Hostile coexistence: invasion of camalote grass in sugarcane fields

Camalote grass (Rottboellia exaltata L.f.), originating in Asia, probably India, is widespread in more than 28 countries, infesting 18 crops or more, including sugar cane. The highest incidence is between 23^o N and 23^o S in latitude, but it also occurs in Australia and the United States of North America, where it reaches 75% to 100% of its growth potential. It is estimated that in Central America and the Caribbean it infests more than 3,5 million hectares. In sugarcane farming in Guatemala, it is considered one of the main challenges in weed management today. It is also considered an important weed in West Africa.

In Brazil, its infestation in agricultural crops began to be observed at the end of the 1950s, being introduced into the country, probably contaminating batches of rice seeds imported from Colombia. It currently infests the northern region most frequently, but with outbreaks in São Paulo and the Central-West Region. Due to its high aggressiveness, it causes a significant reduction in the production of industrialized sugarcane stalks and in the number of economically viable cuts from the sugarcane field.

It is considered one of the 12 worst weed species infesting sugarcane fields in the world, due to its high aggressiveness, which gives it competitive advantages. Infestations greater than 10 plants per square meter do not allow the full development of sugarcane plants, making it impossible to “close” between the rows.

The plants are erect in shape, with leaf sheaths densely covered with rigid bristles measuring 1,0m to 2,5m in height. Belonging to the botanical family Poaceae (grasses), it has an annual or perennial life cycle, depending on environmental conditions. It reproduces through seeds and can also be multiplied by pieces of stem, which have buds in their nodes. It presents high ecological adaptability, facilitating its dissemination, and a plant can emit up to 100 tillers and produce more than 16.000 seeds, estimating a production of up to 6.500 seeds per m^2.  This gives it the popular name of “walking” plant in some countries.

Seed dormancy varies depending on climatic and agronomic conditions, and can last for several years. At times of the year with lower average temperatures, the seeds enter a state of dormancy. Therefore, seeds positioned at greater depths in the soil profile remain viable in the environment for longer, resulting in future infestations. N

In a study on the viability of seeds buried at different depths, in areas cultivated with corn crops in the United States of North America, it was found that 40% to 60% of the seeds remain viable for a year, being viable even at 45 cm deep. This aspect can influence chemical control, since herbicides are normally activated in the 10cm layer of the soil profile.

In Costa Rica it was found that under field conditions, after 18 months 43% of the seeds positioned on the soil surface germinate; At a depth of 5 cm, 49,33% was observed, and at 20cm, germination was 59,33%. Therefore, it was found that greater depths favor the germination of camalote grass.

However, different results obtained in other research studies demonstrate that seeds can remain dormant in the soil for up to four years. Likewise, there was less germination at greater burial depths, with seeds positioned on the surface or buried at 5cm and 10cm depth losing their longevity substantially, and at 20cm depth, less than 10% of the seeds remain viable. .

Another research study found that in sandy soil, the greatest emergence occurs at a depth of 5,0 cm. In clayey soil the emergence pattern is similar, however, there is a more pronounced reduction in emergence with the increase in sowing depth. The highest germination rate is observed in soil with a pH ranging from 6,8 to 7,0, with lower germination occurring in unamended soil, with a pH of 5,3. In a study of emergence through observation of the interaction of straw quantity on the soil surface and depth of seed burial, it was found that emergence (1, 3 and 5 cm depth) and straw densities (0 ton/ha and 8 ton/ha) did not influence emergence in a differential way.

Analyzing the literature on the biology of camalote grass, it can be seen that the plant prefers the same ecological conditions as sugar cane, such as: high temperature (27 ºC) and high humidity (> 1.500 mm of rain per year). In this way, the environmental conditions that provide better formation of sugarcane fields also provide better development of camalote grass, resulting in greater germination of this weed, and consequently greater competition with sugarcane crops. Therefore, the coexistence of camalote grass with sugar cane causes significant losses in yield.

Camalote grass is a C4 cycle plant, therefore, it has high photosynthetic efficiency, so this weed plant has a series of characteristics that give it advantages when colonizing certain areas, especially in hot and humid environments, among these characteristics we can mention the low CO compensation point₂, high rate of photosynthesis when in an environment with high light availability and low photorespiration rate. Due to these characteristics, camalote grass has vigorous vegetative growth and a rapid reproductive cycle.

The fact that camalote grass performs photosynthesis through the C4 cycle favors the presence of the species in sugarcane production areas. This crop also has a C4 photosynthetic cycle, which makes it a highly competitive species with the weed community. Therefore, the weeds most capable of competing with sugarcane, in general, have high photochemical efficiency and rapid space occupation, as is the case with several sedge and grass species, including camalote grass.

