March 4, 2002 http://commodities.caes.uga.edu/fieldcrops/cotton
IN THIS ISSUE:
Cover Management in Conservation Tillage
Insect Management Decisions Before and at Planting
Early Management Decisions for Controlling Disease and Nematodes in Cotton
Avoiding 2,4-D Injury to Cotton
Cover Management in Conservation Tillage. (Brown) Winter vegetation management in conservation tillage systems requires consideration of several factors. The primary goal is to establish a reasonable stand of cotton, and that single goal influences all other decisions related to cover management.
The following are potential benefits or desirable characteristics of vegetative covers.
Provide soil and water conservation in winter months. Vegetation can be winter weeds or small grain or legume cover crops. Winter weeds are no-cost inputs which frequently provide erosion control during winter months but rarely persist enough to offer wind or soil protection once the crop is established. The USDA definition for “conservation tillage” is a system that provides a residue cover on the soil surface of at least 30 percent immediately after planting. Seeded cover crops in descending order of use in Georgia include wheat, rye, oats, and crimson clover. There are only a few plantings of lupine and vetch.
Ease of elimination through natural desiccation or application of burndown herbicides. Among the most critical aspects of successful cotton establishment and early season growth is the elimination of all existing vegetation. Weeds or cover crops that are hard to kill with burndown herbicides disrupt the entire production system. Vigorous winter weeds, cover crops, and emerged warm season weeds pose a serious competitive threat to seedling cotton. Winter weeds such as cutleaf eveningprimrose, horseweed, and wild radish are difficult to kill in April and May with standard burndown products (glyphosate or paraquat), but 2,4-D applications in February to early March are highly effective and economical on these troublesome broadleaf species. In general, small grains are easier to eliminate with burndown herbicides than legumes, though in some systems, early maturing crimson clover may produce seed and decline sufficiently early to make burndown treatments unnecessary. Another generalization is that when small grains are actively growing, glyphosate is probably superior to paraquat treatments, but as seed heads appear, paraquat is equally effective. Ryegrass is an easy to establish, aggressive cover but it is sometimes very, very difficult to kill in the spring.
Produce direct or indirect cash value in grazing, seed production, or N fixation. Cover crops can provide significant value in addition to soil and water conservation and wind protection. Small grains can be used for grazing or grain. Compaction associated with cattle grazing has not been a limiting factor in strip-till cotton. Double crop systems have worked well in south Georgia, but once every 7 to 10 years, an early frost curtails the cotton season. Legumes provide a source of “free” N (at least 30 lb/A) through the natural process of fixation, and crimson clover has worked well in a reseeding system in which the legume matures and produces seed for future years prior to the establishment of cotton.
Provide soil and water conservation and wind protection in the spring/summer months. Persistence of residue is important in maintaining conservation properties of covers into the cotton season. Small grain straw is more persistent than clover or vetch residue. For small grains, the more aggressive, earlier growth of rye provides more residue than wheat. Termination date of small grain covers, which obviously affects the amount and persistence of residues, must balance the calendar in respect to target date for planting cotton and the amount of residue. From the standpoint of cover desiccation and potential problems with insects, burndown applications should be made at least 3 weeks prior to planting cotton. Early termination allows the cover to dry down; it permits re-treatment if necessary; and it removes host plants for insects. How much residue is too much? Excessive cover or residue can physically interfere with strip-till and planter operations. With experience and tinkering, some growers can successfully work in an incredible amount of rye residue. However, those just moving into conservation tillage might consider burning down rye before it exceeds 3 to 4 ft in height and wheat before it reaches 2 or 3 ft. Generally, size and growth dictate termination date for rye, while the calendar (cotton planting date) determines the time to treat wheat. Terminating too early greatly diminishes residue even to the point of its total disappearance shortly after cotton emergence.
