WATER DEFICIENCY ON THE MAIZE GRAIN YIELD DECREASE IN THE CATARINENSE FAR WEST

Maize is of fundamental social economical importance for familiar agriculture in the west of the State of Santa Catarina. In this region, the decrease of grain yield of the culture is frequently related to periods of water deficiency. The purposes of the present study were to assess the maize grain yield decrease due to water deficiency, identify the sowing time with a lower risk of water deficiency and determine and probability of risks of grain yield decrease. The grain yield decrease was calculated using the Isareg model, based on the ten-day water balance, for the period between1989/90 to 2010/11, simulated for nineteen sowing dates and three cycles of maize maturation. The model estimated an average grain yield decrease of 25.1%. The biggest decrease took place in the initial sowing of August and the smallest in the late sowings between January and February, with a maximum variation of 3.9% between the assessed maturation cycles. The Isareg model is appropriate to estimate the grain yield decrease, however it overestimates its value when the relative evapotranspiration is above 0.8. The levels of 40% of grain yield decrease present 10.5% of probability of occurrence.

Maize is the main component of the productive chain of swine and poultry in Santa Catarina.The west of the State of Santa Catarina, characterized by familiar agriculture, presented in the harvest of 2012/2013 the third highest area planted with maize (59.1 thousand ha) in the state, with an average grain yield of 6.626 kg ha -1 (Epagri, 2013).However, this region is the most affected by droughts, with the occurrence of water deficiency mainly during three months of the year (January, July and December), usually associated to the La Niña phenomenon, causing considerable variations in the maize production (Epagri, 2013).
The water availability is considered one of the most important factors to express the potential of the maize culture production in the South of Brazil (Massignam, 2005;Wagner et al., 2013).The impact resulting from the water deficiency in the maize culture provokes its grain yield decrease, which has as an important variable the relative evapotranspiration defined when the real growth evapotranspiration (ETr) is smaller than the culture evapotranspiration (ETc), determining the risk of grain yield decrease of the culture (Nied et al., 2005;Cancela et al., 2006).
The estimate of grain yield decrease in the maize culture is important for planning and decisionmarking from the farms to state level.One way to measure these losses due to water deficiency has been through computer simulation (Massignam, 2005).
Isareg is a software of simulation model of hydric balance, easily parameterized, which demands the essential soil hydraulic characterization, weather data and basic parameters of the culture, besides adopting simplified functions from the relation water-yield to assess the effects of water deficiency in the grain yield decreases (Teixeira & Pereira, 1992;Cancela et al., 2006).Several studies (Victoria et al., 2005;Popova & Pereira, 2011;Chaterlán et al., 2012;Saraiva et al., 2013)

Material and Methods
The study was carried out in the west of the The Isareg model (Teixeira & Pereira, 1992) was used to calculate the water balance, in a tenday scale, and estimate the grain yield decrease in the maize culture.1), "p" factor "p" of water availability in the soil, root system depth, crop coefficient, total of rainfall and reference evapotranspiration calculated by the Penman-Monteith/FAO methodology followed by Araújo et al. (2007)  Zoning (Brasil, 2012) availability factor in the soil (p) are described in Doorenbos and Kassam (1979).
The identification of sowing period less subject to water deficiencies harmful to the maize was assessed through estimate of grain yield decrease, according to Stewart's monophase model modified by Doorenbos and Kassam (1979) expressed by a linear relationship between the production deficit and the evapotranspiration deficit through the equation: (

Results and Discussion
The analysis between the grain yield decrease estimated based on data from the IBGE and the mean ratio ETr ETc -1 (Figure 1) indicates that 83.3% of the relative evapotranspiration values (ETr ETc -1 ) were highest than 0.7 and qualified as low risk ones, while the other results present ratings between 0.6 and 0.7 considered as medium risk, according to the classification proposed by Matzenauer et al. (2002).
The low risk of severe water deficiency able to cause high grain yield decreases was also noticed, considering as limit the critical value lower than 0.6 for the ratio ETr Eto -1 admitted by Leivas et al. (2006).An agreement of these results was observed in relation to sowing time recommended by the rules of agricultural zoning of climate risk for the maize culture in the Santa Catarina State, rating as eligible the counties which presented in at least 20% of their territory a relative evapotranspiration (ETr ETc -1 ) highest or similar to 0.55 with a frequency of 80% in the assessed years and a risk of occurrence of frost similar to 20% (Brasil, 2012).However, there was a small variation of the culture grain yield decrease between the sowing time and between the maturation cycles (average of up to 3.9%).is a highest probability that the events concerning the culture critical period and lower rainfall occurring together, which damages the pollination and reduces the number of grains by ear.
Small variations in the grain yield decrease between the three maturation cycles, of about 1%, were observed for the maize sowings carried out until December 10 th (Figure 3).Differences of up to 10% of probability of highest water deficiency occurrence in the critical subperiod for three maize maturation groups were obtained by Nied et al. (2005)

