UGA Cotton Newsletter
May 25, 2001 http://www.griffin.peachnet.edu/caes/cotton
Crop Progress 1
Cotton Response to Early-Season Drought 1
Monitor Early Season Insects 2
Applying Dual Magnum in Georgia Cotton 3
Glyphosate (Roundup, others) or Staple Plus (glyphosate + Staple)
in Roundup Ready Cotton 4
Crop Progress. (Brown) As of May 21, Georgia cotton producers had planted 63 percent of the intended 2001 crop, putting us slightly behind the average on that date of 72 percent. Without question, drought has delayed planting and is having a profound impact on emergence. Officials from the Boll Weevil Eradication Program anticipate plantings to exceed 1.5 million acres.
How soon and how much rain comes will determine final stands. Planted in dry soil, seed can remain viable in the ground for several weeks. Light showers that marginally wet the seedbed will make for challenging decisions on final plant stands.
Cotton Response to Early-Season Drought. (Jost) I was really hoping that when I came to Georgia, the drought would disappear, but unfortunately, dry conditions linger. Some areas of the state recently received significant rainfall but others remain terribly dry.
At this time of the year, a key focus is managing plant health and growth below ground, especially since there is presently little above-ground biomass. Most fields are not yet squaring. Plant resources are primarily being directed to the development of root systems intended to sustain plants through the season.
The tap root of the cotton plant is pre-formed in the cotton seed and begins to grow when there is sufficient moisture and warm temperatures. By the time that the cotyledons emerge from the soil, the root tip is 5 to 8 inches deep. However, the predominant site of water absorption by the root is actually 2 to 3 inches behind the root tip. In this zone, root hairs and lateral roots are formed. The lateral roots are the ones that are critical for seedling vigor.
Tap roots and lateral roots can grow in two ways. One, they can grow at the tip, basically becoming longer. Secondly, they thicken by adding new cells from the inside and produce even more lateral roots.
It is important to note that roots only grow where there is moisture, they do not actively seek moisture. As the soil dries from the top down, root growth also occurs in the same way. When the soil dries in the upper profile, roots in that area cease to grow. The roots already present deeper in the soil, and in the presence of moisture, grow more quickly.
These root growth patterns are critical when considering irrigation initiation. Currently, the recommendation for the initiation of pre-bloom irrigation is to wait until the plants are wilting at midday. This recommendation is based on plant physiology and the response to water stress. A plant has three basic lines of defense against drought. The first is to reduce leaf expansion. While leaf expansion may be reduced in the early stages of water stress, the photosynthetic capacity of leaves is much less affected. Thus, the reduction in leaf expansion allows the energy produced from photosynthesis to be allocated to other areas of growth such as roots. This enhanced root growth is considered the second line of defense. Since roots do not grow in dry soil, this allows roots in moist soil to grow even more.
The third line of defense is stomatal closure, which ultimately results in leaf wilting. Thus, it is advantageous to wait until the plant shows stress above ground to initiate irrigation. If the upper profiles of the soil are wetted prior to this time, then roots continue to grow mostly in this area of the soil. If at all possible, we want the cotton plant to have roots deep in the soil where soil drying due to evaporation will occur much more slowly.
This response of allocating photoassimates to the roots is much more prominent in the early vegetative stage of cotton growth. As the plant begins to flower and set bolls, these structures become much stronger sinks for the products of photosynthesis than the roots. Therefore, water stress during bloom and boll fill periods may have a dramatic impact on yield but limited impact on root growth.
Monitor Early Season Insects. (Roberts) Thrips populations have increased significantly during the last 7 to 10 days. Compounded with dry soil conditions which are not conducive for plant uptake of preventive, at-plant insecticides, some plant injury is being observed. All fields should be monitored for thrips and damage, even if a preventive insecticide was used at planting. Symptoms of thrips injury include the characteristic crinkling of expanding leaves, plant stunting, and in severe cases loss of apical dominance and even stand loss. Foliar treatments are recommended if 2 to3 thrips are found per plant, especially if immature or wingless thrips are found. Once plants reach the 5-leaf stage and are growing rapidly, damage is less likely to occur.
