Cotton -- The University of Georgia

February 7, 2002                                                                                           



Variety Selection

Another Look at Variety Selection

2,4-D Application in February for Conservation Tillage Cotton

It’s Not too Early to Think About Controlling Nematodes and Seedling Disease


Variety Selection:  (Jost)  It is once again time to start gearing up for the cotton season.  Variety selection is one of the foundational decisions for the entire production system.  The Official Variety Trial (OVT) data is now available on the UGA Cotton Page.  The link is in the Breaking News section near the top of the page.  Yield and quality reports are available by location and combined over locations and years.


Variety selection continues to be a hotly debated issue.  The questions asked often revolve around “transgenic” variety performance and whether they are as high yielding and high quality as “conventional” varieties.  It is extremely difficult to address this issue.  Growers have many strong and varying opinions.  Probably the best way to tackle this question is to use data collected over several locations over several years where many varieties are grown side-by-side, such as that found in the OVTs. 


The following figure compares 17 varieties which were evaluated in the OVTs during the 2000 and 2001 seasons.  There are some fairly large differences in average yield across these 17 varieties.  However, YIELD is only part of the equation--LINT Quality is also important in total crop value.  Furthermore, instead of looking at length or micronaire or uniformity individually, the entire fiber package must be considered as a whole, and the value of each parameter must be weighted accordingly.  The line in this graph indicates the value per acre of each variety.  Value per acre was calculated by beginning with a loan rate of 51.92 cents/lb and then factoring in all premiums and discounts for length, micronaire, uniformity, and strength.  It was assumed that all varieties had a base color grade of 41-4.

Cotton -- The University of Georgia


These data indicate that the higher yielding varieties are not always the ones with the highest value.  In addition, if the values of the varieties from the same “family” are compared, they appear to be fairly consistent whether RR, Bt, stacked, or non-transgenic.  The argument could be made that varieties containing the Bt gene tend to be higher yielding and thus, for that reason, have a higher value.  However, if “transgenics” as a whole are compared to newer “conventional” varieties, there are some striking differences.  Some of the newer varieties have significantly greater value than transgenic options.  This is good news though, as eventually some of these conventional varieties will be available with technology.


It is still difficult to determine which variety is the “best.”  This decision must be made on a grower and field basis factoring in weed pressure, past pest problems, and management limitations along with the expected lint premiums/discounts.  Again, it is also wise not to plant everything to one variety.  Use some of what has worked well in the past, along with some newer selections based on reliable data.


Another Look at Variety Selection:  (Brown)  The following article was submitted to the Georgia Cotton Commission for publication in its quarterly 100 Percent Cotton newsletter published in the Southeast Farm Press.  The overwhelming acceptance of transgenic technology in cotton has changed the starting point for variety selection.  The initial question in the selection process is no longer yield but TECHNOLOGY:  Does X variety possess built-in pest management properties expressed by Bollgard (Bt), Roundup Ready (RR), or Bollgard/Roundup Ready (BR) “stacked” genes?


Technology comes with a cost.  Sometimes the costs are paid at both the front and back ends of the season.  We pay more for the seed, do we get more in the end?  Labor savings and convenience make transgenic offerings attractive, but are they the path to better profits?  We must evaluate not only the input costs or savings of a variety/technology package but also the net return associated with lost or gained yield and fiber quality.


In terms of pure yield potential, the best conventional, Bt, and BR varieties are superior to the best RR varieties.  In trials in high yield situations, RR varieties are rarely near the top.  It is not a matter of glyphosate adversely affecting yields of RR cotton.  Repeated tests confirm that when properly applied, glyphosate does not reduce yield of RR cotton.  It has to do with maximum yield potential.  A review at the recent Beltwide indicated that in Asystems@ trials, RR cultivars averaged about 100 lb/A less than comparable BR varieties.  No doubt, if comparisons of elite varieties (those with the highest yield potential) are made with RR varieties, the difference is even greater.  Is RR technology worth a decrease of 100 lb/A or more in high yield environments?  Arguably, RR offerings are a reasonable option in dry land production.  In such conditions, factors other than genetic potential limit yield.


An interjected caution is warranted.  Data and experience continue to confirm the dangers of mis-application of glyphosate in RR cotton.  Tardy over-the-top and sloppily applied directed treatments can reduce yields considerably.  Yield losses greater than 50 percent have occurred in grower fields.  Sloppy post applicationsBthose in 8-inch plus cotton that cover substantial portions of the stem and lower canopyBmay be the most harmful.  Glyphosate can be absorbed through woody bark.  Directed applications should be made with great precision toward the base of the crop plant with almost no contact with the stem or foliage, or preferably, conventional directed and layby herbicides can be substituted for glyphosate.