Vegetative covers sometimes adversely affect cotton. Covers can harbor pest organisms and negatively affect soil moisture levels in the spring. Some legumes are a host for nematodes, a group of pests which are having more and more of an impact across the state. Legumes, and to a lesser degree small grains, can be a reservoir for insects such as cutworms, false chinch bugs, and occasionally grasshoppers. Residues may create a few more problems with seedling diseases, though this has proven to be a minor issue. Degradation of legumes can release certain compounds that are harmful to young cotton plants, hence the benefit of an interval of 3 weeks between burndown application and planting. Spring growth of cover crops can significantly deplete soil moisture, a serious problem in years with deficit rainfall in the winter and spring. Early termination partially alleviates this problem. In wet years, covers, especially legumes, can maintain a wet mat at the soil surface and thus complicate strip-tillage and planting.
The total value of cover crops goes well beyond immediately measurable benefits. Long term advantages include sustained soil and water conservation, organic matter buildup, increased water infiltration, improved soil tilth, reduced crusting, etc. Such benefits become increasingly apparent after several years in conservation tillage systems and undoubtedly enhance land productivity and overall management.
Insect Management Decisions Before and at Planting - (Roberts) Variety selection may be the single most important decision a producer makes in a given year. In addition to factors such as yield and quality, growers must also give consideration to available transgenic traits such as Roundup Ready and Bollgard. From an insect management standpoint, decision to plant Bt or not-Bt cotton will greatly influence IPM for the upcoming year. Factors to consider concerning whether or not to plant Bt cotton include tobacco budworm populations, the threat of pyrethroid resistance, and your ability to treat tobacco budworm and corn earworm in a timely manner in the presence of high pressure.
Pyrethroid resistant tobacco budworm was first documented in Georgia during 1997 in Decatur County. Since that time we have observed problems with control and suspect resistance or increased tolerance in other parts of the state. Alternative insecticides such as Tracer and Steward are effective tools for managing pyrethroid resistant tobacco budworm. However, foliar treatments must be applied on a timely basis. Larvae less than 1/4 inch in length (3 days of age) must be targeted to achieve good control. Non-Bt acres should not exceed what can be sprayed in 3-4 days. If we experience high pressure, multiple insecticide applications of insecticide may be needed on a five- day interval. Rain and other delays may inhibit our ability to be timely and reduce the effectiveness of insecticide applications. We have the tools to manage tobacco budworm and corn earworm on non-Bt cotton, but management will be more intensive. Regardless of technology, a good scout is a must. Good decisions can only be made when we have good scouting information.
Many growers have increased acreage of conservation tillage during recent years. Variations in insect complexes can vary by tillage system. Many insects are unaffected by tillage system and include most mid-late season bug and caterpillar pests. Early season thrips populations are generally lower in reduced tillage but a preventive insecticide at planting is still recommended. However, the risk of attack from some insect pests, especially cutworms, is increased in conservation tillage. The risk of cutworm attack can be significantly reduced if winter cover crops or weeds are terminated at least 3 weeks prior to planting. If the cover is not terminated, cutworms may become established on plants and move to emerging cotton seedlings as host plants dry down. Observations and data suggest timely termination is especially important when legumes or winter weeds are used as the cover. In addition to cutworms, problems with false chinch bugs have been observed in recent years. Most problem fields have been in reduced tillage and high populations tend to be associated with fields which were not burned down in a timely fashion. In situations where the risk of cutworm attack is high (i.e. following legumes or green vegetation present at planting) consideration should be given to applying a preventive cutworm treatment at planting. An economical approach would be to band a pyrethroid behind the planter.
On a more limited basis we have observed some fields with high populations of grasshoppers. Timely termination of covers will help here but will not eliminate the threat of stand reduction from grasshopper feeding.
