Planning can prevent frost losses in wheat

A winter forecast with mild temperatures and rainfall at the historical average, it can favor the cultivation of wheat in the South Region, which concentrates 90% of the cereal's production. But the scenario also requires sowing planning to avoid losses due to frost during periods of greatest crop risk.

The wheat sowing period in the South Region (southern PR, SC and RS) runs from May to July, considering regional differences in climate, soil and cultivar cycle. The dates are defined by the Agricultural Climate Risk Zoning (ZARC) with the aim of reducing the risk of crop losses caused by climatic factors. 

In wheat, different causes associated with the climate can affect yields, such as frost during flowering or heading, drought after sowing and during the grain filling phase, excess rain at harvest, in addition to the occurrence of pests and diseases. The subtropical climate of southern Brazil challenges the production of winter cereals as it is a region that is normally humid and subject to low temperatures in winter and early spring, with the formation of frosts, and, at the same time, it can be hit by temperatures high elevations and dry climate.

This year, the expectation is for a favorable climate for winter crops, with mild temperatures and rainfall at the historical average, above 100 mm/month. In this scenario, the risk may be associated with low temperatures, capable of causing frosts which, depending on the stage of the crop, can result in significant losses in wheat.

Frost losses

In general, frost occurs when the temperature drops to the negative range on the thermometer scale and can be observed mainly in lowlands in the countryside, where cold air is concentrated, or even on slopes facing south. However, depending on the intensity of the phenomenon, frost can occur everywhere, whether in native, cultivated areas or even in urban areas. 

The damage to wheat can be seen about a week after the occurrence of frost, when the plant actually shows burn damage to the leaves or damage to the ears, which easily come off the plants or appear whitish. The biggest losses occur during wheat heading and flowering, when, in addition to failures in graining, total death of the ear can occur. During the initial phase of wheat, even under intense frost, there may even be loss of some plants but, due to the compensation of tillers, the crop's yield is not compromised. 

Escape strategies

The main strategy for climate risk management is sowing planning, which must consider the risk of frost, especially during wheat ears, which is the most critical period. This guidance is followed by the team of researchers that develops the Agricultural Climate Risk Zoning (ZARC), indicating the wheat sowing periods (by ten days), for each municipality producing the cereal in Brazil. Based on historical climate series, modeling and risk simulation, ZARC allows you to identify sowing periods in which there is a lower chance of crop failure due to extreme weather events.

ZARC is updated every year, following improvements in the risk simulation system, the expansion of the database and the emergence of new production technologies. The application ZARC – Right Planting allows you to access the best date for planting in each municipality via cell phone. 

Adjusting to cultivars

However, the ZARC tool still does not reach microclimate characteristics within the same municipality. To help the producer with an even more assertive sowing, Embrapa Trigo researcher Ricardo Castro suggests defining the sowing date for wheat based on the average number of days of development of the cultivar (information provided by the breeder). The calculation mainly considers the sub-period until heading, known as the vegetative sub-period (leaves), which comprises the period from emergence to the heading of plants. 

In Passo Fundo, RS, the beginning of September is defined by ZARC as having a low risk for frost, although the occurrence of this phenomenon cannot be ruled out. Using an early cycle cultivar, such as BRS Reponte, an average of 133 days is expected from plant emergence to harvest. From sowing to harvesting it takes about 79 days. To define the most precise sowing date, the calculation must consider 79 days retroactive to the first week of September, which indicates the sowing of BRS Reponte wheat in Passo Fundo, RS, for approximately June 15th, with the lowest risk of frost losses. 

Ricardo Castro warns that this calculation may vary depending on temperature: “In warmer regions or years, wheat development will certainly accelerate, while in colder areas this period will be longer. Therefore, even making adjustments for each cultivar, the producer must always follow the agricultural zoning, based on studies that consider both the geographical indication and the history of the climate in the region”.

According to the researcher, it is also important to highlight that losses due to frost in wheat are subject to coverage by agricultural insurance and, certainly, crops that followed the indications of agricultural zoning, with staggering sowing and using cultivars from different cycles, will have fewer problems. with adverse weather conditions.