UGA COTTON NEWSLETTER
April 25, 2001 http://www.griffin.peachnet.edu/caes/cotton
Do Pecan Trees Ever Lie 2
Seed Tag Germination, Warm Germ/Cool Germ Values 2
The Low-Down on In-Furrow Fungicides 3
Applying Prowl or Pendimax After Planting 4
Should I Wait Until 4-Leaf Roundup Ready Cotton Before Applying Glyphosate 4
Roundup Ultra and Roundup UltraMAX Do Not Require Additional Surfactant 5
Planting. (Jost) The time for cotton planting has arrived for some growers and is rapidly approaching for others. There are a couple of considerations surrounding cotton planting that have been discussed at length in the past, but a reminder is always good. This is especially true since the investment of dollars at planting is ever increasing (seed costs + technology fees).
1. Cotton is a warm-blooded animal, and needs heat to grow.
Remember, prior to planting, soil temperatures at a 4-inch depth should be at least 65oF for several days in a row. Cotton typically requires an average of 4 to 14 days to emerge after planting. Cooler temperatures will push the time of emergence to the longer of this range. Delayed emergence in cool temperatures can be related somewhat to the heat unit (or DD60's) requirement for cotton emergence. Most sources indicate that cotton requires 50 DD60's to emerge, and the cooler the air temperatures are the longer it takes to accumulate these DD60's. Soil temperatures, which may be significantly above or below air temperatures, are not taken into consideration in the DD60 calculation system. Therefore, when making the decision to plant, look at soil temperatures and the extended forecast. It is especially important to remember that soils will warm slower than air.
Temperatures below 50 to 55oF may cause chilling injury to germinating seedlings. This chilling injury can result in a damaged root system or seedling death in severe situations. Again, paying attention to the forecast, should help to prevent this occurrence.
The base temperature for cotton growth is 60oF. While soil or air temperatures of 60oF will not cause chilling injury, they are not conducive to rapid growth. A slow growing plant will be more susceptible to diseases and early season insects. Cotton will emerge from the soil and grow more quickly as soil and air temperatures increase from 60oF.
2. Replant decisions are difficult (and no fun) to make.
Trying to determine what will be gained by replanting or lost by keeping a less than optimal stand is about as speculative as marketing the crop. Remembering that cotton is a heat-requiring plant may help to avoid this situation. But, if despite your best efforts, a skippy stand is what you end up with due to cool temperatures, lack of moisture, or too much rain, there are some things to consider when contemplating the decision to replant.
Optimal stands of cotton range from 30,000 to 40,000 plants per acre, yet research has shown that good yields can be obtained with stands greater or less than this range. Generally, the consideration to replant a marginal stand should revolve around the presence of skips. If there are skips greater than 2 to 3 feet between plants replanting may be the way to go. This is especially true if the rows next to the skip also have a poor stand. A uniform stand, even if the plant population is low, will most likely be able to produce good yields due the ability of the cotton plant to modify its growth habit.
In fields with low, but uniform populations, plants should be inspected for indications of overall plant vigor. Plants without cotyledons will not survive. Plants with healthy root systems will be more apt to be good producers and able to survive subsequent stresses, than those with discolored, unhealthy roots. Also, the time of year plays a role. A stand that you might consider replanting on May 1, may seem more acceptable on June 10.
Do Pecan Trees Ever Lie? (Brown) Nature provides many clues about the time to plant warm season crops such as cotton. One accepted sign is pecan trees-when they leaf out, the danger of cold weather is past and it is safe to plant cotton. Pecan foliage was visible by the second week in April 2001, and yet scattered frost occurred the following week. In southern portions of the state, soil temperatures remained in the 60os F despite the fact that air temperatures fell into the 30os F. No harm done. What little cotton planted has fared well. According to Georgia Ag Statistics, only 5 percent (compared to the historical average of 9 percent) of our cotton acreage was planted by April 23 .
Seed Tag Germination, Warm Germ/Cool Germ Values. (Brown) By law, commercial seed
sold in Georgia must meet certain standards. The minimum for germination is 80 percent. More accurately, "germination" refers to "the standard warm germination test" which measures seed viability by subjecting seed in moist paper to a constant 86o F or alternating temperatures of 86/68o F. Seed are evaluated at 4, 8, and 12 days. The minimum requirement is 80 percent, and even if warm germ tests results are higher, "80 percent germination" is what is printed on the bag. The warm germ is rarely exactly 80 percent. For a given lot of seed, the actual warm germ test results (and other data) can be obtained from the company.
