Original URL : http://www.cottoninc.com/ConservationTillage/homepage.cfm?page=2278

Third Annual National
Conservation Tillage
 Cotton & Rice Conference

PROCEEDINGS
sponsored by: National Conservation Tillage Digest

Herbicide Resistant Crops and Weed Species Shifts
Presented by: Dr. Robert M. Hayes, Professor, University of Tennessee

     Weed shifts are part of ecological succession. When selection pressure is applied to a population, change occurs. With the earliest use of herbicides, weed shifts occurred. For example, when no herbicide is used in cotton, annual grasses predominate. Yet when herbicides to control annual grasses became commonplace, dicot (broadleaf) weeds flourished. More recently, emphasis has focused on controlling cocklebur and morning glory, only to see sicklepod and prickly sida increase.

     With the introduction of BXN cotton and Buctril, producers typically reduced soil-applied herbicide rates, creating a situation for nutsedges, sicklepod and pigweed escapes, particularly Palmer amaranth.

     With the introduction of Roundup Ready crops, shifts are inevitable. We need only to look at early preplant burndown programs in conversation tillage systems with Roundup to see these shifts. Cutleaf eveningprimrose, fleabanes, and smartweed increased with repeated Roundup use, leading to tank mixtures or alternative treatments to achieve control. Similarly, increases in spurges, prickly sida, hemp sesbania, Florida pusley, pitted morning glory, barnyard grass, maypop passionflower, and bermudagrass will likely increase since common use rates do not provide adequate control.

     In addition, control of Roundup Ready crops in Roundup Ready rotation crops presents another difficult challenge. Roundup Ready soybean in Roundup Ready cotton is especially challenging. We were unable to achieve adequate control with MSMA or Staple applied overtop. Soybeans, once established, have tolerance to many residual cotton herbicides (Goal, Bladex/Cypro, and even Cotoran/Meturon). Residual preemergence herbicides such as Cotoran/Meturon and Caparol/Cotton Pro are the best approach to control Roundup Ready soybeans in cotton. Another option is the use of post-directed sprays with Caparol/Cotton Pro + MSMA or Karmex/Direx + MSMA. However, it is often difficult to control volunteer soybeans in the cotton row with directed sprays if no height differential exists.

     How do we as producers prevent weed shifts? The simple answer is to not do the same thing year after year. Unfortunately, most cotton producers do not practice crop rotation, and if they do it is short rotation. With Roundup Ready technology now in both corn and soybean, herbicide rotation may not seem practical. However, crop rotation with herbicide rotation utilizing herbicides with different mode of action but effective on the species that tend to escape should prevent weed shifts from becoming serious problems.

     For example, prickly sida, velvetleaf, and spurred anoda are not readily controlled with Roundup in cotton or soybeans. Consequently, Command as a preemergence and Zorial preemergence or preplant incorporated in cotton can prevent the proliferation of these species. Staple postemergence controls velvetleaf and spurred anoda, but only partially controls prickly sida. Corn rotations with atrazine will generally provide adequate control of these species. Likewise, rotation with soybean and the use of products containing metribuzin (Lexone, Sencor, and Canopy) will control these species. Unfortunately, one of the popular postemergence soybean tank mixes, Roundup plus Classic, does not adequately control prickly sida. Other products such as Basagran, Frontrow, and Storm provide postemergence control. Python controls prickly sida in soybean, but it has an 18-month crop rotation interval before planting cotton.
Bermudagrass, barnyardgrass and other grasses can be controlled in cotton with the overtop grass herbicides (Poast, Assure II, Select, etc.) in either cotton or in soybeans. In fact, rotation to narrow row soybeans and intense crop competition alone can greatly reduce bermudagrass density.

     Perennial vines, while suppressed, by in-season Roundup applications, will in most instances survive. Additional pre-harvest and post-harvest applications of Roundup for two or more consecutive years will control trumpetcreeper. Post-harvest fall application of Banvel provides suppression of redvine, curly dock and trumpetcreeper. Repeated annual applications improve results.

     In conclusion, weeds shifts are sure to occur. Awareness of weed escapes, proper planning and rapid response can abate or minimize the impact to producers. The worst strategy is to think that weed shifts will not occur even though the same weed control system is used year after year. There is too much diversity among weed populations for us to think that somehow weeds will not escape even our best eradication attempts. Weeds have even better survival strategies than the boll weevil!

