The "drought" phenomenon from an agronomic perspective

“Drought” is a complex phenomenon resulting from the interaction of several factors. It includes more than a simple comparison between rainfall and historical rainfall averages. Its developments and consequences involve a wide range of aspects of a technical, economic, social and environmental nature, which must be considered in a systemic way. Nonetheless...

Every year, the theme is remembered. In general, this occurs when the phenomenon has already established itself and the drops in production are irreversible, especially in summer crops. On these occasions, due to a lack of planning and preventive measures, two actions are normally taken. One of them is to do nothing and wait for the final results, which can be disastrous for the producer, depending on the intensity of the phenomenon. Another attitude is to try solutions to face the problem. “That’s where the danger lies”, as they say. In general, simplistic and easy-to-apply solutions are sought, but devoid of responsibilities. These reactions are understandable because they are typical of human nature, but they obscure the capacity for discernment, even of technicians who deal with the problem, who have the information, but (often) lack the knowledge and conditions necessary for its application.

To avoid falling into the trap of discussing “drought” in a simplistic way and exposing false hope to farmers (ultimately, those most affected), it is necessary to consider some basic aspects. First of all, it is necessary to analyze the relationship between the conditions that the environment offers and the water needs of crops, especially spring-summer crops. On the one hand, crops vary their water consumption throughout the cycle, as this demand increases as the leaf area grows. On the other hand, each crop has its critical periods in relation to lack of water, during which water deficiency causes intense and irreversible losses in grain production. Research reveals that the critical period for corn is only around 10 days. In other words, a rain of 40 mm can make the difference between harvesting and not harvesting grains. From a climatic point of view, the water requirement of large spring-summer crops is greater than the rain that occurs during most of the season. In other words, in summer, climatic average rainfall is lower than the amount of water that plants need. Furthermore, there is great spatial and temporal variability in summer rainfall in Rio Grande do Sul. This is the normal weather and climate condition. And, therefore, when meteorology predicts rain in normal conditions, it means that there will be a lack of rain for summer crops in the State.

How does drought establish itself in plants? When is it harmful to crops?

It is simple to understand that plants' water demand increases with the growth of the leaf area, due to greater transpiration. On the other hand, an increase in leaf area implies greater photosynthesis, especially if there is high availability of solar radiation. But, in general, the maximum leaf area also coincides with the most critical period for lack of water, which goes from flowering to the beginning of grain filling, in most crops that produce grains. Therefore, if the maximum leaf area occurs during the period of greatest availability of solar radiation, transpiration will also be the maximum possible, that is, there will be a greater demand for water precisely in the critical period for grain production. Therefore, crops are doubly susceptible to droughts at the beginning of the reproductive period, because they have maximum water consumption (transpiration) and require optimal water conditions to produce the yield components.

In the countryside, the water deficit increases during times of greatest atmospheric evaporative demand, as the incidence of solar radiation and air temperature increase. Excessive loss of water through the leaves causes the plant to close its stomata to prevent complete drying of the tissues. Therefore, it needs to absorb water from the soil to recover the ideal water condition, open the stomata again and allow the entry of CO2 for photosynthesis. The drought condition intensifies when the plant is unable to absorb from the soil the amount of water necessary to replace losses, even at night. In this condition, it goes into stress, with the potential to cause significant damage to grain yield.

Another aspect that must be considered refers to the management of the soil-culture system. The water retention capacity and availability varies in different soils, depending on their texture and, mainly, their structure. Improving soil structure is closely associated with organic matter content. This is why conservation systems, such as the direct planting system, are important. On the other hand, the direct planting system favors the concentration of nutrients in the surface layer of the soil. Furthermore, if the system is poorly implemented or managed, it can encourage the emergence of compacted layers just below the surface. Thus, if there is a concentration of nutrients close to the soil surface and the presence of impediment layers below, root development may be limited to the first 5 to 7 cm deep. Under these conditions, even during short droughts, plants enter water stress with the risk of significant losses in grain yield if they are in a critical period. Therefore, fertilization strategies only on the surface or in the sowing line at a depth of less than 10 cm must be carefully planned so as not to aggravate the problem of short-term droughts or more intense droughts.

The extent of damage caused by droughts and/or droughts also depends on other aspects of crop management. Inadequate sowing times, outside the agricultural zone, are a factor in increasing the risk of productivity declines for most agricultural crops, especially if there is drought during the critical period. When choosing cultivars, it is important to explore genetic diversity in relation to tolerance to water deficit, as well as adjusting sowing density to water conditions and soil management. The production model adopted on the property is also important in this aspect. Lack of crop rotation, use of cultivars from a single cycle (all early, for example), sowing times concentrated in a few days are high-risk strategies. Even though the droughts are short, the lack of concentrated water during the critical period can seriously compromise agricultural activity. A production system based on diversity of crops and genotypes, scheduling of sowing times and diversity of activities is much safer to reduce direct and indirect risks of adverse phenomena such as drought and drought.

“Drought” is a complex phenomenon and should not be treated in a simplistic way. It is a problem that requires serious and knowledgeable treatment by the agronomic segment. Actions to live with short droughts and drought can be effective if they are properly planned, not immediate. Initially, the diversity of crops, the diversification of cultivars with at least two cycle lengths and the diversity of sowing times in the same harvest are essential in the same harvest. These strategies, combined with others, require efforts from numerous sectors that operate in agribusiness, such as genetic improvement, soil management, management of agricultural crops, planning and monitoring of production systems, storage and marketing of products, management of water resources and irrigation, environmental management, among others. The search for knowledge must be an indispensable premise in the evaluation and formulation of measures to control and mitigate the effects of droughts, as the dynamics of the processes are intense and diverse on a space-time scale, which demands a systemic and continuous evaluation.

Genei Antonio Dalmago

Embrapa Wheat researchers -

Homero Bergamaschi

Professor at the Faculty of Agronomy at UFRGS