In some parts of the Cotton Belt, cotton is often planted under marginally cool conditions. Stand establishment in cold soil taxes the biological limits of the seed of this sub-tropical, perennial plant. The "cool germination test" is an attempt to measure how seed will fare under such conditions. Some also consider the test a measure of seed vigor. The cool germ test is conducted at a constant 64.4o F. Seeds are evaluated at 7 days and only those having a length of at least 1.5 inches from the tip of the radicle (the young root) to the attachment point of the cotyledons are counted as positives.
The industry standard for acceptable cool germ test value is generally 60 percent. Some states calculate a "Vigor Index," which is simply the addition of warm and cool germ test values. The desired Vigor Index is at least 140 (for example, 80 percent warm germ, 60 percent cool germ,
80 + 60 = 140). Cool test results are not printed on the seed tag nor are there specific minimum values required by law. The primary reasons are (1) cool test values are often not repeatable from one lab to the next, and (2) cool test values may change over time.
The latter point has been the case with DP 458 B/RR, a small seeded cultivar with limited seed vigor. It apparently has some innate dormancy that is only overcome by time. Last year significant amounts of DP 458 B/RR with cool test values between 50 and 60 percent were planted, and fortunately, there were almost no reports of problems with stand establishment. This year's seed are similar in quality, which means that this variety should be planted only when conditions are ideal for rapid germination and emergence.
The Low-Down on In-Furrow Fungicides. (Kemerait) By the time you read this, many growers will have begun planting or will do so shortly. At this time of the year, there is renewed interest in the use of in-furrow and hopper box fungicides to reduce the severity of seedling disease and to increase stand counts. Also, articles on seedling diseases and in-furrow fungicides frequent trade magazines. These articles are very informative and can also provide case studies on the benefits of in-furrow fungicides. However, growers in Georgia, particularly in southern Georgia, should realize that the situation we face here (for example, weather conditions and types of pathogens) may differ from where the articles are written.
Growers may have questions about their need to use the at-plant fungicides beyond the commercial seed treatments already on the seed. (Note: growers should ALWAYS plant seed that has been treated with fungicides. Essentially all commercial seed treatments include some fungicide). Seedling diseases are most severe when soils are cool and wet. Growers can do a lot to reduce the risk of seedling diseases by planting in warm soils (they should be warm now). Warmer soils produce rapid germination and vigorous early growth, usually enabling cotton seedlings to escape serious infection.
Since being in Georgia, I have only seen the seedling disease know as "soreshin" which is caused by Rhizoctonia solani. It is easily identified by the brown lesions that girdle the hypocotyl near the soil line that may cause the seedling to fall over. Pythium is another common pathogen across the Cotton Belt. Although I am sure that it is present in the state, I haven't seen it and believe it to be of minor importance. Pythium causes seed and root rots and preemergence damping off. It is more environmentally sensitive than Rhizoctonia and probably will only be a factor in soils that are wet and during periods of prolonged cool. You may read about the seedling disease caused by Thielaviopsis; this pathogen doesn't affect our crop. Also, Fusarium has been associated with seedling diseases, but it is of minor importance.
The bottom-line: If a grower wants to use an in-furrow fungicide or a hopper box treatment, I would not discourage him, especially if he has had trouble getting a stand in the past. However, I would probably recommend lower rather than higher rates of the fungicide. Also, given that Pythiuim is not likely to be a problem in southern Georgia, it is probably sufficient to use a product that is active only against Rhizoctonia as it is likely less expensive. Studies this season will take a closer look at seedling disease in the northern areas of the state. If a grower is asking your advice on whether to use a fungicide or not, consider their individual situation and field history. It is very common in field trials to find that there is no significant yield increase when fungicides are applied. This is in part because of the fact that if a few seedlings are killed by disease, neighboring plants will grow larger and compensate for the potential yield loss. Perhaps the main reason that a grower will choose to use additional fungicide is the additional stand insurance that it provides.
Applying Prowl or Pendimax After Planting. (Culpepper, Brown). Growers often choose to plant cotton and then apply soil-applied herbicides such as Prowl or Pendimax in separate operations as a means to facilitate efficiency. This method of applying Prowl or Pendimax after planting is often very effective assuming no weeds have emerged since planting and a timely rainfall or irrigation occurs within 5 days of the herbicide application. However, what often occurs is the cotton actually begins "cracking" or emerges by the time Prowl or Pendimax is applied. In this situation, should a grower continue to make the herbicide application?
