High impact: damage caused by dairy farmers to soybeans

In Brazil, the milkman or wild peanut (Euphorbia heterophylla) is one of the main weed species that affect the yield of cultivated plants. Milkweed is a plant with an annual cycle, with a large seed production capacity. In the 70s, when soybean cultivation was just in its infancy in Brazilian areas, milkweed was considered a secondary weed species. The use for a long time of the association of the herbicides trifluralin and metribuzin, which do not control milkweed, in soybean areas, ended up selecting it, making it one of the main weed species in Brazil, affecting important cultivated species. The great impact caused by the milkman occurs due to several causes, among which the biological aspects of the species, its competitive capacity and the resistance of populations to herbicides stand out.

Figure 1 – Soybean crop with heavy milkweed infestation (E. heterophylla). Photo: Michelangelo Muzell Trezzi

Biological characteristics of the milkman

Among the biological characteristics that make it difficult to control milkweed in crop areas, the rapid initial development, emergence in flows, large seed production and resprouting capacity stand out. Rapid initial development is a typical characteristic of pioneer species capable of draining resources from the environment for themselves, making it difficult for them to access cultivated plants.

Milkweed seeds are disseminated in an “explosive” way and, from the soil, emergence occurs in flows, which makes chemical control more difficult, as post-emergence applications of plants are capable of reaching the first flow, but in general they do not control subsequent flows. Emergence in flows occurs because the process is mainly dependent on soil temperature and solar radiation. Thus, temperature alternations of 25/30ºC stimulate milk germination and seed germination can be sensitive or radiation insensitive, depending on the time of year the seeds matured on the mother plant. The practical result of this is that large quantities of straw may be needed on the soil surface to contain milkweed emergence. For example, 23 tons are needed per hectare of sorghum straw to reduce milk emergence by 50%, while for guanxuma and picão-preto, for example, around 1,5 tonnes per hectare to 2 tonnes per hectare are enough to obtain the same effect. This certainly contributes to this species becoming increasingly widespread, even in areas that have been under direct planting with straw for many years.

Dairyman competition with cultivated species

The competitive potential of dairy farming with cultivated species contributes greatly to the negative impact of this species, as it imposes significant daily losses on crops. Experiments indicate daily losses of 5,2kg/ha to 6,5kg/ha of soybeans resulting from interaction with the milkman. About 32 plants per m² are enough to generate a 50% reduction in soybean yield.

The early emergence of dairy crops in relation to grain crops can increase the damage caused as the weed species advances and becomes more competitive than the cultivated species. An experiment conducted by the research group of the Núcleo de Investigações em Ciência das Plantas Daninhas (Niped), at UTFPR Campus Pato Branco, with the bean crop shows that milk plants that emerge 12 days before the crop result in a maximum loss of 63%, while emergence together with beans results in a loss of only 44% (Figure 2). In other words, choosing the appropriate desiccation strategy can bring positive results, avoiding losses in grain yield and increasing the farmer's income.

Figure 2 – Percentage loss of bean yield depending on different densities and times of emergence of E. heterophylla. DAS = days before sowing the beans. UTFPR, Pato Branco Campus

Figure 3 – Dairy competition with bean plants. Photo: Adriano Machado

Impact of the species' genetic variability

The milkman is native to the tropical and subtropical regions of the Americas and the probable center of origin is in Paraguay and Southwest/West of Paraná. Therefore, in this region this species has high genetic variability, translated into great morphological variation (leaf shape, hairiness on the stem, cycle, etc.) and also physiological/biochemical variation (variation in the absorption, translocation and metabolization of herbicides, insensitivity of enzymes to some herbicides etc).

The conjunction of intensification of the use of certain herbicides without proper rotation, areas with high populations and high genetic variability of the milkweed determined the most harmful aspect of this species, herbicide-resistant populations (Table 1). The first cases of resistant milkweed in Brazil occurred in the 90s, due to the mechanism of action of ALS inhibitors (herbicides Pivot, Scepter, etc.), which became widespread in crops in the southern region of Brazil. In 2004, the Niped research group recorded more worrying cases, of multiple resistance to ALS and Protox inhibitors (herbicides Flex, Cobra, etc.), in the southwest region of Paraná. This was the first case of a weed with multiple resistance in Brazil. In 2013, Niped confirmed the existence of two new cases of milkmen with multiple resistance to ALS and Protox inhibitors, in Medianeira (Paraná) and Vilhena (Rondônia). If, with the resistance of a weed population to a mechanism of action, farmers are prevented from using several herbicides from the same mechanism, with multiple resistance producers also lose other alternatives. In comparison, it would be equivalent to losing many antibiotics capable of controlling resistant bacteria in hospitals.