One very obvious difference in conservation tillage is the build up of fire ant populations due to the lack of soil disturbance. Fire ants are an important predator of bollworm eggs and larvae. We consider fire ants to be one of the three most important predators which would also include bigeyed bugs and minute pirate bugs. However, there is a slight negative associated with fire ants. Fire ants feed on honeydew excreted by aphids and will protect or farm aphids so as to increase the presence of honeydew. The end result is that aphids tend to build populations quicker in the presence of fire ants, but these same fields are generally the first in which aphids crash due to the naturally occurring fungus. The benefits of fire ants far outweigh this slight negative. From an IPM standpoint, termination of the winter cover at least three weeks prior to planting will reduce the risk of early season insect problems.
A preventive insecticide at planting is recommended for early season thrips control. There are several options available which include in-furrow granules, in-furrow sprays, and seed treatments. A large percentage of the acreage in Georgia is treated with Temik in-furrow. Temik at 3.5 lbs product per acre has proven to be a consistent performer. Commercial seed treatments offer convenience at planting and are an option for early season thrips management. Orthene treated seed provide control for about 7-10 days. Gaucho and Cruiser (formerly Adage) will provide more extended control. However, these seed treatments have tended to be erratic in performance when compared with Temik. This erratic performance appears to be more common on April and early May plantings when thrips populations are generally high and plant growth may not be rapid. Regardless of treatment applied at planting, it is important that growers scout fields at least weekly and react in a timely fashion if the treatment is failing. Foliar sprays of Orthene, Bidrin, or dimethoate are recommended when thrips number 2-3 per plant. Foliar treatment is rarely necessary after plants have 5 true leaves and are growing rapidly.
Early Management Decisions for Controlling Disease and Nematodes in Cotton - (Kemerait) Diseases and nematodes are likely to cause at least some concern for many of Georgia=s cotton growers in 2002. One of the most difficult issues in the management of these problems is that decisions must be made before (sometimes long before) symptoms appear in the field. For example, though we are still a couple of months from planting, growers are likely committed to a crop rotation scheme for the 2002 season and either did or did not pull nematode samples last fall. If they did pull the samples, they have some idea of nematode populations in their field, otherwise they must rely on notes or memories from previous seasons.
At this point, growers must decide whether or not to use a nematicide to reduce damage from root knot, reniform, sting, and Columbia lance nematodes. Obviously, if they haven=t had a problem in the past, or are coming off a very good rotation (e.g. several years out of a host crop), then they may not need a nematicide. However, if they are going into a field with a history of damage and loss, then they need to decide 1) if they can afford to put out a nematicide (can they afford not to?), 2) which nematicide to use (probably Telone II or Temik 15G), and 3) which rate to use. Once the seed is covered, it is too late to make many other nematicide decisions. Breaking up an existing hard pan before planting and adequate irrigation during the season will also help reduce losses to nematodes.
Seedling diseases are generally not an important problem for most growers in the state. However, seedling diseases hit a few farmers hard every year and these growers need to think about using a fungicide treatment in addition to what already is formulated on the seed. They also need to determine if previous losses have been from Rhizoctonia or Pythium (or both), as these pathogens are not necessarily controlled by the same fungicides. Seeds planted into cool, wet soils are most at risk; growers can minimize the risk by waiting to plant until conditions improve. Data from 2002 showed that in-furrow fungicides such as Terraclor and Terraclor Super X, as well as hopper box treatments, often improved stand counts, but corresponding increases in yield did not occur. At-risk growers should carefully weigh their options to reduce seedling disease before they plant. Again, once the seed is covered, there aren’t any future options.
Fertility plays a role in boll rot and in Stemphyllium leaf spot, both of which can be devastating. Growers should carefully follow fertility recommendations to avoid excessive amounts of nitrogen that can lead to rank growth and boll rot. Growers should also insure adequate levels of potassium as low potassium levels are the primary cause of Stemphyllium leaf spot.
Avoiding 2,4-D Injury to Cotton - (Culpepper) Cotton injury from 2,4-D continues to be a common and unnecessary problem. The problem can result from sprayer contamination, spray drift, and vapor drift. Special thanks to Dr. Alan York of NC State University for help in preparation of this section.
