Lawrence E. Steckel

Palmer Amaranth ( Amaranthus palmeri ): A Review

Abstract In little over 20 yr, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult-to-control weed, including its rapid growth rate, high fecundity, genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems and a fascinating model weed. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts, and management of this weed, and we suggest future directions for research. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA. Resumen En poco más de 20 años, Amaranthus palmeri ha salido de una relativa oscuridad a su estado actual como una de las malezas agrícolas más ampliamente distribuida, más problemática y económicamente dañina en el sureste de los Estados Unidos. Numerosos factores le han permitido a A. palmeri convertirse en una maleza tan dominante y difícil de controlar, incluyendo su rápida tasa de crecimiento, alta fecundidad, diversidad genética, habilidad para tolerar condiciones adversas, y su facilidad para evolucionar resistencia a herbicidas. Es una amenaza para varios sistemas de cultivos en los Estados Unidos, pero también es una maleza modelo fascinante. En este artículo, revisamos la cantidad creciente de literatura sobre A. palmeri para resumir el estado actual de conocimiento sobre la biología, impactos agrícolas, y manejo de esta maleza, y sugerimos futuras direcciones para su investigación.

The Dioecious Amaranthus spp.: Here to Stay

There are nearly 75 species in the genus Amaranthus, part of the Amaranthaceae family, worldwide. In this large genus, there is a distinct group of 10 species that are dioecious (separate male and female plants). In contrast to the monoecious Amaranthus spp. which are represented by species endemic to every continent, the dioecious Amaranthus spp. are all native to North America. The Amaranthus spp. have a long documented history of being fellow travelers with humans. In recent years, three dioecious Amaranthus spp.: Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and tall waterhemp [Amaranthus tuberculatus (Moq) Sauer] have become major weeds to row crops in North America. Palmer amaranth has become a very troublesome weed to cotton (Gossypium hirsutum L.), corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in large parts of the southern United States, and the waterhemp complex is now a major weed pest in corn and soybean production in the midwestern United States (Horak and Loughin 2000; King 1966; Mabberly 1997; Robertson 1981; Sauer 1950, 1955, 1972; Wax 1995).

Palmer Amaranth (Amaranthus palmeri) in Tennessee Has Low Level Glyphosate Resistance

Many agricultural producers apply glyphosate to glyphosate-resistant crops to control weeds, including Palmer amaranth. Populations of this weed in Tennessee not completely controlled by glyphosate were examined. Field and greenhouse research confirmed that two separate populations had reduced biomass sensitivity (1.5× to 5.0×) to glyphosate compared to susceptible populations, although the level of resistance was higher based on plant mortality response (about 10×). Shikimate accumulated in both resistant and susceptible plants, indicating that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was inhibited in both biotypes. These results suggest that an altered target site is not responsible for glyphosate resistance in these Palmer amaranth biotypes. Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA

Confirmation and Control of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) in Tennessee

Abstract Seeds of a suspected glyphosate-resistant giant ragweed biotype from Lauderdale County, TN, were collected from a continuous cotton field in fall 2007 after plants were nonresponsive to multiple glyphosate applications. The objectives of this research were to (1) confirm resistance by quantifying the response of the putative resistant biotype to glyphosate compared to a susceptible biotype from a nonagricultural area, (2) quantify shikimate accumulation over time in both biotypes, and (3) determine the effectiveness of POST-applied herbicides labeled for use in cotton in controlling both biotypes at three growth stages. The susceptible biotype had a 50% lethal dose of 407 g ae/ha of glyphosate compared with 2,176 g/ha for the resistant biotype when treated at the four-node stage, a 5.3-fold level of resistance. The resistant biotype accumulated 3.3- to 9.8-fold less shikimate than the susceptible biotype at 1 to 7 d after treatment. The resistant biotype was less responsive to glyphosate as treatment was delayed past the two-node stage, much more than the susceptible biotype. Glufosinate, MSMA, and diuron controlled both biotypes by at least 90%, regardless of size at application. Prometryn, flumioxazin, carfentrazone-ethyl, fomesafen, and trifloxysulfuron controlled both biotypes by at least 89% when applied at the two-node stage, but control generally diminished with later application timings. Pyrithiobac was not effective in controlling either biotype, regardless of size at application. Hence, there are effective herbicide options for controlling glyphosate-resistant giant ragweed in cotton, and the resistant biotype does not appear to exhibit multiple resistances to other herbicides. Nomenclature: Carfentrazone-ethyl; diuron; flumioxazin; fomesafen; glufosinate; glyphosate; MSMA; prometryn; pyrithiobac; trifloxysulfuron; giant ragweed, Ambrosia trifida L. AMBTR; cotton, Gossypium hirsutum L

Biotic and abiotic factors influence horseweed emergence

Abstract Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r2 = 0.21), air temperature (r2 = 0.45) or rainfall (r2 = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m−2 to a high of > 1,500 plants−2 at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

Integrated pest management and weed management in the United States and Canada.

