteers and avoid pollen flow. However, winter wheat cultivars differ in seed dormancy. In Nebraska, seedlings of one cultivar still emerged two years after harvesting (Wicks 2001), therefore, a three-year interval before the next wheat crop may be more appropriate.

Tactics to minimize selection pressure on weeds

Regardless of what herbicide is used, imposing selection pressure by continuous use of a herbicide will lead to weed species that are more difficult to control. Initial focus in the scientific community has been on weed resistance, but tolerance in weed species also is a concern. Tolerance is the ability of a weed species to survive normal-use herbicide treatments; higher rates are needed for control. For example, with glyphosate, it is hypothesized that development of resistance will be rare because of its unique mode of action (Powles et al., 1997). However, continuous use of glyphosate is selecting for tolerance in weeds; biotypes of toothed spurge (Euphorbia serrata), horseweed (Conyza canadensis), and wild buckwheat (Polygonum convolvulus) require double the normal-use rates for control (Wicks, 2001). Even if not resistant, tolerant weeds increase input costs.

Herbicide options will remain viable if selection pressure is minimized. Strategies that minimize selection pressure have been identified, enabling producers to devise weed management systems that control weeds yet avoid favoring resistant and/or tolerant weed species (Figure 1).

Figure 1. Management tactics that minimize selection pressure of

herbicides on weeds.

Varying modes of action

A key strategy to reduce selection pressure is rotating herbicides with different modes of action (Gressel and Segel, 1990). Devising rotations comprised of a diversity of crops offers numerous herbicide options to ensure that selection pressure by one mode of action class of herbicides is not imposed for two consecutive years.

Herbicide mixtures

A second strategy is to use mixtures of herbicides with different modes of action (Powles et al., 1997). In the past, herbicides were combined to broaden the spectrum of weeds controlled. However, with resistance management, the goal of mixtures changes; herbicides in mixtures should control the same spectrum of weeds. This approach eliminates weeds that are resistant to one herbicide with other herbicides in the mixture.

Off - years

In Manitoba, weeds resistance to herbicides that inhibit the ACCase enzyme have proliferated in the last decade (Bourgeois and Morrison, 1995). This class of herbicides, called Group I in Canada’s classification of herbicides, includes Hoelon (diclofop) and Poast (sethoyxdim). Resistance development can be delayed if a herbicide of this class is used only once every three years; the two-year interval allows susceptible biotypes to produce seed and replenish the seed bank in soil. This reduces frequency of resistant biotypes in the weed community.

Cultural practices

If crop competitiveness can be improved, producers may be able to eliminate herbicide use in some crops. Cultural practices such as fertilizer placement or higher seeding rates, when used alone, usually improve crop’s tolerance of weeds marginally. However, integrating several practices into cultural systems synergistically improves crop growth such that herbicides may not be needed. For example, a cultural system comprised of tall cultivars, increased seeding rates, fertilizer placement, and delayed planting increased proso millet competitiveness with weeds 15-fold (Anderson, 2000). Yield loss due to weeds was reduced from 30% to less than 2%. This approach enables producers to eliminate herbicide selection pressure in proso millet.

 

A rotation design based on a cycle-of-four crops

helps manage herbicide resistance

Producers in the Central Great Plains are diversifying the winter wheat-fallow rotation by including summer annual crops such as corn, sorghum, or sunflower. A successful rotation design is to alternate winter annual crops with summer annual crops. Producers are arranging crops and fallow in a cycle-of-four design; this approach increases grain yield and economic returns (Anderson, 1998).

The cycle-of-four design also helps producers integrate strategies that minimize herbicide resistance with their rotations. If three or four different crops are used, it is easy to achieve a three-year interval out of a crop, thus improving control of crop volunteers as well as minimizing pollen transfer among similar plants. Furthermore, crop diversity expands the arsenal of herbicides available, therefore producers can rotate herbicides with different modes of action.

 

Planning triangle for herbicide-resistant crops

A well-planned rotation with herbicide-resistant crops should address three issues; controlling crop volunteers, avoiding pollen flow, and minimizing selection pressure on the weed community (Figure 2). A key concern in designing rotations with herbicide-resistant crops is the

following crop; producers may lose benefits gained with herbicide-resistant crops if weed control

Figure 2. Planning triangle to guide design of rotations with herbicide-

resistance crops.

in the next crop is either ineffective or more expensive. Producers in the Central Great Plains have explored rotations with herbicide-resistant corn; some rotations successfully address each component of the planning triangle, thus accruing the benefits of herbicide-resistant crops without favoring negative consequences.

