What are the consequences of La Niña for the soybean harvest in Mato Grosso do Sul?

The American scientific agency National Oceanic and Atmospheric Administration (NOAA), by monitoring the sea surface temperature (SST) of the Equatorial Pacific Ocean, in the region called Niño 3.4 (5ºN-5ºS, 120º-170ºW), highlighted the trend formation of the La Niña phenomenon (The girl, in Spanish). This phenomenon is nothing more than a cooling beyond normal of these waters, therefore differing from its “little brother”, El Niño (The boy, also in Spanish). This, in turn, consists of the opposite phenomenon, the warming of the waters.

Most of planet Earth is occupied by oceans and they interact significantly with the atmosphere that surrounds the entire Earth's surface. This interaction is quite strong, complex and difficult to be explained precisely by man, even with all the advances already achieved by science.

In practice, the oceans have a strong influence on the climate we experience in our daily lives. In South America, the El Niño and La Niña phenomena are considered the main ones capable of causing climatic conditions to diverge from those considered normal. When they occur, different regions may experience different temperature and rainfall conditions, which may be above or below normal and even within normal limits.

Current scenario and recent history of below-normal TSM

The quarterly monitoring of SST in the Niño 3.4 region indicates that in the last quarter (Jun-Jul-Aug) the measured temperature was 0,4 ºC below normal, highlighting a cooling process that has been taking place since the quarter Jan-Feb-Mar, when it was 0,6 ºC above normal. The drop, therefore, has already been 1 ºC and, according to NOAA, it is expected to continue, culminating in the formation of La Niña. It is worth noting that a La Niña period is only considered to occur when the SST is at least 0,5 ºC below normal for a period of seven consecutive months (equivalent to five quarters in the NOAA assessment methodology).

Since 1950, NOAA data shows that the quarter Jun-Jul-Aug, the one that precedes soybean sowing in Mato Grosso do Sul, had SST below normal in 32 of the 71 years (45%). In an even more recent period, since 2004, this occurred in eight of the 17 years (47%), with 2020 being the 8th year (Figure 1). From there it can be seen that this is a relatively frequent condition, and not an exception.

The most likely sowing dates in years similar to 2020 were identified between 21/09 and 16/10, which is considered a normal period without delays. This implied defining the critical periods as being between 11/11 and 11/02, depending on the crop and when it was sown. The 2008/2009 and 2011/2012 harvests were the only ones that had a total rainfall volume in the complete cycle of less than 550 mm. In these two harvests, precipitation was concentrated within 22 to 24 days. On the other hand, in the others, the rain was distributed between 26 and 33 days. Bearing in mind that the lack of water in the phase of greatest sensitivity is responsible for decisively penalizing soybean productivity, it is convenient to put a magnifying glass on this analysis, focusing on the supply of rain in the critical period. 

Thus, it is noted that the same harvests of 2008/2009 and 2011/2012 had the lowest rainfall in the critical period, with 211 mm and 282 mm, respectively. In terms of rainy days, these years were also very low, even if they were not the lowest (11 and 12 days, respectively). The 2005/2006 harvest was the shortest, with ten days, even though it was the second rainiest harvest in the critical phase with 425 mm, showing the concentration of rain in a few days. Checking productivity, the 2008/2009 and 2011/2012 harvests were those with the lowest productivity, followed precisely by the 2005/2006 harvest. These three crops had lower productivity than the municipality's average in the period between 2004 and 2018, which is equal to 2660 kg ha-1. Differently, all other years had higher than average productivity, varying between 2700 kg ha-1 and 3470 kg ha-1.  

Effect of below-normal SST on soybean crops

Climatic data measured at the Embrapa Agropecuária Oeste agrometeorological station in Dourados/MS and average soybean productivity data in the municipality provided by the Brazilian Institute of Geography and Statistics (IBGE), from 2004 to 2018, were used to highlight the possible effects of low SST over soybean crops in Mato Grosso do Sul (Table 1).

For the analysis, it was considered that 45 mm of rain were needed to make sowing viable after the long and typical winter dry period. Thus, considering the climatic data, the most likely sowing times were defined for each crop. Considering a cycle of 132 days between soybean sowing and harvesting, and that the phase of greatest sensitivity to water deficiency occurs between the 52nd and 118th day after sowing, the rainfall supply in this critical period in each harvest was evaluated. and its effects on soybean productivity.