Data generated at two locations in 2000 strongly suggest that Prowl or Pendimax applied broadcast overtop of emerging or emerged cotton can cause significant injury (Table 1). In these trials, cotton emergence was occurring during the second half of day 3 up until mid-day of day 5. Insignificant injury was noted when Prowl was applied prior to cotton emergence with applications being made 0 or 2 days after planting. However, Prowl applied 4 days after planting injured cotton 21% and reduced cotton height 34% compared to Prowl applied the day of planting. Delaying the application of Prowl to 6 or 8 days after planting when cotton was fully emerged, caused even more injury.
Thus, it is clear that Prowl or Pendimax must be applied prior to cotton "cracking" or emergence. In most situations, it is recommended that these herbicides be applied within 2 days of planting.
|Table 1. Cotton response to Prowl applied 0, 2, 4, 6, and 8 days after planting.*|
|Application timing**||Visual injury
|day of planting||3 c||3.8 a|
|2 days after planting||2 c||3.7 a|
|4 days after planting||21 b||2.5 b|
|6 days after planting||48 a||1.7 b|
|8 days after planting||47 a||1.7 b|
| *Data pooled over two locations in 2000. Data provided by Brown and Culpepper.
**Cotton emergence occurred between 3.5 and 5 days after planting.
Should I Wait Until 4-Leaf Roundup Ready Cotton Before Applying Glyphosate (Roundup, Others)? (Culpepper). Often, the most critical time frame for a cotton field to be maintained as weed free as is feasible is the first six weeks of cotton development. However, with the release of Roundup Ready cotton, many fields are now heavily infested with weeds for several weeks during the first six weeks of cotton growth. Although most of these weeds can be controlled by a glyphosate application, potential yield loss due to early-season weed competition is a concern. The weeds present, their densities, the length of weed competition with cotton, and environmental conditions all play a role in the potential yield loss from weed competition during this critical time period.
When a dinitroanaline herbicide (Pendimax, Prowl, Treflan, etc.), Cotoran, or Staple is applied at planting, the glyphosate application can be delayed until the 4-leaf stage of cotton development in MOST situations. However, if the soil-applied herbicide is not effective (due to weather or application method) or when a grower chooses not to use a soil-applied herbicide, early-season weed competition reduces cotton yield in approximately 50% of our fields when the glyphosate application is delayed until the 4-leaf stage of cotton based on research across the Southeast. This yield loss occurs even in situations where all weeds are controlled completely from the 4-leaf stage until harvest as the cotton never recovers from weed competition occurring between emergence and the glyphosate application at the 4-leaf stage of cotton.
Thus, data suggest that in situations where no soil-applied herbicide is used or when the soil-applied herbicide is ineffective, then two postemergence over-the-top applications of glyphosate may be needed. Remember, when making two over-the-top applications of glyphosate, there must be two nodes of cotton growth and 10 days separating the applications with both being administered prior to the 5th leaf of cotton being the size of a quarter.
"Roundup Ultra and Roundup UltraMAX Do Not Require Additional Surfactant." (Culpepper). In last month's Georgia Cotton Newsletter, tables comparing different glyphosate formulations, their use rates, and adjuvant recommendations were presented. In light of new information, there are changes that need to be made. First, the adjuvant recommendations for Roundup Ultra and Roundup UltraMAX were incorrect according to the newest labels for the 2001 growing season. At this time, "Monsanto will label and promote only brands such as Roundup Ultra and Roundup UltraMAX that do not require surfactants and other additives until their interaction with Roundup Ready cotton is better understood".
Secondly, the number of glyphosate products being sold for use in Roundup Ready continues to increase. Glyphosate brand labels that I have received since last month's Georgia Cotton Newsletter include GLY-4, GLY-4 Plus, and Mirage. I am sure there are many others also being sold in our area. Carefully read all glyphosate labels to make sure the proper rate and adjuvant are being applied. Also, be careful to apply only those glyphosate formulations over-the-top of Roundup Ready cotton that are labeled for this method of application.
Steven M. Brown, Extension Agronomist-Cotton
Philip H. Jost, Extension Agronomist-Cotton
Stanley Culpepper, Extension Agronomist-Weed Science
Bob Kemerait, Extension Plant Pathologist