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- Since Palmer amaranth was first discovered in the Douglas district in the Northern Cape early in 2018, the SAHRI has led mitigation efforts against this potentially ruinous botanical foe.
- The hype around Palmer amaranth, which is an annual summer plant, is justified by its exceptional ability to compete aggressively with basically all annual crops.
- Palmer amaranth is an annual plant and completes its life cycle in a single calendar year, thus it competes aggressively with annual crops that are arguably less driven by nature for survival than weedy plants.
- US data shows that Palmer amaranth emerging with the crop attains a 10cm height within about 17 days after emergence; thereafter, its growth rate exceeds 5cm per day.
- Continued effective herbicide use is nowhere more challenged than in no-till systems with glyphosate-tolerant crops.
The previous article on Palmer amaranth (Amaranthus palmeri), which appeared in the March issue of Oilseeds Focus, marked four years since the confirmation of the United States’ (US) ‘number one weed’ in South Africa in 2018. To date, no data exists on the effects this weed will undoubtedly have on local crop production. Because Palmer amaranth is indigenous to the US, where its origin (endemism) is the Sonoran Desert, most of the knowledge about it was accumulated there.
Since about 2010, numerous scientific studies in the US have revealed the reasons behind this weed’s ability to cause economically crippling yield losses in diverse annual cropping systems, and the threats it poses for the economic viability of crop production (DJ Mahoney et al., Agronomy 11, 2021, 1734).
Continuous eradication efforts
Since Palmer amaranth was first discovered in the Douglas district in the Northern Cape early in 2018, the South African Herbicide Resistance Initiative (SAHRI) based at the University of Pretoria has led mitigation efforts against this potentially ruinous botanical foe by means of active advocacy and research.
The hype around Palmer amaranth, which is an annual summer plant, is justified by its exceptional ability to compete aggressively with basically all annual crops. Because the sexes are on separate plants, female plants are of particular interest since they carry seed and have greater biomass than male plants. Plant biomass is positively correlated with competitive ability. Studies in the US report that Palmer amaranth, without competition, can attain plant heights of 160 to 270cm, and female plants produce more than 600 000 seeds per plant. In the Potchefstroom district in South Africa, such heights were observed in dense stands of the weed located outside crop fields (Photo 1). Crops on their own tend to fight back and reduce the growth of Palmer amaranth (Photo 2).
It is an annual plant and completes its life cycle in a single calendar year, thus it competes aggressively with annual crops that are arguably less driven by nature for survival than weedy plants. Palmer amaranth adapts to different cropping systems – the seed-carrying female plants tend to grow just a bit taller than the crop it infests (Photo 1).
Extremely rapid growth rate
US data shows that Palmer amaranth emerging with the crop attains a 10cm height within about 17 days after emergence; thereafter, its growth rate exceeds 5cm per day. Even if this weed is 100% controlled for the first three weeks after planting the crop, the growth rate of new cohorts exceeds 4cm per day. Therefore, the window for applying post- emergence herbicides at the ideal growth stage of weeds is extremely narrow in the case of Palmer amaranth.
In US cotton production, the final biomass of female Palmer amaranth plants was reduced between 50 and 80% when the weed was 100% controlled for the first three weeks after planting the crop and then left to grow uncontrolled to maturity. This proves why it is important to include effective pre-emergence herbicides in the weed control programme.
Maize and soya bean have denser canopies than both cotton and groundnuts; therefore, these crops compete differentially with the weed for sunlight. Plant height of Palmer amaranth which was 100% controlled for the first three weeks after planting the crop and subsequently left to grow to maturity, was reduced by 87, 32, 25, and 15% in maize, soya bean, cotton, and groundnut, respectively.
This confirms the ability of crops to perform weed suppression on their own. Besides its exceptionally strong interference with crop growth and development, Palmer amaranth is incomparable because of its superior ability, which is ascribed to high genetic diversity to evolve resistance relatively rapidly to a wide range of herbicide mechanisms of action.
Resistance to over 70 herbicides
As of October 2022, a total of 72 cases of herbicide resistance have been reported globally for Palmer amaranth (www.weedscience/org). This weed has proven resistant to a staggering total of nine extremely important herbicide modes of action or sites of action (SOA).
The following herbicide groups, according to the Herbicide Resistance Action Committee (HRAC), are implicated:
- Group 2: Acetolactate synthase inhibitors, specifically chlorimuron- ethyl, imazethapyr, imazaquin, flumetsulam, halosulfuron-methyl, and sulfometuron methyl.
- Group 3: Microtubule assembly- inhibitors, specifically trifluralin and pendimethalin.
- Group 4: Auxin mimics, specifically 2,4-D and dicamba.
- Group 5: Photosystem II inhibitors, specifically atrazine.
- Group 9: 5-enolpyruvylshikimate- 3-phosphate inhibitors, specifically glyphosate.
- Group 10: Glutamine synthetase inhibitor, specifically glufosinate- ammonium.
- Group 14: Protoporphyrinogen oxidase inhibitors, specifically fomesafen.
- Group 15: Long-chain fatty acid inhibitors, specifically s-metolachlor.
- Group 27: 4-Hydroxyphenylpyruvate dioxygenase inhibitors, specifically mesotrione, tembotrione and topramezone.
The above herbicide resistance scenario paints a particularly grim picture for the future of glyphosate use in glyphosate- tolerant crops such as maize, cotton, and soya bean that are infested with Palmer amaranth. From a crop perspective, the weed control prospects are equally dire because of the serious limitations that weed resistance in general places on herbicide options. At present, there are 514 confirmed cases of herbicide-resistant weeds globally, with 267 weed species and 165 different herbicides involved (www.weedscience.org).
Continued effective herbicide use is nowhere more challenged than in no-till systems with glyphosate-tolerant crops. The worst-case scenario in the US regarding herbicide resistance of Palmer amaranth was recorded in 2016 in Arkansas in cotton and soya bean plantings, with multiple resistance to five SOAs, i.e., HRAC Groups 2, 14, 9, 3, and 15.
In this case, the herbicides implicated were flumetsulam, fomesafen, glyphosate, imazethapyr, pendimethalin and s-metolachlor. The most common herbicide resistance of Palmer amaranth in the US is multiple resistance to the combination glyphosate (HRAC Group 9) and ALS-inhibitors (HRAC Group 2). As with any highly competitive weed, Palmer amaranth must be effectively controlled during the first three weeks after planting the crop. Pre-plant and pre-emergence herbicides are important tools in this regard.
Palmer amaranth emerges in flushes throughout the growing season, therefore post-emergence herbicides and persistent herbicides with residual action must also be included in the control programme. Zero tolerance to seed production, especially in the case of herbicide- resistant weeds, must be the basic objective in order to prevent fresh seed deposits into the soil seedbank.
Readers are encouraged to contact Prof Charlie Reinhardt about sightings of real or suspected Palmer amaranth. The Manual for Identification and Reporting of Palmer amaranth (Amaranthus palmeri) in South Africa is available free of charge in Afrikaans or English. Prof Reinhardt works in the agricultural sciences programme at North-West University, Potchefstroom campus, and is also research leader at the South African Herbicide Resistance Initiative at the University of Pretoria. For enquiries, contact him at 083 442 3427 or firstname.lastname@example.org.