Table 1. Analysis of crops with sea surface temperatures in the Niño region 3.4 below normal since 2004 and their effects on soybeans cultivated in Dourados/MS.

crop	Without	PC	ChuvaCC	DCC	ChuvaPC	DCPC	Productivity
2005/2006	03/10	23 / 11 the 28 / 01	794	28	425	10	2280
2007/2008	10/10	30 / 11 the 05 / 02	732	32	354	15	2700
2008/2009	02/10	22 / 11 the 28 / 01	455	22	211	11	1920
2010/2011	21/09	11 / 11 the 17 / 01	864	33	428	18	3300
2011/2012	21/09	11 / 11 the 17 / 01	533	24	282	12	2000
2013/2014	26/09	16 / 11 the 22 / 01	671	26	384	12	2880
2016/2017	16/10	06 / 12 the 11 / 02	597	29	382	21	3470

crop

Without

PC

ChuvaCC

DCC

ChuvaPC

DCPC

Productivity

2005/2006

03/10

23 / 11 the 28 / 01

794

28

425

10

2280

2007/2008

10/10

30 / 11 the 05 / 02

732

32

354

15

2700

2008/2009

02/10

22 / 11 the 28 / 01

455

22

211

11

1920

2010/2011

21/09

11 / 11 the 17 / 01

864

33

428

18

3300

2011/2012

21/09

11 / 11 the 17 / 01

533

24

282

12

2000

2013/2014

26/09

16 / 11 the 22 / 01

671

26

384

12

2880

2016/2017

16/10

06 / 12 the 11 / 02

597

29

382

21

3470

Note: No, Probable sowing date; PC, critical period of water need; RainCC, rainfall in the complete cycle (mm); DCCC, number of days with rain above 5 mm in the complete cycle; RainPC, rain in the critical period (mm); DCCC, number of days with rain above 5 mm in the critical period; Productivity, average productivity of areas cultivated with soybeans in the municipality of Dourados/MS (kg ha-1).

The most likely sowing dates in years similar to 2020 were identified between 21/09 and 16/10, which is considered a normal period without delays. This implied defining the critical periods as being between 11/11 and 11/02, depending on the crop and when it was sown.

The 2008/2009 and 2011/2012 harvests were the only ones that had a total rainfall volume in the complete cycle of less than 550 mm. In these two harvests, precipitation was concentrated within 22 to 24 days. On the other hand, in the others, the rain was distributed between 26 and 33 days.

Bearing in mind that the lack of water in the phase of greatest sensitivity is responsible for decisively penalizing soybean productivity, it is convenient to put a magnifying glass on this analysis, focusing on the supply of rain in the critical period. Thus, it is noted that the same harvests of 2008/2009 and 2011/2012 had the lowest rainfall in the critical period, with 211 mm and 282 mm, respectively. In terms of rainy days, these years were also very low, even if they were not the lowest (11 and 12 days, respectively). The 2005/2006 harvest was the shortest, with ten days, even though it was the second rainiest harvest in the critical phase with 425 mm, showing the concentration of rain in a few days.

Checking productivity, the 2008/2009 and 2011/2012 harvests were those with the lowest productivity, followed precisely by the 2005/2006 harvest. These three crops had lower productivity than the municipality's average in the period between 2004 and 2018, which is equal to 2660 kg ha-1. Differently, all other years had higher than average productivity, varying between 2700 kg ha-1 and 3470 kg ha-1.

Conclusions about the possible impacts on the 2020/2021 soybean harvest in Mato Grosso do Sul

 
From what was exposed and analyzed, it was concluded that:
 
1) Although the waters are colder than normal, we are not yet in La Niña, but rather in the process of forming this phenomenon;
2) The volume of rainfall and also the number of rainy days tends to be lower and cause significant frustration for soybean farmers only in years in which the waters already come from a long cooling process prior to the Jun-Jul-Aug quarter, characterizing La Niña , as occurred in the years 2008/2009 and 2011/2012. This is not the case this year 2020;
3) In years similar to what is occurring in 2020, the volume of rainfall and its distribution throughout the cycle and, above all, in the phase of greatest sensitivity tends to be satisfactory, to the point of enabling productivity in most years above average ;
4) In harvests like the one we are starting in Mato Grosso do Sul, their success seems to be conditioned by the distribution of rainfall in the critical period, even more than their volume, the critical period being that between November 11th and 11th /February. This should be the key to success for this 2020/2021 harvest!