“In addition to competing for water, light, nutrients and space, weeds can disrupt crop practices”
Ana Ligia Giraldeli is a professor at the State University of Londrina, agronomist and M.Sc in agriculture and environment from the Federal University of São Carlos, Ph.D in phytotechnics and specialist in agribusiness at the Luiz de Queiroz College of Agriculture – USP.
AgriBrasilis – What are the main invasive species in sugarcane plantations?
Ana Giraldeli – In the sugarcane crop, we can separate the main invasive species in three groups: the traditional ones with widespread occurrence; the traditional ones with regional occurrence; and those emerging with the system of production and mechanized harvesting of sugarcane.
In the traditional ones with widespread occurrence, we have carurus (Amaranthus spp.), signal grass (Brachiaria decumbens), marmalade grass (Brachiaria plantaginea), Bermuda grass (Cynodon dactylon), coco-grass (Cyperus rotundus), crabgrass (Digitaria horizontalis), Indian goosegrass (Eleusine indica), guanxumas (Sida santaremnensis), purslane (Portulaca oleracea) and Guinea grass (Panicum maximum).
In the traditional ones of regional occurrence, we have Johnson grass (Sorghum halepense), fedegoso (Cassia spectabilis) and itchgrass (Rottboellia cochinchinensis).
In emerging ones, we have black velvet bean (Mucuna spp.), castor bean (Ricinus communis), bitter-melon (Momordica charantia), corda-de-viola (Ipomea spp.), siratro (Macroptilium atropurpureum), loofah (Luffa aegyptiaca) and milkweed (Euphorbia heterophylla).
AgriBrasilis – The nutsedge (Cyperus rotundus) is among the most difficult plants to control in sugarcane crops. Why does this occur? What practices should be adopted against this pest?
Ana Giraldeli – The nutsedge (Cyperus rotundus) is among the ten most difficult weeds to control in the world. This happens because of its propagation form, that makes it difficult to control. This species has its reproduction through seeds, bulbs, tubers and rhizomes.
It is difficult to achieve control of nutsedge in just one harvest. For control, some important points must be observed: weeding, mowing and turning the soil favor the plant. Some herbicides for control are: sulfentrazone, diclosulam, imazapic, imazapyr, glyphosate, MSMA, halosulfuron and ethoxysulfuron.
It is also important to clean the machinery so that parts of the sedge plants are not taken to other areas.
AgriBrasilis – In addition to weed competition, what other problems can be caused by the permanence of weeds in the environment?
Ana Giraldeli – In addition to competing for water, light, nutrients and space, weeds can disrupt crop practices. Species such as “corda-de-viola” disturb the harvest because they end up causing the machine to clog, for example. Species that contain thorns make manual harvesting unfeasible.
Some species reduce the quality of the product, as in the case of weed seeds attached to the cotton fiber or even in the case of nutsedge tubers growing inside potato tubers. Some species are hosts of pests, diseases and nematodes. In addition to agricultural areas, weeds can also be a problem in non-agricultural areas, such as on highways, railway lines, lakes, rivers and under power lines.
AgriBrasilis – How is the chemical management against weeds carried out in the sugarcane culture?
Ana Giraldeli – The sugarcane production system has a wide range of conditions and factors related to soil, climate and crop management.
In relation to the soil, we must analyze the adsorption capacity of chemical molecules used in the soil, that is, organic matter contents and clay contents and quality for the use of pre-emerging herbicides. In general, the higher the clay and organic matter content of a soil, the higher the doses needed to control weeds.
Another issue is the humidity and temperature conditions. For times of higher temperatures and humidity, we will probably have a higher density of weeds in the area. The herbicide choice must consider the humidity and the period without rain. For this, some products can be used in times of drought. These are generally herbicides with higher water solubility and lower Kow (octanol-water partition coefficient).
It is also necessary to consider the layer of straw left on the soil, as herbicides with greater solubility in water and lower Kow will pass through the layer of straw easily and reach the soil.
AgriBrasilis – What are the differences between pre-emergence and post-emergence controls? Why?
Ana Giraldeli – In pre-emergence control, the target of the herbicide is the bank of soil propagules. The herbicide needs to reach the soil and reach the layer from 0 to 10 cm deep, where most of the weed seeds are concentrated.
Pre-emergent herbicides need moisture in the soil to activate and actually reach the soil solution and be taken up by the seeds in the process of germination and emergence.
Post-emergence herbicides are applied after crop and/or weed emergence. When applied in post-emergence of the crop, they need to be selective in order not to cause severe damage to the crop. The target in this case is the weed leaves. Therefore, it is important that there is no rain within a period of two to six hours after application (depending on the product).
In general, there is a recommendation for the use of adjuvants together with the herbicide in post-emergence, but this will also depend on the product, as some already come with adjuvants in the formulation.
AgriBrasilis – What are pre-sprouted sugarcane seedlings? What are the impacts of this planting system on weed management?
Ana Giraldeli – Pre-sprouted sugarcane seedlings are a multiplication technology that contributes to the fast production of seedlings, associating a high standard of plant health, vigor and planting uniformity, with a reduction in the number of seedlings that go to field.
In this system, as the seedling goes to the field once it is formed, the differential is that there is contact between the sugarcane root system and the soil layer treated with pre-emergent herbicide.