To Bt or not to Bt?  From a yield standpoint, the few remaining Bt varieties are solid performers and several of the BR varieties rank high in most variety trials.  The real question: What is the value of in-plant control of tobacco budworm and bollworm?  Insect populations are less predictable than weeds.  Worm pressure has generally been light since 1997, but there have been areas of intense activity.  No one can say beforehand what will be the severity of worm infestations in a given field, and thus the value of Bollgard technology for worm control is a guess.  Predictions are enhanced by an objective view of production history; specifically, by considering past levels of pest pressure, difficulty of control, and the possibility of pyrethroid resistance.  Recent developments--new insecticides, reduced pyrethroid prices, and other pest issues (i.e. stink bugs)--provide motivation to consider conventional varieties, as does the potential for improved yield and fiber quality with certain conventional options.  If a producer is consistently spraying less than three times for worms in non Bt fields, conventional varieties deserve a hard look.  Conversely, Bollgard technology might also be considered insurance.  Entomologists cringe about the idea of not planting Bt cotton when they consider what MIGHT happen.  They wonder if we truly remember how to “fight” insects.  Can we get over our entire acreage in 3 days or less with a timely insecticide application...and do it again 3 days later?  Who knows what pressure we=ll see this year?  If 2002 is a year of widespread worm pressure and/or pyrethroid resistance, Bt cotton will have even greater value than in past seasons.


Fiber length has been a major issue in recent seasons.  Increased problems with short staple have occurred with increased planting of transgenic cotton.  Without question, there is an environmental link to short fiber.  Many of the transgenic varieties that have dominated our acreage have marginal fiber length, especially when subjected to drought stress.  Longer staple is already available in conventional varieties such as PSC GA 161, Delta Pearl, and Fiber Max offerings and with some of the new transgenic Fiber Max varieties.  More improvements in fiber length are on the horizon.


One good thing about the 2002 variety decision is supply.  There are ample quantities of seed of the most popular varieties, both transgenic and conventional.  Some of the newer transgenics that were planted on limited acreage last year are much more available.  All this makes for competition and the opportunity to spread risks by “not putting all our eggs in one basket.”


2,4-D Application in February for Conservation Tillage Cotton:  (Culpepper)  Controlling cutleaf eveningprimrose and wild radish has proven difficult and often unsuccessful because these weeds are tolerant to glyphosate (Roundup, others) and paraquat (Boa, Gramoxone Max).   The addition of a tank mix partner with glyphosate or paraquat or making sequential herbicide application is recommended for controlling intense populations of these weeds in conservation tillage cotton.


The most consistent and effective herbicide program for Georgia growers, with or without a covercrop, is an application of 2,4-D in early to mid-February followed by either glyphosate or paraquat near planting.  The 2,4-D application in February may be applied alone or mixed with glyphosate or paraquat depending on weeds and or cover crop present.


2,4-D is a very economical and effective herbicide; however, be especially aware of drift to off-target crops as well as spray tank contamination.  For those growers who do not choose to make a 2,4-D application in February, many other herbicide choices are available but these applications are usually less effective on primrose and wild radish.


It’s Not too Early to Think About Controlling Nematodes and Seedling Disease:  (Kemerait)  Although we are still a couple of months away from the start of the cotton season, growers may still want to begin thinking about steps to control nematodes and seedling disease.  Research at The University of Georgia over the past couple of years has shown that while in-furrow fungicides and hopper box fungicide treatments may improve stand counts, they rarely significantly increase yields.  This indicates that our seeding rates are high enough to offset some loss to diseases, such as soreshin.  Growers who will be planting into fields with a history of severe seedling disease or into cold, wet soils are the ones most likely to benefit from extra fungicide help.  Growers can also help themselves by insuring that the seed they plant has a COLD germination value of at least 60%, but the higher, the better.


Problems with nematodes, especially reniform and root-knot, continue to increase across the state.  Lack of suitable non-host crops for rotation is the primary reason.  Hopefully, growers who suspected that they might have a problem in the 2002 season pulled soil samples from their fields last fall and have an idea where they need to use a nematicide.  Telone II at 3 gal/A is our most effective treatment, but also the most costly.  If a grower is using Telone, and if he has records from nematode damage in previous seasons, he may be able to spot treat sections of a field.  Many growers will likely choose to use Temik in furrow at 5-5.5 lb/A.  This rate offers greater protection than the thrips rate of 3.5 lb/A, and often offers just as much protection as 6 or 7 lbs/A.  A sidedress application of Temik at 5 lb/A at pinhead-square has been effective in some, but certainly not all, trials.  Control of nematodes early in the season will also help to reduce Fusarium wilt later in the season.



Prepared by:

Cotton -- The University of Georgia

Steven M. Brown, Extension Agronomist-Cotton

Philip H. Jost, Extension Agronomist-Cotton & Ag Crops

Stanley Culpepper, Extension Agronomist-Weed Science

Bob Kemerait-Extension Plant Pathologist



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