Energy cane: tolerant or susceptible to herbicides?

Energy cane (Figures 1 and 2) is the result of crossing commercial sugar cane hybrids with the species Saccharum spontaneum, giving rise to cultivars with greater tillering and hardiness, thinner stalks, low sugar content, high fiber content and productive potential. Commercially, it constitutes the raw material for the production of second-generation ethanol (2G) or cogeneration of electrical energy. The final product (ethanol or electrical energy) is a source of renewable energy that contributes to the national energy matrix.

In this sense, sugarcane and energy cane cultivation in Brazil minimizes the environmental problems caused by non-renewable energy sources, generally derived from petroleum. In this scenario, energy cane is one of the main alternatives for the sugarcane sector in generating clean energy using biomass, as it reduces CO2 emissions during its life cycle. The production of energy cane has increasingly contributed as another raw material option for the generation of electricity and ethanol production.

There are few energy cane cultivar options on the market, with Vertix Type 1 and Vertix Type 2 being the first materials available on the market. According to Granbio (2018), these materials have a productivity 1,5 to 2 times higher than sugar cane. Therefore, while a sugar cane field has a productivity potential of 100 t ha-1, energy cane has a potential of between 150 and 200 t ha-1.

In addition to productivity, there are other characteristics to be considered, such as greater fiber production in the stalks. Energy cane materials produce up to 25% of the weight of the stalks in fiber, compared to 15% for sugar cane. They also have greater resistance to diseases, greater longevity of ratoons (8 to 10 years) and adapt to production environments that are less favorable to crops (restrictive environments). Even though they are grown in soil and climate environments restricted to agricultural production (called D and E, according to Prado, 2018), energy cane materials receive the same management applied to conventional sugar cane.

Regarding weed control, the diversity of flora is diverse because unfavorable (restrictive) environments have not received management that could minimize the seed bank in the soil over the years. As a result, the coexistence of crops and weeds is intense and when chemical management with herbicides is applied, the molecules may have their dynamics hindered in the soil (because it is unfavorable to agriculture), resulting in inadequate selectivity to the crop and deficient control. It is worth noting that the coexistence of weeds and sugarcane can result in a reduction in productivity of up to 85% (Graciano & Ramalho, 1985).

In Brazil, field observations (visual) found that energy cane cultivars are sensitive to herbicides when applied in the same way as conventional sugar cane, but technical-scientific studies are needed to prove this information. 

 Selectivity is a prerequisite for the use of herbicides, i.e., it is essential for the success of chemical weed control. Selectivity can be defined as the ability of a herbicide to control weeds without affecting the plants of interest, i.e., without reducing the productivity of the crop, in this case energy sugarcane. 

Cultivars can be considered susceptible or tolerant to the same herbicide molecule, with susceptible cultivars being more likely to be poisoned by herbicides and tolerant cultivars being less harmed by herbicides. However, it is also possible for herbicides to negatively impact crop productivity without necessarily presenting visually detectable symptoms (“phytohides”), in the same way that other products can cause severe injuries without significantly impacting productivity. Therefore, in experiments with herbicide selectivity, it is recommended to evaluate both the phytotechnical parameters and physiological changes in the plant. Thus, as with conventional sugarcane, energy cane requires studies that prove whether or not these plants are sensitive to herbicides.

Regarding selectivity, in trials conducted in recent years at the IAC Sugarcane Center in Ribeirão Preto, SP, the Vertix 2 cultivar and an energy cane clone showed mild symptoms of poisoning when compared to the control. The values ​​assigned were very close to zero because the symptoms were barely visible. The symptoms were given scores of a maximum of 11% on a scale with a maximum value of 100% (plant death), as can be seen in Figure 4. In the field, the symptoms are mild and practically imperceptible, particularly in plants 60 days after application (DAA). Producers would certainly call it “phyto zero” (Figure 4). Coincidentally, the few symptoms observed were whitening of the leaf blade, typical of the mechanism of action of isoxaflutole (carotenoid inhibitor), which is easily confused with the symptoms of leaf scald (Figure 3). 