Even geographically very distant populations present similar characteristics of resistance to ALS and Protox inhibitor herbicides. Furthermore, they are resistant to herbicides that have never been previously applied in the areas. It is already known that the dairy plant's resistance to ALS-inhibiting herbicides is due to a modification of the ALS enzyme, which makes it insensitive to herbicides and, consequently, the enzyme continues to produce amino acids essential for plant survival. Currently, the cause of milkman resistance to Protox inhibitors is being investigated.

Table 1 – Cases of resistance in dairy farming confirmed around the world, such as country and year of discovery, crops in which they were found and mechanisms of action/herbicides

City	Vintage	Crops in which resistance has been found	Mechanisms of action/herbicides
Equador	1990	corn and soy	Photosystem II-linuron inhibitors
Paraná - Rio Grande do Sul/ Brazil	1993	Soybean	ALS inhibitors - chlorimuron-ethyl, cloransulam-methyl, imazamox, imazaquin and imazethapyr
Itapua/Paraguay	1995	Soybean	ALS inhibitor - imazethapyr
Parana Brazil	2004	corn and soy	Multiple resistance to ALS and Protox inhibitors - acifluorfen-sodium, cloransulam-methyl, diclosulam, flumetsulam, flumiclorac-pentil, medosafen, imazethapyr, lactofen, metsulfuron-methyl and nicosulfuron

City

Vintage

Crops in which resistance has been found

Mechanisms of action/herbicides

Equador

1990

corn and soy

Photosystem II-linuron inhibitors

Paraná - Rio Grande do Sul/ Brazil

1993

Soybean

ALS inhibitors - chlorimuron-ethyl, cloransulam-methyl, imazamox, imazaquin and imazethapyr

Itapua/Paraguay

1995

Soybean

ALS inhibitor - imazethapyr

Parana Brazil

2004

corn and soy

Multiple resistance to ALS and Protox inhibitors - acifluorfen-sodium, cloransulam-methyl, diclosulam, flumetsulam, flumiclorac-pentil, medosafen, imazethapyr, lactofen, metsulfuron-methyl and nicosulfuron

Source: Adapted from The International Survey of Herbicide Resistant Weeds. www.weedscience.org, 2014.

How long do resistant populations persist in crops?

When a resistant population emerges, one of the first consequences is that farmers no longer use herbicides that select resistant weeds. Replacing herbicides with more efficient ones, however, does not mean that bank propagules are eliminated from crop areas. As the use of old herbicides may be a necessity for farmers, it is necessary to know how long the seeds remain in the areas.

In 2013, Niped investigated the persistence of populations resistant to ALS and ALS/Protox inhibitors in areas of the Southwest region of Paraná, whose resistance to herbicides had been detected in 2002 (Figure 4). It was discovered that, in all areas, populations resistant to ALS and Protox inhibitors remain today, that is, even after ten years. This information is an alert to difficulties that may be encountered by farmers when managing dairy in areas where resistance had previously been found and if they wish to apply selector herbicides again. With this information we have the dimension of the level of knowledge that technicians should have of the plots and crop areas that already had weed species resistant to herbicides over time, that is, it is very important that they have the history of the areas, a situation that is aggravated in cases of multiple resistance.

Figure 4 - Control levels of dairy populations resistant to ALS inhibitor herbicides after herbicide application: A) control overview, B) control with Protox inhibitors (fomesafen), C) with EPSPS inhibitors (glyphosate), D ) with ALS inhibitors (imazethapyr). Photo: Elouize Xavier

Final considerations

The existence of favorable biological characteristics, high competitive capacity and high genetic variability has increased the impact of dairy farming on crops and has also worsened the difficulty in controlling this weed species. Research, technical assistance and farmers must prepare to face the problem, which presents an increasing degree of complexity and, consequently, requires greater complexity in the solutions presented. It is important to highlight that basic principles of weed management are essential to reduce the negative impacts of this species and other weeds. Highlights include the integration of management methods (preventive, cultural, chemical), with the aim of reducing weed populations in the areas; crop rotation and herbicide action mechanisms; the control of weeds in early development stages and in periods when these plants have less impact on crops; not using underdoses of herbicides and the use of appropriate application technology, among others.

This article was published in issue 183 of Cultivar Grandes Culturas magazine. Click here to read the edition.