COMMON PIGWEED is a growing threat

October 2025

PROF JUAN VORSTER, SOUTH AFRICAN HERBICIDE RESISTANCE INITIATIVE (SAHRI), UNIVERSITY OF PRETORIA.FIRST PUBLISHED IN SA GRAAN/GRAIN, SEPTEMBER 2025.

COMMON PIGWEED (AMARANTHUS HYBRIDUS) IS PROBABLY ONE OF THE MOST WIDESPREAD AND COMMON WEEDS ACROSS SOUTHERN AFRICA. SINCE THE DISCOVERY OF HERBICIDE-RESISTANT POPULATIONS OF PALMER AMARANTH (AMARANTHUS PALMERI) IN 2018, A LOT OF FOCUS HAS BEEN PLACED ON THIS SPECIES, BUT IN THE LAST FEW SEASONS COMMON PIGWEED HAS EMERGED AS A SIGNIFICANT CHALLENGE TO FARMERS IN THE SUMMER RAINFALL REGION OF SOUTH AFRICA, ESPECIALLY IN CROPS LIKE MAIZE AND SOYBEAN.

Not only are we seeing an increase in cases of glyphosate resistance (HRAC group 9/G) but also populations resistant to active ingredients in the ALS-inhibitor group (HRAC group 2/B) and even cases of multiple resistance to both groups. This article explores some of the latest research findings from the South African Herbicide Resistance Initiative (SAHRI) at the University of Pretoria, the threat posed by herbicide-resistant populations of common pigweed as well how farmers can manage the problem.

THE THREAT
Common pigweed is a fast-growing, highly competitive weed that can significantly reduce crop yields if left uncontrolled. It belongs to the Amaranthaceae family, a group of weeds known for their adaptability and rapid reproduction. Common pigweed competes with crops for vital resources like light, water, and nutrients. It is especially adapted to the warmer and wetter regions of the summer production areas but also has the ability to tolerate drought conditions. Seeds germinate in a wide band of temperatures ranging from 15°C to 41°C with an optimum between 30°C and 35°C. 

Its aggressive growth habit and ability to produce hundreds of thousands of seeds per plant make it particularly difficult to control. The ability to produce large amounts of seed can lead to a rapid accumulation of seeds in the soil. These seeds can remain dormant in the soil for extended periods, allowing the weed to re-emerge in future growing seasons. 

Seeds also germinate in flushes throughout the season allowing it to potentially escape early-season control with pre-emergence herbicides. If left unchecked, it can reduce crop yields by up to 90%, causing severe financial losses for farmers (Figure 1). Its high seed production rate is also linked to its ability to rapidly develop herbicide resistance. Resistance to glyphosate is especially concerning when trying to control it in Roundup Ready crops, especially broadleaf crops like soybean. Populations showing resistance to multiple herbicide modes of action (glyphosate and ALS-inhibitors) are even more concerning.

A recent study conducted by researchers at SAHRI confirmed the first cases of glyphosate-resistant populations of common pigweed in South Africa. Populations of this weed collected from Bergville and Winterton in KwaZulu-Natal were resistant to glyphosate, even at four to eight times the recommended rate (Figure 2). Investigating the mechanism of resistance revealed a rare triple mutation in the EPSPS enzyme (normally inhibited by glyphosate) that prevents the glyphosate molecule binding to the target enzyme.

Both the Bergville and Winterton populations also showed resistance to the ALS-inhibiting herbicide active chlorimuron-ethyl at four times the recommended field rates. When we investigated the resistance mechanism, various mutations in the target enzyme that confers resistance to a variety of the active ingredients in the ALS herbicide group were identified. 

HERBICIDE RESISTANCE IS A GROWING PROBLEM
Glyphosate is a broad-spectrum, non-selective herbicide that has been used extensively in South Africa due to the widespread adoption of glyphosate-resistant (GR) Roundup Ready (RR) crops like maize and soybeans. However, over-reliance on this herbicide has led to the emergence of a range of glyphosate-resistant weed species. Among those listed in South Africa are Palmer amaranth (A. palmeri), hairy fleabane (Conyza bonariensis), rigid ryegrass (Lolium rigidum), buckhorn plantain (Plantago lanceolota) and, more recently, common pigweed (A. hybridus).

ALS-inhibiting herbicides are also commonly used in South Africa. ALS-inhibitors are comprised of five chemical families, namely imidazolinones (IMIs), sulfonylureas (SUs), pyrimidinyl benzoate, triazolinones, and triazolopyrimidines (TPs).

At least ten different mutations have been identified in the ALS gene that can confer various levels of resistance to one or more of these families. In South Africa the following species have been recorded as resistant to ALS-inhibiting herbicides: Palmer amaranth (A. palmeri), wild oat (Avena fatua), rigid ryegrass (L. rigidum), wild radish (Raphanus raphanistrum), little-seeded canary grass (Phalaris minor), common chickweed (Stellaria media) and now common pigweed (A. hybridus) as well.

MANAGING RESISTANCE
While the spread of herbicide-resistant common pigweed poses a significant challenge, there are several strategies that South African farmers can adopt to manage this weed effectively. Preventing resistance from developing or spreading between fields is the most effective strategy, and preventing the spread of pigweed is critical.

Farmers should clean their equipment between fields to avoid transporting weed seeds from one area to another.

Also manage field borders and non-crop areas to prevent the weed from establishing and spreading into cultivated fields.

Rotation of herbicide modes of action and herbicide mixtures is essential. The use of pre-emergence herbicides with residual activity is essential, especially in crops with limited post-emergence options and where tillage is not used for weed control.

Early detection of resistance is important to prevent resistance from spreading. Ongoing resistance screening at SAHRI has been able to identify cases of resistance to glyphosate and ALS-inhibitor herbicides across the summer rainfall season.

Early intervention can prevent resistant populations from building up seed banks in the soil. Preventing seed set from any remaining plants in the field is more effective than trying to control large populations in the next season.

Crop rotation in fields where resistance is present is important, with maize offering more alternative chemical control options compared to soybean.

Planting cover crops like sorghum or rye can help suppress the growth of pigweed by outcompeting it for resources.

By implementing integrated weed management strategies, rotating herbicides, and preventing the spread of resistant weeds, farmers can protect their crops and livelihoods from the damaging effects of pigweed. Now more than ever, it is crucial to stay informed and proactive in the approach to weed control. The future of South African agriculture 
depends on it.

Publication: October 2025

Section: Pula/Imvula