Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m−1of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m−1row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m−1of cotton row for NAWF and 0.8 m−1or 1.6 m−1of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m−1row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m−1row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss.

Goosegrass is a problematic summer annual weed in cotton, soybean, and corn production in the southern United States. Glyphosate is labeled for POST control of goosegrass in glyphosate-resistant (GR) cotton, soybean, and corn production. A population of goosegrass in west Tennessee not controlled by glyphosate was examined in greenhouse and laboratory studies. At 21 days after treatment (DAT), a glyphosate-susceptible (SS) biotype was controlled > 90% with glyphosate at rates greater than 210 g ae ha−1. Comparatively, the GR biotype was only controlled 12% at 210 g ae ha−1. Using goosegrass control data, I50values for GR and SS biotypes were 868 and 117 g ae ha−1, susceptibility, resulting in a resistance factor (RF) of 7.4. Treatment with glyphosate at 210 g ae ha−1reduced fresh weight biomass of the SS biotype to 5 g per pot compared to 36 g for the GR biotype. A total of 3,360 g ae ha−1glyphosate was required to reduce fresh weights of the GR biotype to ∼5 g per pot. Using fresh and dry weight biomass data, I50values for the GR biotype were 3 to 10 times greater than the SS biotype. On each date from 1 to 6 DAT the SS biotype accumulated higher concentrations of shikimate than the GR biotype. Future research should evaluate strategies for managing GR goosegrass with alternative modes of action. To prevent the spread of resistance, additional research evaluating programs for managing glyphosate-susceptible goosegrass in GR crops is also warranted.

Glyphosate-resistant (GR) weeds, especially GR Palmer amaranth, are very problematic for cotton growers in the Southeast and Midsouth regions of the United States. Glufosinate can control GR Palmer amaranth, and growers are transitioning to glufosinate-based systems. Palmer amaranth must be small for consistently effective control by glufosinate. Because this weed grows rapidly, growers are not always timely with applications. With widespread resistance to acetolactate synthase-inhibiting herbicides, growers have few herbicide options to mix with glufosinate to improve control of larger weeds. In a field study using a WideStrike®cotton cultivar, we evaluated fluometuron at 140 to 1,120 g ai ha−1mixed with the ammonium salt of glufosinate at 485 g ae ha−1for control of GR Palmer amaranth 13 and 26 cm tall. Standard PRE- and POST-directed herbicides were included in the systems. Glufosinate alone injured the WideStrike® cotton less than 10%. Fluometuron increased injury up to 25% but did not adversely affect yield. Glufosinate controlled 13-cm Palmer amaranth at least 90%, and there was no improvement in weed control nor a cotton yield response to fluometuron mixed with glufosinate. Palmer amaranth 26 cm tall was controlled only 59% by glufosinate. Fluometuron mixed with glufosinate increased control of the larger weeds up to 28% and there was a trend for greater yields. However, delaying applications until weeds were 26 cm reduced yield 22% relative to timely application. Our results suggest fluometuron mixed with glufosinate may be of some benefit when attempting to control large Palmer amaranth. However, mixing fluometuron with glufosinate is not a substitute for a timely glufosinate application.

Also known as great ragweed, horseweed, horse-cane, richweed, bitterweed, bloodweed, blood ragweed, tall ragweed, palmate ragweed. Classification and Description:Giant ragweed is an erect summer annual that is native to the U.S. and it can be commonly found throughout many parts of the country. It can reach heights from 3 to more than 16 feet. Giant ragweed is a member of the Asteraceae, or sunflower, family of plants. Seedling giant ragweed has a purple hypocotyl and cotyledons that are round to oblong and thick. The first true leaves do not have lobes but do have toothed margins and are lanceolate (long and thin) in shape. Subsequent leaves are opposite, blades simple, hairy and large (4-10 inches long and up to 8 inches wide). Leaves occur on petioles and most often have three prominent lanceolate-shaped lobes, although they can occasionally have five lobes. The lobes originate from the same point (palmate). These large, three-lobed leaves make giant ragweed a very distinctive plant. Leaf margins are serrated. Stems can be reddish and are erect, branching above, rough and sometimes hairy. Stems can be reddish. Giant ragweed has separate male and female flowers. Male flowers occur in slender racemes (columns) in the upper terminals. Female flowers occur in clusters in leaf axils below the male flowers. All flowers are small and greenish-yellow. Fruit is a large, black, woody achene that is egg-shaped, except the widest part is towards the end instead of in the middle. The widest end has one single short beak and other shorter projections, which make it resemble a crown. Seed is small and enclosed in the fruit. Reproduction is by seeds. Weed Status and Injury:Giant ragweed can readily be found along fence rows of agronomic crop fields and pastures in Tennessee. Increasingly, it is becoming established in agronomic crop fields. Herbicides commonly used on agronomic crops, like glyphosate, only provide partial control, and so giant ragweed is becoming an increasing problem in row crops. It can also be found in pastures, low woods and young Seedling giant ragweedGiant ragweed in a fence row W119Programs in agriculture and natural resources, 4-H youth development, family and consumer sciences, and resource development. University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides equal opportunities in programs and employment.

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