Chapter 1. Benefits of Triazine Herbicides. Leonard P. Gianessi

Transcription

1 Chapter 1 Benefits of Triazine Herbicides Leonard P. Gianessi National Center for Food and Agricultural Policy, 1616 Ρ Street, NW, Washington, DC Widespread use of the triazine herbicides for weed control on corn, sorghum, sugarcane and fruit crop acreage has resulted in efficient, low-cost effective weed control on tens of millions of acres for the past thirty years. Many growers make a single application of a triazine herbicide at planting. The triazines stay active in the soil for five to six weeks and control most important broadleaf weeds and several important grass weeds. By mixing the triazine herbicides with an herbicide active on grass weeds, the single application is often all that is required for season-long weed control. The triazines cost approximately $8-10 less per acre of corn than available alternatives. For the minor acreage crops, such as sugarcane andfruitcrops, for which there are fewer alternatives, the savings of using triazines amount to $50-70 per acre. One reason that growers readily adopt the triazines is their excellent crop safety record. U. S. farmers have used the triazine herbicides on a majority of U. S. crop acres for thirty years because of the weed control benefits they provide at an economical cost. Triazine herbicides are widely-used to control weeds in U. S. crop production. Since 1976, atrazine has been used consistently on approximately two-thirds of the nation's corn acreage (1,2). Atrazine also is used on approximately 90% of the acreage of sugarcane and 67% of the acreage of sorghum on a yearly basis. In addition to being used on about 25% of the nation'sfieldcorn acreage, cyanazine is used on 25% of the nation's cotton acreage. Although registered for use on field corn, simazine is most widely-used to control weeds in treefruitand nut orchards and in vineyards. From 25% to 50% of the acreage of the nation's almonds, avocados, blueberries, citrus, hazelnuts, grapes, nectarines, peaches, pears, apples, 1998 American Chemical Society 1

2 2 raspberries and walnuts receive simazine applications. U. S. farmers use the triazine herbicides because of the weed control benefits that they provide. The benefits of the triazines result from four basic characteristics: A lengthy period of residual control of germinating weed seeds; A broad spectrum of weed species that are controlled; A record of excellent crop safety; and Economical pricing. These benefits can be discerned clearly by examining the choices that farmers have in selecting weed control alternatives. Basically, available alternatives offer a shorter period of residual control, control a narrower spectrum of weed species, are more likely to damage the crop and cost more to use. The benefits of the triazines are examined further in the following sections that focus on particular crops and uses: field corn, sugarcane, grapes, citrus, cotton, sweet corn, fallowland and sorghum. Sugarcane In Louisiana, sugarcane is planted in August/September, goes dormant in the winter and starts to grow again in the spring. The first harvest for sugar is in the fall of the following year. A wide range of cool-season weeds often becomes established in sugarcane fields after planting. At one time, winter weeds were not considered competitive with sugarcane since they grow when sugarcane is dormant and are removed early in the spring (3). Experiments with triazine herbicides applied in October provided residual control of germinating weeds as they emerged throughout the fall and winter. Research demonstrated that sugarcane cannot recover completely from intense winter weed competition and that the residual weed control provided by the triazine herbicides increased sugarcane yield by 12% (4). Sugarcane is related closely to many of the weeds that infest it, and it has been problematic to find herbicides that control the weeds without damaging the sugarcane. Extensive tests in Texas in the 1970's showed that under the worst case conditions, atrazine would lower sugarcane yields by about 5% while alternatives would lower sugarcane yields by 20-40% (5). Alternatives to atrazine in Louisiana have been estimated to cost from $4 to $5 more per acre. In Texas, Hawaii and Florida, alternatives to atrazine are estimated to be $30 to $70 more per acre. Alternatives generally have less residual control and/or are less efficacious on particular weed species. As a result, combinations of active ingredients or multiple applications would be required as effective atrazine replacements (6). Grapes Simazine use in grape vineyards is limited to the berm - the raised area on either side of the vine trunks. The remainder of the soil area, the middles, is either disked or mowed. Prior to the development of simazine and other residual herbicides in the 1960's, the chief methods of weed control in the vine row were hand pulling and the use of the French plow, that plows the soil out of the vine row. It has a trip