Due to this high competitive ability, dispersion in the various Brazilian agricultural regions is also becoming a growing problem for the country's agriculture, especially when the species is infested in less competitive crops, with a photosynthetic cycle of the C3 type, such as soybean and beans, for example.

Its aggressiveness makes it feared among farmers around the world.  When present in agricultural areas, camalote grass directly interferes through competition for water, light, nutrients and space. In a study on the growth and development of camalote grass¹⁵ it was observed that throughout the development of the research the plants Rottboellia exaltata they remained quite vigorous and showed rapid initial growth. At 28 days after sowing (DAS) they were already in the tillering stage, remaining under full vegetative development until 45 DAS, when the reproductive phase began with the definition of the phenological stage of rubbering/pre-flowering. Flowering occurred at 49 DAS with the emission of the first floral racemes; the plants remained flowering, through the continuous emission of new racemes by the main stem and later by the tillers, until the end of the experiment.

In the same research it was also found that there is a large accumulation of fresh mass, thus demonstrating a high capacity to produce phytomass. At 49 DAS, at the time of flowering, the plants had, on average, values ​​close to 26,5 g of total fresh mass, with approximately 20 g corresponding to the share of the fresh mass of the aerial part and the remaining 6,5 g corresponding to the participation of fresh root mass. These data are relevant, as they demonstrate that this plant presents rapid initial development linked to the ability to exploit a considerable volume of soil. These data are important, as plants that germinate first in an agricultural environment and colonize in a pioneering way the space destined for production agricultural crops will have advantages in relation to competition with crops of agronomic interest, such as sugar cane. 

The total accumulation of macronutrients increases throughout the development cycle of the Camalote grass plant, and the period in which the greatest accumulation of nutrients is observed is before flowering, thus demonstrating that this weed species has a high capacity to extract resources of the agricultural environment, which can result in severe weed competition with crops of agronomic interest.

In sugarcane regions, camalote grass stands out for causing significant productivity losses. Studies demonstrate reductions in productivity in sugarcane fields of up to 80% for ratoon sugarcane areas, and up to 100% in sugarcane plants. The interference caused by the presence of weeds in sugarcane cultivation can reduce the quantity of stalks harvested or even the number of economically viable cuts. In addition to direct damage, camalote grass can also cause disorders related to cultural practices., as workers often refuse to enter sugarcane plots with a high infestation of camalote grass, as this plant has simple, multicellular siliceous trichomes (joçal), which, in contact with the skin, penetrate like sugar needles. glass, break and cause allergic reactions.

 Its dispersion becomes extremely worrying for sugar cane producers, in areas not yet infested, since the plants could adapt to new conditions, causing a reduction in productivity, mainly because this species is difficult to control. In this sense, there are already reports of the adaptation of camalote grass biotypes to the most varied Brazilian environmental conditions, which can result, on some occasions, in small morphological changes in the plant's structure. These occurrences are related to the intense polymorphism of the species, correlated with the existence of polyploidy. The practical consequence is greater aggressiveness of camalote grass infestations in crops of agronomic interest and in sugarcane fields due to the adaptation of this weed to production conditions, which can result in the worsening of the problem.

The competition between populations of camalote grass and sugarcane cultivation is something that has been gaining prominence, mainly due to the rapid dissemination and aggressiveness that this plant represents. Management is costly, due to the need to use up to six herbicide applications during the crop cycle. This situation contributes to camalote grass becoming a plant that is difficult to control, as there are not many products recommended for its management and most of the herbicides that control this plant can cause toxicity to the sugar cane crop. Another pertinent aspect is the fact that camalote grass can germinate all year round (with a maximum peak in September), meaning that several applications are needed to control it.

The management of camalote grass must be carried out based on knowledge of the growth and development of the species, that is, control measures must be administered in a period prior to approximately 40 days of plant development, as from this point onwards, it can flowering occurs with rapid seed dispersal to the soil. Likewise, rapid initial growth requires management measures to be carried out on young plants so that the best results are obtained, since plant development makes control difficult; and also so that competition between the weed and sugar cane does not occur or even does not have a significant impact on production.

Therefore, it is noted that camalote grass is a plant with several aggressive characteristics, rapid initial development and high potential for dissemination, which contributes to it becoming increasingly recurrent in areas cultivated with sugar cane and other crops. of agronomic interest. Therefore, knowing the aspects related to the biology of this plant makes it possible to prevent its spread, in addition to contributing to the development of control strategies. The urgent development of new technologies, especially herbicides, is necessary, otherwise the relative importance of this weed will be great and sugarcane farming will have a great challenge ahead.

 The article appears in issue 189 of Cultivar Grandes Culturas.