Some additional early season insects of concern include cutworms and grasshoppers. Isolated fields have had significant stand loss to one or both of these pest. Grasshoppers and cutworms are most likely to occur in conservation tillage fields, but some cutworm problems have been reported in conventional tillage. Grasshopper and cutworm damage appears similar in some situations. Grasshoppers will feed on leaves, but stand loss may occur if feeding occurs on the stem during or after emergence. Unlike cutworms, grasshoppers will not completely cut the stem. A portion of the stem may be removed during feeding which weakens the stem and plants eventually die or break over at the feeding site. Be sure to monitor all fields as stands become established.
Cotton scouting schools are being offered at several locations across the state. In addition to the annual scout schools at the RDC in Tifton, several programs are being held locally. Registration is requested for the scout schools in Tifton, contact your county agent or call 912 386-3424 for more information.
May 29 Crisp/Dooly/Wilcox Counties
May 31 Jeff Davis County
June 2 (Saturday) RDC, Tifton
June 4 RDC, Tifton
June 5 Evans County
June 18 Burke County (Midville)
Applying Dual Magnum in Georgia Cotton. (Culpepper). Dual Magnum (1.0 to 1.3 pt/acre broadcast) is labeled for postemergence overtop or directed application to cotton in Georgia. When making a herbicide application including Dual Magnum, cotton should be 3 to12 inches tall. The label clearly states to not use this product on sand or loamy sand soil. It also states to NOT apply the product over-the-top with any adjuvant or with fluid fertilizer.
The following mixtures for postemergence-directed applications to 3- to 12- inch cotton are mentioned on the Dual Magnum label: Dual Magnum + MSMA, Dual Magnum + Caparol + MSMA, and Dual Magnum + Cotoran + MSMA.
Although mixing glyphosate (Roundup, others) and Dual Magnum is not discussed on the label, many questions have been asked regarding this combination. Research over the past several years has indicated only minor cosmetic injury; this injury was noted with purple, pinhead-sized specks on the cotton foliage when applied directed or postemergence over-the-top of cotton. Studies are being conducted this year to investigate the hypothesis that injury from glyphosate plus Dual Magnum combinations is only cosmetic.
Postemergence applications of Dual Magnum do not control existing weeds. However, combinations of glyphosate and Dual Magnum provide both postemergence and residual control of problem species such as pigweed, yellow nutsedge, and annual grasses. Although the residual control from Dual Magnum may be beneficial in some instances, it is of little benefit if TIMELY late postemergence-directed applications of appropriate conventional herbicides follow.
To reiterate, there is no mention of mixing glyphosate with Dual Magnum on any available label, and the label states that the product should not be used on sand or loamy sand soils.
Glyphosate (Roundup, others) or Staple Plus (glyphosate + Staple) in Roundup Ready Cotton? (Culpepper). Early postemergence weed control options in Roundup Ready cotton are generally limited to two practical labeled choices at this time. Choosing between glyphosate alone or glyphosate plus Staple is usually a field by field decision considering weed control, cotton tolerance, and cost of the herbicide treatments.
Weed control advantages with Staple Plus are generally observed on dayflower species, hemp sesbania, and large morningglory species (except tall morningglory). Selecting glyphosate alone or glyphosate plus Staple for morningglory control can be difficult. Frequently, small morningglories (2.5 inches or less) can be controlled very effectively with glyphosate alone. However, as morningglories become larger (4+ inches), the advantage of Staple plus glyphosate (except on tall morningglory) becomes more apparent. Staple, if activated by rainfall or irrigation, also provides residual weed control of susceptible weed species.
Over the past two seasons, applications using glyphosate plus Staple occasionally caused Aspeckling@ of the cotton and rarely caused leaf drop within 48 hrs of application. Research in 2000 suggested this injury was transient, but additional trials are being conducted this year to further address this concern.
Steven M. Brown, Extension Agronomist-Cotton
Philip H. Jost, Extension Agronomist-Cotton & Ag Crops
Stanley Culpepper, Extension Agronomist-Weed Science
Phillip Roberts, Extension Entomologist-Cotton