3 3 wire that hydraulically moves the plow around the vine. Residual herbicides, including simazine, largely replaced hand hoeing and the French plow in California vineyards because of the following factors: 1) herbicide use requires less labor; 2) mechanical injury to the vine is eliminated; and 3) herbicides provide longer lasting residual weed control (7). Simazine provides four to six months of residual control of most germinating weed species in California vineyards. Simazine controls 36 weed species that are important in grape vineyards. The other available residual herbicides, oryzalin and oxyfluorfen, control 26 to 27 weed species (8). Simazine needs to be applied only once at a cost of $10 per acre. To match the broad spectrum control, the other two residual herbicides would have to be combined in an application. The cost would double to $21 per acre. Another possibility is to substitute a non residual herbicide, glyphosate, that would require three applications at an increased cost of $15 per acre (9). Grapes are produced by organic methods on several thousand vineyard acres in California (10). Weeds are controlled without herbicides, and there is litde or no yield loss (77). Organic grape growers use the French plow with the risk that the implement can knock down a vine or two as it progresses. In addition, organic grape growers use hand laborers with hoes to remove the weeds in the row. Since there is no residual control with these methods, they must be repeated to remove new flushes of weeds. The University of California has prepared budgets of the costs of producing organic grapes. The budgets assume two hand hoeings at a cost of $50 to $100 per acre plus an in-row cultivation at $18 per acre (72). The use of simazine and other chemical herbicides is considerably less costly. Citrus Florida's subtropical temperatures and 54 inches average annual rainfall are conducive to rapid germination of weed seeds and vigorous weed growth. Until residual herbicides, including simazine, became available in the 1960's, citrus groves were tilled or hand weeded. The widespread use of simazine and other herbicides instead of tillage and hand weeding is mainly the result of: 1) lower machinery and labor costs; 2) the need to minimize damage to tree trunks and surface feeders in root systems; 3) the unsatisfactory length of acceptable control of weed species by tillage; and 4) research during the early 1960's that showed significantly better tree growth, earlier production and less physical damage to trees under herbicide programs than those under tillage programs (13). Experiments in a young grove showed that trees treated with simazine, starting one year after planting, made significantly more growth than trees cultivated with tillage equipment (14). When they began bearing, simazine-treated trees yielded significantly more than cultivated trees. Simazine typically is applied in citrus orchards in the fall and provides several months of residual weed control. In addition to providing a wide spectrum of weed species control, simazine also prevents the establishment of many troublesome vines that, once established, cannot be controlled by available post-emergence herbicides.

4 4 In Texas, research has demonstrated that simazine has no equal in controlling broadleaf weeds. Substituting other products for simazine would likely lead to less effective weed control with resulting citrus yield reductions of 20% (75). In Texas citrus orchards available herbicide treatments are estimated at $30 to $50 per acre greater than current simazine usage (75). Sorghum Early weed control is essential for efficient sorghum production. Sorghum grows slowly in the seedling stage and is especially susceptible to weed interference in the first three to four weeks after emergence (76). Weeds must be controlled during this period to achieve maximum yields. Atrazine is the major component in tank mixes for sorghum because it is more effective than all the other herbicides in controlling broadleaf weeds. A recent survey of sorghum farmers indicated that the major alternatives that would be used if atrazine were no longer available are cultivation and 2,4-D with an associated sorghum yield loss of 33% (77). 2,4-D and cultivation are used post-emergence to weed growth and do not prevent weed seed germination. As a result, early weed interference would not be prevented and yield losses would result. Research has indicated the 2,4-D injures sorghum in most years. 2,4-D makes sorghum plants brittle for several weeks after application and plants are extremely subject to breaking by winds (18). Field Corn The introduction of 2,4-D in the mid 1940's ushered in the modem weed control era. It provided good post-emergence control of broadleaf weeds and reached its peak in the 1950's with nearly half of the corn acreage treated. However, 2,4-D had some inherent problems with occasional injury to corn and movement outside the target area. The introduction of atrazine in the late 1950's provided corn growers with a very economical and effective herbicide with excellent corn tolerance. By the 1970's, it had the major market share and has continued its dominance, being used on over two-thirds of the U. S. corn acreage in 1993, more than all the other herbicides for broadleaf weed control combined (19). One of the key advantages for atrazine is its flexibility. It can be soil applied prior to planting, either on the surface or incorporated, and provides season-long control of germinating weed species. It can be applied pre-emergence, at planting or soon after, or it can be applied post-emergence. Not only does atrazine control most annual broadleaf weeds usually found in corn, it also has some activity for control of grass weeds to complement other herbicides that are used primarily for control of grass weeds (19). Atrazine is used quite advantageously with essentially every other herbicide for field corn. If some of these other herbicides were to be used without atrazine, there is a likelihood of a decrease in spectrum of control, increased risk of corn injury, increased risk of movement outside the target area, a decrease in length of control and increased cost (79).