Orginal URL http://www.cottoninc.com/ConservationTillage/homepage.cfm?page=3166#Burndown

Conservation Tillage
 Cotton & Rice Conference
January 24-25, 2002 — Tunica, Mississippi
sponsored by: National Conservation Tillage Digest

Burndown or Burn-up?
Presented by Dr. Robert Hayes
Professor, University of Tennessee

          Control of existing vegetation before, at, or immediately after planting is essential to successful conservation tillage production systems. While there are some species (common cocklebur, crabgrass, etc.) that can be controlled with postemergence herbicides, these weeds can consume soil moisture critical for the germination and emergence of the young cotton plants. There are other species (cutleaf eveningprimrose, curly dock, smartweed, etc.) that are difficult to nearly impossible to control once the cotton emerges.

          Prior to the introduction of Roundup Ready cotton, most producers use a 'one-two' punch of early preplant glyphosate followed by Gramoxone + Cotoran, Caparol or Bladex at or near planting. Many producers are now omitting this latter treatment in lieu of an early postemergence glyphosate application. In some instances, conservation tillage cotton is being planted without complete control in anticipation that the early postemergence glyphosate treatment will provide complete control. Unfortunately, this has not always been the case. Instead of complete 'burndown', this creates a situation where the landlord, producers and/or lenders are hot enough to 'burn up'!

          Back to basics. Perhaps the most important step is proper identification of the weeds present in a field and then selecting the most effective strategy for control, including herbicide(s), rates, timing of application, spray gallonage, adjuvants (surfactants, ammonium sulfate, etc.), and proper calibrations and recommended nozzles for spray coverage and minimization of off-target movement (drift). The one-two punch is still the most reliable approach to control of a broad spectrum of grass and broadleaf weeds. Unfortunately, it can be rather costly, and given the current economic situation, producers are searching for ways to reduce production costs, of which weed control costs are targets for reduction.

          2,4-D. Recently, producers have expressed interest in 2,4-D as a component in the 'burndown' herbicide arsenal. While 2,4-D is relatively inexpensive and effective on many winter annual broadleaf weeds, it is not labeled for application within 90 days of planting cotton. Still, several studies have demonstrated that there is little risk to cotton when application is made more than 30 days before planting cotton. A key is the soil degradation of 2,4-D that begins following the first rainfall after application. Unfortunately, the disadvantages are sprayer contamination (virtually impossible to sanitize a sprayer sufficient for subsequent use in cotton) and drift to sensitive plants (tomatoes, grapes, roses, greenhouse plants, etc.). Research is underway comparing the amine and ester formulations of 2,4-D and to possibly shorten the interval before planting.

          Harmony Extra. Harmony Extra is labeled for curly dock, smartweed and many winter annuals broadleaf weeds. It can be tank-mixed with glyphosate (Roundup, Touchdown, etc.) to provide a broader spectrum of control. Combinations with paraquat (Gramoxone Max, Boa) or glyphosate improves control of Carolina geranium, curly dock and cutleaf eveningprimrose. Application may be made from late fall through early spring, but no later than 45 days prior to planting, although research indicates no cotton injury or yield reduction when applied 15 or more days before planting.

          Glyphosate resistant horseweed. Often erroneously called mare's tail, horseweed is common in fields not tilled after harvest in the southeastern United States. Estimates of seed per plant range from 50,000 to 250,000. There have been reported biotypes with resistance to paraquat (Gramoxone Max). In 2000, a farmer in Lauderdale County, TN noted lack of control of horseweed with Roundup Ultra at 1 quart per acre, and a subsequent application at 2 quarts per acre produced numerous survivors. Trials in 2001 confirmed the lack of control with numerous glyphosate formulations, application timings, adjuvants, and spray volumes, growth stages, etc. Several other producers from Lauderdale County to Gibson County reported horseweed control failures with glyphosate containing products. Subsequent application of glyphosate after cotton emergence also failed to provide control. Staple postemergence stunted the horseweed to the point that post-directed diuron (Direx, Karmex) plus MSMA was able to provide acceptable control in most situations. Overtop MSMA suppressed the horseweed but did not provide complete control. Further research is in progress to evaluate alternative strategies to manage this weed. Producers should insure complete control of horseweed, cutleaf evening primrose, ryegrass, and dock before cotton begins to emerge. Otherwise, achieving in-crop control may be very difficult and costly at best, if not impossible.

          Synergism and Antagonism. With 'burndown' products that cause leaf burn may interfere with (antagonize) the uptake and translocation of systemic herbicides like glyphosate (Roundup, Touchdown), culminating in failure to control the growing points of the weeds. Most wettable powder and flowable formulation will also tend to reduce activity, especially at the lowest use rates. In contrast, photosynthetic inhibitors (Direx, Karmex, Cotoran, etc.) may synergize paraquat, by inhibiting two photosynthetic processes.