Wheat - corn - sunflower - fallow

This is the most effective rotation to include herbicide-resistant corn. Grass herbicides such as Poast easily control corn volunteers in sunflower, and diversity of crops allows rotating herbicides with different modes of action. Growing corn only once every four years minimizes pollen flow with other hybrids.

An observation with corn resistant to non-residual herbicides such as glyphosate is that weeds emerge after the last application and produce seeds (Wicks, 2001). This response is more prominent in semiarid regions where crop populations are lower; the crop canopy is not as competitive. Weed seeds produced in corn enter the soil’s seed bank, resulting in more weeds in following crops.

A characteristic of Central Great Plains production systems is that non-crop periods between crops are crucial for precipitation storage; thus weeds during these periods need to be controlled. Some herbicides (such as glyphosate) are used to control weeds between crops; if glyphosate is used in corn, other more-expensive herbicides will be required for non-crop periods. In some situations, the economic trade-off can be detrimental to net returns.

Wheat - corn - proso millet - fallow

A weakness with this rotation is lack of herbicides to control volunteer corn in proso millet. However, if planting of proso millet is delayed until June 5-10 in a no-till system, producers can control volunteers yet still maintain grain yield. A concern, however, is that weeds resistant to the ALS enzyme inhibitors [metsulfuron (Ally); prosulfuron (Peak), imazethapyr (Pursuit)] are common in the region, especially kochia. Metsulfuron and prosulfuron are used in winter wheat and proso millet, respectively, so producers avoid use of corn hybrids resistant to imazethapyr to minimize selection pressure for ALS-resistant biotypes.

Wheat - corn - corn - fallow

Some producers are planting imidazolinone-resistant corn in the first year, followed by glufosinate-resistant corn in the second year, with weeds controlled during non-crop periods with glyphosate. A drawback with this rotation is high input costs; seed is more expensive whereas insecticides are required to control corn rootworm in the second corn crop. Furthermore, yield in the second corn crop is usually 20 to 30% less than first-year corn.

Wheat - wheat - corn - proso millet

Producers are evaluating rotations that eliminate fallow. Herbicide-resistant corn fits this rotation, but weed control in winter wheat is usually ineffective. Growth of winter wheat planted after proso millet harvest is usually 30 to 40% less because of less soil water; weeds such as kochia, Russian thistle, and winter annual grasses easily establish and produce seeds. Consequently, the second wheat crop requires high inputs to manage weeds. Herbicide-resistant wheat would be most appropriate with the second wheat crop, as this would avoid pollen flow and difficulty in controlling volunteer wheat the next year.

Key suggestions for managing herbicide-resistant crops

ˇ Consider using a herbicide-resistant crop only once every four years

ˇ Consideration of crop following the herbicide-resistant crop is also crucial for success

References

Anderson, R. L. and D. C. Nielsen. 1996. Emergence patterns of five weed species in the Great Plains. Weed Technol. 10:744-749.

Anderson, R. L. 1998. Designing rotations for a semiarid region. p. 4-15, in Proceedings, 10th Annual Meeting, Colorado Conservation Tillage Association. Sterling, CO. 70 pages.

Anderson, R. L. 2000. A cultural system approach can eliminate herbicide need in semiarid proso millet. Weed Technol. 14:602-607.

Bourgeois, L. and I. Morrison. 1995. Weed resistance in Manitoba: an update. In Proceedings, 17th Annual Manitoba-North Dakota Zero Tillage Association Workshop. Pages 67-71.

Gressel, J. and L. A. Segel. 1990. Modelling the effectiveness of herbicide rotations and mixtures as strategies to delay of preclude resistance. Weed Technol. 4:186-198.

Hall, L., K. Topinka, J. Huffman, L. Davis, and A. Good. 2000. Pollen flow between herbicide-resistant Brassica napus is the cause of multiple-resistant B. napus volunteers. Weed Sci. 48:688-694.

Powles, S. B., D. F. Lorraine-Colwill, J. J. Dellow, and C. Preston. 1998. Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia. Weed Sci. 46:604-607.

Powles, S. B., C. Preston, I. B. Bryan, and A. R. Justin. 1997. Herbicide resistance: impact and management. Advances in Agronomy 58:57-93. Academic Press, Inc. New York.

Seefeldt, S. S., R. Zemetra, F. L. Young, and S. S. Jones. 1998. Production of herbicide-resistant jointed goatgrass (Aegilops cylindrica) x wheat (Triticum aestivum) hybrids in the field by natural hybridization. Weed Sci. 46:632-634.

Wicks, G. 2001. Personal Communications. Weed Scientist, University of Nebraska’s North Central Research Center, North Platte NE.