5 5 Cyanazine was used on 20% of the field corn acreage in 1993, with the majority used in combination with atrazine. This combination has been relatively popular for broad spectrum control. For no-till corn acreage, the combination of cyanazine and atrazine provides good burndown of existing weeds, as well as giving appropriate length of residual control. Atrazine helps to improve pigweed control, while the addition of cyanazine helps to increase control of velvetleaf and some annual grasses. Atrazine costs about $3 per acre. Other alternatives average approximately $10 more in cost per acre. In many areas, since the length of residual control is shorter with atrazine alternatives, two applications would have to be made to substitute for a single atrazine application. Numerous research studies have been conducted to examine the potential of weed control options as substitutes for cyanazine and atrazine. In Wisconsin, growers have faced restrictions on the use of atrazine that are more stringent than any other state. A three-year study in Wisconsin was conducted to identify the best alternatives and to quantify the cost of the atrazine restrictions to Wisconsin growers. Net returns for all 13 alternative treatments were lower than for the atrazine standard. All alternatives were more expensive and resulted in measurably lower com yields (20). The least loss in net return ($10/A) resulted from the use of pendimethalin and dicamba. However, growers may be hesitant to use this treatment because of the potential for crop injury. Sweet Corn Weeds are more competitive with sweet corn than with field corn since sweet corn does not grow so rapidly nor so tall as field corn (21). Considerably fewer herbicide alternatives exist for sweet corn in comparison to field corn because sweet corn cultivars are only partially tolerant of many herbicides. Many of the "super-sweet" corn hybrids are particularly sensitive to newer herbicides registered for field corn acreage (22). 2,4-D and linuron are registered for sweet com; however, both can be phytotoxic to the sweet corn plants. Other herbicides registered for sweet corn, such as butylate, do not have an equivalent broad spectrum of control of broadleaf and grass weeds. In addition, in Florida, where much sweet corn acreage is on soils high in organic matter, alternative herbicides often are rendered ineffective. Atrazine has proven to be an effective herbicide for use in Florida's high organic (muck) soils. Atrazine has been the basis of almost all weed control programs in sweet corn for many years. It controls a wide range of common weeds, can be used in many different combinations and can be applied in many different ways (23). Sweet corn has a high tolerance to atrazine. Cultivation of sweet corn does not control weeds within the crop row. Also, cultivation is only possible when the soil is dry. During wet periods of the year, it is impossible to cultivate. Also, in Florida, weed generation and growth is extremely rapid. In most cases it is potentially impossible to cultivate the number of times necessary to control weeds during the year.

6 6 In Massachusetts, experiments with cultivation of sweet corn indicated that four cultivations were required at a cost that was double the cost of using herbicides (24 ). Fallowland Almost all of the wheat in states such as Colorado, Montana and Nebraska is grown in an annual rotation with fallow. A wheat/fallow rotation is also practiced on 50% of the acreage in Kansas. By keeping the acreage out of crop production for an entire year, the soil stores moisture that is then available for the following year's wheat crop. Uncontrolled weed growth can deplete soil moisture during the fallow year. Growers used to cultivate fallow ground to kill emerged weeds. However, the practice of cultivation dried out the soil, depleted moisture and promoted soil erosion. Research indicated that by applying herbicides to fallow ground, weed growth would be controlled, soil moisture would increase and the following year's wheat yield would increase. The combination of atrazine and cyanazine provided weed control (more than 85% of the plot weed free) for 386 days followed by atrazine alone with 369 days of control (25). Atrazine use produced wheat yields 39% higher than the conventional tillage treatment. Cotton In the late 1980's, the use of cyanazine on U. S. cotton increased significantly: from 7% acreage treated to 20% acreage treated (1,2). Cyanazine is one of the most cost effective herbicides cotton farmers can use, particularly for late and layby post-directed herbicide treatments. Cyanazine provides burndown of weeds that are present at the time of application, plus residual control of weeds that germinate after the application. At the same time, the residual control of cyanazine is short enough not to restrict farmers' rotational crop options. While other herbicides may be able to out perform cyanazine in a single feature, such as providing longer residual control of a specific weed, no other herbicide available to cotton farmers offers all of these benefits in one product at a reasonable cost per acre. Most cyanazine used on cotton is applied as a layby treatment. This application is made when the cotton plants are over twelve inches tall, but have not yet "canopied," or bushed out to the point where they cover the middle of each row and their leaves deprive any weeds under them of the sunlight needed to grow. At the layby stage, cultivation and application equipment is still able to get through the field without doing damage to the cotton plants. The reason cotton farmers apply a layby treatment to cover the entire row width is that this application is the last opportunity they have to control grasses and broadleaf weeds before the cotton gets too big to spray or cultivate. After the layby application, farmers do not have any good options to come back and clean up any weed problems that arise.