There is interest in more diverse weed management tactics because of evolved herbicide resistance in important weeds in many US and Canadian crop systems. While herbicide resistance in weeds is not new, the issue has become critical because of the adoption of simple, convenient and inexpensive crop systems based on genetically engineered glyphosate-tolerant crop cultivars. Importantly, genetic engineering has not been a factor in rice and wheat, two globally important food crops. There are many tactics that help to mitigate herbicide resistance in weeds and should be widely adopted. Evolved herbicide resistance in key weeds has influenced a limited number of growers to include a more diverse suite of tactics to supplement existing herbicidal tactics. Most growers still emphasize herbicides, often to the exclusion of alternative tactics. Application of integrated pest management for weeds is better characterized as integrated weed management, and more typically integrated herbicide management. However, adoption of diverse weed management tactics is limited. Modifying herbicide use will not solve herbicide resistance in weeds, and the relief provided by different herbicide use practices is generally short-lived at best. More diversity of tactics for weed management must be incorporated in crop systems.

Assessment of Weed Management Practices and Problem Weeds in the Midsouth United States—Soybean: A Consultant's Perspective

Abstract Soybean consultants from Arkansas, Louisiana, Mississippi, and Tennessee were surveyed by direct mail and by on-farm visits in fall 2011 to assess weed management practices and the prevalence of weed species in midsouth U.S. soybean. These consultants represented 15, 21, 5, and 10% of total soybean planted in Arkansas, Louisiana, Mississippi, and Tennessee, respectively, in 2011. Collectively, 93% of the total scouted area in these four states was planted with glyphosate-resistant (RR) soybean. The adoption of glufosinate-resistant (LL) soybean was greatest in Arkansas (12%), followed by Tennessee (4%), Mississippi (2%), and Louisiana (< 1%). Only 17% of the RR soybean was treated solely with glyphosate, compared with 35% of LL soybean treated solely with glufosinate. Across four states, average cost of herbicides in RR and LL soybean systems was US

Glyphosate-resistant Horseweed (Conyza Canadensis) Control with Glufosinate Prior to Planting No-till Cotton (Gossypium Hirsutum)1

78 and US

Tillage, Cropping System, and Soil Depth Effects on Common Waterhemp (Amaranthus rudis) Seed-Bank Persistence

91 ha−1, respectively. Collectively across states, total scouted area under conventional tillage was 42%, stale seedbed was 37%, and no-tillage was 21%. Palmer amaranth and morningglories were the most problematic weeds in all four states. Additionally, barnyardgrass and horseweed were the third most problematic weeds of Arkansas and Tennessee, respectively, and Italian ryegrass was the third most problematic weed in Louisiana and Mississippi. Glyphosate-resistant Palmer amaranth infested fewer fields in Louisiana (16% of fields) than it did in the remaining three states (54% collectively). Average Palmer amaranth hand-weeding costs in the midsouth was US

Adoption of Best Management Practices for Herbicide-Resistant Weeds in Midsouthern United States Cotton, Rice, and Soybean

59 ha−1. Three-fourths of the midsouth consultants stipulated the need for continued research and education focused on management of glyphosate-resistant and glyphosate-tolerant weed species. Nomenclature: Glufosinate; glyphosate; barnyardgrass; Echinochloa crus-galli (L.) Beauv.; horseweed; Conyza canadensis (L.) Cronq.; Italian ryegrass; Lolium perenne L. ssp. multiflorum (Lam.) Husnot; morningglory; Ipomoea spp.; Palmer amaranth; Amaranthus palmeri S. Wats.; soybean; Glycine max (L). Merr. Resumen Asesores en soya de Arkansas, Louisiana, Mississippi, y Tennessee fueron encuestados vía correo y visitas en finca en el otoño de 2011 para evaluar las prácticas de manejo de malezas y la prevalencia de especies de malezas en la producción de soya en el Sur medio de los Estados Unidos. Estos asesores representaron 15, 21, 5 y 10% del total de soya plantada en Arkansas, Louisiana, Mississippi, y Tennessee, respectivamente en 2011. Colectivamente, 93% del total del área evaluada en estos cuatro estados fue sembrada con soya resistente a glyphsoate (RR). La adopción de soya resistente a glufosinate (LL) fue mayor en Arkansas (12%), seguida por Tennessee (4%), Mississippi (2%) y Louisiana (<1%). Solamente 17% de la soya RR fue tratada únicamente con glyphosate, al compararse con 35% de soya LL que fue tratada solamente con glufosinate. En los cuatro estados, el costo promedio de herbicidas en sistemas de soya RR y LL fue US