Cotton injury can occur from minute residues of 2,4-D (or 2,4-DB) in a sprayer. It is recommended that any sprayer previously used to apply 2,4-D not be used in cotton. If such a sprayer must be used, it should be washed thoroughly before spraying cotton. Special attention should be given to sprayers used to apply Roundup Ultra or emulsifiable concentrates because these products seem to be particularly effective at pulling 2,4-D residues out of a sprayer.
The following procedure is suggested for washing out sprayers that have been used to apply 2,4-D. Keep in mind this procedure may not totally remove 2,4-D residues. Dispose of rinsates in an approved manner.
1. Remove nozzles, nozzle strainers, and in-line strainers. Using a soft brush, wash the nozzles and strainers with soapy water. Be sure to remove any visible deposits.
2. Before replacing nozzles and strainers, fill sprayer tank with water and add a strong detergent such as 4 pounds of trisodium phosphate per 50 gallons of water or a commercial spray tank cleaner. Agitate for 15 minutes and then flush about one-fourth of the water-detergent mixture through the lines. Replace nozzles and strainers and flush remainder of water-detergent mixture through the nozzles.
3. Spray diesel fuel on the inside surfaces of the tank. Start the sprayer to fill the lines, and let the diesel fuel sit in the lines for several hours, preferably overnight. Then spray out the diesel fuel. Note: this step is suggested only if the sprayer has previously been used to apply an ester formulation of 2,4-D.
4. Fill the tank with water and add household ammonia at the rate of 1 quart per 25 gallons of water. Agitate for 15 minutes, spray a few gallons of the mixture through the nozzles, and let the remainder sit in the tank and lines for several hours, preferably overnight. Then spray out the remainder of the ammonia-water mixture.
5. Fill the tank with water and detergent. Agitate for several minutes and spray it out.
6. Fill the tank with fresh water and spray it all through the nozzles.
Spray drift means movement of spray droplets by wind. As opposed to vapor drift (described below), spray drift can occur with any formulation of 2,4-D (or any other product). Spraying during windy conditions and using nozzles and pressures that result in the creation of fine spray droplets increase the risk of spray drift.
Except in extreme cases, such as spraying in very windy conditions and using nozzles and pressures that create very fine droplets, spray drift normally is observed only over short distances. A buffer of 200 feet or more between the area being sprayed and the susceptible crop usually is adequate to prevent injury from spray droplet drift unless it is very windy. If there is no wind or if the wind is blowing away from the cotton field, a shorter buffer is acceptable.
Most cases of 2,4-D injury to cotton result from vapor drift of an ester-containing formulation of 2,4-D. Vapor drift results when the herbicide volatilizes and the vapors move to a susceptible crop such as cotton. Hot temperatures, moist soils, and thermal inversions all increase the potential for vapor drift. Injury from vapor drift can occur at rather long distances from the sprayed area.
Vapor drift can be avoided simply by refraining from the use of ester-containing formulations of 2,4-D. Ester formulations should not be used within a mile of any cotton field during the months that cotton is in the field. Most commercially available ester formulations are considered Alow volatile.@ These formulations are still volatile, and their use can lead to cotton injury. Weedone 638 and any other formulations containing a mixture of 2,4-D ester and 2,4-D acid also should be avoided in cotton-producing areas. Vapor drift usually is not a problem with amine formulations of 2,4-D.
Ester and ester-acid formulations of 2,4-D are popular because they mix well with liquid nitrogen. Amine formulations also can be mixed with liquid nitrogen if the 2,4-D is premixed with water before adding it to the liquid nitrogen.
Steven M. Brown, Extension Agronomist-Cotton
Stanley Culpepper, Extension Agronomist-Weed Science
Bob Kemerait, Extension Plant Pathologist
Phillip Roberts, Extension Entomologist
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