7 7 Summary Uncontrolled weeds would lower significantly U. S. crop yields through competition for sunlight, space, nutrients and moisture. Prior to the development of residual herbicides, U. S. farmers used mechanicaltillageand hand hoeing methods for weed control. Residual herbicides, such as the triazines, were adopted because they produce a long period of weed control at a significantly reduced cost in comparison to earlier methods of weed control. For most U. S. crops, more than one herbicide active ingredient is registered to control a similar spectrum of weeds. For field corn, numerous registered alternatives exist (greater than 20 active ingredients). The U. S. herbicide market is extremely competitive, and numerous products have been introduced for corn during the past 30 years. The dominant share of treated acres that triazine herbicides have maintained is indicative of their benefits. In comparison to alternatives (other chemical or non-chemical methods), the triazine herbicides provide superior weed control benefits in terms of spectrum of control, length of control, crop safety and cost. Literature Cited 1. USDA, Agricultural Chemical Usage: Field Crops Summary ( National Agricultural Statistics Service. 2. USDA, Inputs; Outlook and Situation, Economic Research Service, IOS-2, October Matherne, R. J., et al., Culture of Sugarcane for Sugar Production in t Mississippi Delta, USDA Agriculture Handbook No. 417, Millhollon, R. W., "Influence of Winter Weeds on Growth and Yield of Sugarcane," Proceedings of International Society of Sugarcane Technolo Reeves, Sim Α., Jr., "Evaluation of Selected Pre- and Postemergence Chemicals on Weed Control and Phytotoxicity to Eight Sugarcane Varieties," Proceedings Southern Weed Science Society, Dusky, J. Α., and D. L. Colvin, Weed Management in Sugarcane , University of Florida, Cooperative Extension Service, SS-AGR-09, December 1995, revised. 7. Lange, A. H., etal.,chemical Weed Control in Vineyards, California Agricultural Experiment Station, leaflet 216, June UCIPM Pest Management Guidelines, Division of Agriculture and Natural Resources, University of California, Publication Kempen, Harold M., letter to USEPA, Public Docket OPP , March 18, "Bugs, Weeds, and Fine Wine," Business Week, August 10, "Organics on the Rise," Fruit Grower, October Klonsky, Karen, etal.,sample Costs to Produce Organic Wine Grapes in the North Coast, UC Cooperative Extension Service, December 21, Tucker, D. P. H., et al., "Two Weed Control Systems for Florida Citrus," Proceedings Florida State Horticultural Society, 1980.

8 8 14. Mersie, W., and M. Singh, "Benefits and Problems of Chemical Weed Control in Citrus," Reviews of Weed Science, Swietlik, D., letter to USEPA, Public Docket OPP , February 14, Stahlman, Phillip W., letter to USEPA, Public Docket OPP , February 16, Morrison, W. M., et al., The Biologic and Economic Assessment of Pesticides in Grain Sorghum, USDA, National Agricultural Pesticide Impact Assessment Program, Regehr, David, letter to USEPA, Public Docket OPP , February 26, Knake, Ellery L., letter to USEPA, Public Docket OPP , February 13, Harvey, R. Gordon, "Weed Control Options without Atrazine or Bladex," Proceedings 1996 Wisconsin Fertilizer, Ag Lime and Pest Management Conference, Volume Doersch, R. E., Weed Control in Commercial Sweet Corn Production, University of Wisconsin Cooperative Extension Service, Fact sheet A2345, July Beste, C. Edward, letter to USEPA, Public Docket OPP , February 6, Cooperative Extension Service, Western Washington Weed Control Guide: Sweet Corn, Washington State University, EM 3981, July Hazzard, Ruth, ed., Proceedings of the Northeast Farmer to Farmer Information Exchange: Sweet Corn Meeting, Northeast Organic Farming Association, Anderson, Randy L., and Darrryl E. Smika, Chemical Fallow in the Central Great Plains, Colorado State University Experiment Station, Bulletin 5883, January 1984.