In most cases, visual symptoms evolve within a maximum of 90 DAA and without compromising other phytotechnical variables, such as plant height and stand (Figure 4). However, if the sugarcane or energy cane cultivar is susceptible to herbicides, phytotechnical losses will certainly also be observed.  

The cultivar and clone treated with imazapic + isoxaflutole had reduced height compared to the control (Figure 4), although the visual symptoms of intoxication were mild (11% score). It was also observed that the clone was more susceptible than the cultivar Vertix 2 because the difference between the height of the plants in the herbicide treatment and the control was greater for the clone (Figure 4). 

Another damage caused by herbicides was to the plant stand (Figure 4), even though the symptoms of poisoning were almost imperceptible visually (Figure 3). Energy cane has high tillering and in field research, approximately 100 tillers are normally observed for each meter of furrow, compared to 15 to 18 for conventional sugar cane. The herbicide treatment reduced the stalk stand (between 10 and 20) compared to the control, at 120 DAA (Figure 4). 

Since the height and stand of the energy cane cultivar and clone were reduced by herbicide treatment, even with symptoms of mild intoxication (no more than 11%), it can be estimated that both materials are susceptible to herbicides. However, this statement can only be confirmed once the research is completed and productivity is obtained. However, so far, the field observation that energy cane is susceptible to herbicides appears to be true, when observing the height and stand data obtained in the third part of the crop cycle (first 4 months) (Figure 4).

In the field, weeds will be present, unlike the experiment that was maintained by weeding, since the aim was to evaluate only the effect of the herbicide on the crop. In commercial cultivation, possibly in areas with little agricultural affinity, herbicides will have unfavorable soil dynamics. 

Many molecules can more easily ionize and/or move in the topsoil. This will result in lower weed control efficiency, which will lead producers to apply more herbicides. Consequently, materials that are already susceptible to herbicides may become even more toxic.  

When cultivating energy cane, it is important that producers be cautious when using herbicides, preferably applying them before planting the crop to minimize the seed bank. Consequently, the intensity of weed infestation after planting will be lower. When using this technique, a single application of herbicide in each energy cane cycle (which can last between 8 and 10 years) may be sufficient to contain the invasive plants.  

Greater success in weed control can be achieved if the producer applies herbicides considering the observed weed flora, respecting the physical and chemical properties of the herbicide molecules according to the time of application (rainy or dry season) and the dose recommended by the manufacturer according to the type of soil. What should be avoided is applying herbicides with medium and high water solubility during very rainy periods and herbicides with the capacity to be photodegraded and volatilized during dry periods. Molecules with such characteristics exposed to light and temperature conditions will dissipate into the environment before they even begin to work in the soil.

In other research also developed by the matology team at the IAC Sugarcane Center, it was observed that energy cane expressed stress to herbicides differently than sugar cane. In both crops, symptoms of intoxication, reduction in height and stand were observed, but the energy cane stressed by herbicides flowered faster. 

In two experiments, under sugarcane-plant conditions, the energy cane from all herbicide treatments flowered earlier than the energy cane from the control treatment. The plants from the treatments emitted the panicle up to 30 days earlier than the control plants. This is not a typical behavior in sugarcane. 

Flowering in sugarcane is negative in production systems because it “drains” sucrose for the formation and emission of panicles, which leaves the stalks with a higher fiber content. Certainly, sucrose consumption and the increase in fiber content are also similar in energy cane, but these characteristics in the energy cane stalk may be less harmful than in sugar cane.

Energy sugarcane is the raw material for the production of 2G ethanol, which is obtained from specific enzymes that use the cellulose in the stalks as a substrate to produce ethanol. If the stalks had more fiber, the industrial loss might not be as significant.

There is much to know about the physiology of energy cane, particularly about the effects of herbicides on its development.

*Per Matthew A. e Carlos Alberto Mathias Azania, from the Advanced Sugarcane Research Center/IAC