Amaranthus palmeri, Popularly known as caruru or caruru-palmeri, it represents one of the most serious threats to Brazilian agriculture today. Originating from the desert region of the southwestern United States and northern Mexico, this highly aggressive invader stands out for its extraordinary capacity for adaptation, resistance to herbicides and impressive reproductive potential. Its presence in Brazilian crops has caused serious concerns due to the devastating economic impact.
In general terms, studies indicate reductions in productivity that can reach 91% (corn), 79% (soybean) and 77% (cotton). Knowing this species and its management strategies has become essential for producers seeking to protect their crops from this difficult-to-control weed.
In South America, the presence of Amaranthus palmeri It was initially verified in Argentina (there is a report from the last century). It was believed that, because it was found in that country, the South region would be the probable route of introduction of the weed into Brazil.
In 2013, after detection of A. palmeri resistant to glyphosate in Argentina, research institutions in Brazil began to worry about the possibility of the species entering.
Contrary to expectations, the first detection of A. palmeri in Brazil it officially took place in the state of Mato Grosso, in 2015, a region far from the border with Argentina.
Amaranthus palmeri It is a short-cycle annual plant. It completes its cycle (germination, growth, flowering and seed production) in periods of four to six months, depending on environmental conditions.
It develops deep and extensive roots, allowing access to water and nutrients in lower layers of the soil, which makes it highly resistant to moderate droughts.
A. palmeri It presents C4 photosynthesis. It has an efficient carbon fixation mechanism, common in plants adapted to hot climates. This explains its accelerated growth even at high temperatures.
In terms of reproduction and genetics:
• Dioecia: Male and female plants are separated, promoting genetic crossing and variability. This contributes to rapid adaptation to herbicides and adverse conditions.
• Seed production: Each female plant can produce up to a million seeds. Under competitive conditions, the number is usually between 100 and 600. The seeds are small (1–2 mm) and light, making them easy to disperse by wind, water, animals, or agricultural equipment.
• Variable dormancy: some seeds germinate soon after dispersal, while others remain dormant in the soil for ten years or more, forming a persistent seed bank.
Its growth rate can reach 5 cm to 7 cm per day in ideal conditions, surpassing crops such as soybeans and corn in the competition for light.
A single plant can accumulate up to 1 kg of dry biomass, extracting large amounts of nitrogen, phosphorus and potassium from the soil.
Furthermore, it presents phenotypic plasticity. It adapts its morphology (height, leaf area) according to the availability of resources, becoming more robust in fertile soils or open spaces.
Amaranthus palmeri It develops best at temperatures between 28°C and 35°C, but tolerates temperatures from 15°C to 40°C. The plant grows vigorously in long days and with high light intensity, being common in tropical and subtropical regions.
In terms of soil and moisture, it grows in soils with a pH between 5,5 and 8, as long as they are well drained. It is common in sandy soils, but also thrives in clay soils if there is adequate moisture.
Its deep roots allow it to survive in dry periods, but for maximum growth it prefers regular humidity (500–800 mm of annual rainfall). On the other hand, it is sensitive to waterlogged soils.
Germination and establishment of A. palmeri:
• Emergence depth: Seeds germinate up to 5 cm deep, but the ideal is between 0,5 cm and 2 cm.
• Triggers for germination: rainfall or irrigation after dry periods; soil disturbance (e.g. conventional tillage); soil temperatures above 18°C.
• Emergency period: germinates throughout the growing season, making continuous control a challenge.
the management of Amaranthus palmeri requires integrated strategies.
• Prevention: monitor areas to identify infestations early; avoid introducing contaminated seeds into agricultural equipment.
• Cultural methods: crop rotation (using competitive species such as grasses); use of mulch or cover crops to suppress germination; hand or mechanical weeding before flowering; row cultivation to remove young seedlings.
• Chemical control: use registered products; populations resistant to multiple herbicides require rotation of active ingredients and modes of action.
Herbicide resistance, already documented in several countries, makes Amaranthus palmeri one of the worst invaders of the 90st century. Its inadequate management can cause losses of up to XNUMX% in productivity in crops such as soybeans.
The species has demonstrated documented resistance to several classes of active ingredients, such as ALS inhibitors (such as imazethapyr and chlorimuron); PPO inhibitors (such as fomesafen and lactofen); dinitroanilines (such as pendimethalin); HPPD inhibitors (such as mesotrione and tembotrione); and auxinic herbicides (such as dicamba and 2,4-D).
By May 2025, Amaranthus palmeri resistant to glyphosate is officially present in the states of Mato Grosso and Mato Grosso do Sul (Naviraí and Aral Moreira). Other states are monitoring the development of the weed, but there is still no confirmation of new cases.
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