Conclusions
The grain yield decrease due to water deficiency presents an average value of 25.1%, with small variation between the maize sowing time and the maturation cycles for the west of the State of Santa Catarina.The risk of grain yield decreases shows a downward trend from the initial sowing periods in relation to the simulated end periods.
However, there was a great variability of grain yield decreases between the years.The Isareg model was able to estimate the grain yield decrease, but it overestimates the decrease values when the relative evapotranspiration is highest than 0.8.
For the maize cultivations in the spring/summer and summer/fall there is a 50% probability of the culture grain yield to be reduced in 25% in relation to the grain yield potential of the west of the State of Santa Catarina.
used the Isareg model due to a lower requirement of input data concerning the soil and agronomic parameters, the employment of simple procedures to estimate the grain yield decreases, besides presenting good results for different cultures and soil and climate conditions.Considering the soil and climate characteristics in the west of the State of Santa Catarinathe present work aims to assess the impact of water deficiency in the maize grain yield decrease for different maturation cycles (early, normal and late), identify the sowing time with the lowest risk of maize grain yield decrease because of water deficiency, test the Isareg model for the estimate of grain yield decrease and estimate the probability of grain yield decrease resulting from water deficiency.
State of Santa Catarina, Brazil, involving 21 counties belonging to the region of São Miguel do Oeste.This region presents two climate types: humid subtropical with hot summer (Cfa) and humid subtropical climate with mild summer (Cfb), according to Köppen classification.The annual average temperature in this region is 19.3 ºC, in which the average temperature in the coldest month is 14.2 ºC and in the hottest month it is 25.3 ºC, with a monthly average sunshine of 177 h.The annual average rainfall ranges from 1.430 to 2.280 mm, with an annual of rainy days between 118 and 146 days (Epagri, 2013).The meteorological variables with daily data concerning air temperature, relative humidity of the air, rainfall, sun radiation and wind speed were obtained in the representative historical series for the region, for the period of 25 years (1987 to 2011), from the São Miguel do Oeste station (26°47'05''S, 53°30'13''W and 700 m), belonging to EPAGRI/CIRAM.
and Sousa et al. (2010).Phenological phases of culture considered: sowing, beginning of vegetative growth, flowering start, beginning of fruit formation, physiological maturation onset and harvest.The simulations were carried out individually for each year of the 22 years of the historical series considered integrated with each of the 19 sowing dates, for each of the 3 groups of maize physiological maturation (in days until harvest): early group (110 days), normal group (130 days) late group (150 days).The edaphic parameters were obtained by collecting 12 soil samples in the de 0-0.10, 0.10-0.20,0.20-0.40 and 0.40-0.60m layers, according to the methodology used by Kaufmann et al. (2014).Only the soil predominant classes in the region were considered.Were considered basically Cambisol soils, in which the selection of soils collection places was based on information of the Epagri Rural Extension technicians, who identified farms with the predominant soil characteristics and direct sowing system from the region.To apply the Isareg model, concerning agronomic parameters, 19 maize sowing time were considered for the early, normal and late maturation cycles, apart every 10 days, from August 12 th to February 08th, a recommendation period from the Ministry of Agriculture Climate Risk Agricultural

1 )
In which: Y a is the real culture grain yield obtained from the official data set of maize agricultural production according to IBGE(IBGE,   2012), Y m is its maximum grain yield, ETr is the culture real evapotranspiration, ETc is the culture evapotranspiration and ky is the culture response factor to the water deficiency, also called water sensitivity coefficient.The maximum grain yield of maize (Y m ) was adjusted so that the simulations with the Isareg model determined that during the whole crop cycle it would remain under the comfort of the water, with the supply of its evapotranspirative demand to seek the maximization of the grain yield potential of the crop.The relation ETr ETc -1 , defined as relative evapotranspiration, was obtained by using a ten-day water balance.The ky value was kept constant (1.15) for the whole culture cycle, according to the Isareg model parameterization.The grain yield decrease based on data from the Brazilian Institute for Geography and Statistics (IBGE) was calculated using the statistical analysis (Anova) with the use of spreadsheets.The probabilities of grain yield decrease were determined in the highest frequencies on the levels of 10, 25 and 40% of grain yield decrease, levels also studied byNied et al. (2005).The probabilities were calculated through the gamma function (γ), which enabled a better adjustment of the parameters for each of the 19 series of simulated sowing time.The adhesion of the probabilities for the estimated frequencies was checked using the Qui-square testin a 5% level of significance.
The non-linearity between the relative evapotranspiration and the grain yield decrease based on data from the IBGE characterized a limitation of the Isareg model, which overestimated the grain yield decrease when the relative evapotranspiration was highest than 0.8.Above this value of the relative evapotranspiration there was a low risk of grain yield decrease based on data from the IBGE, with all values lower than 10%.According toBergamaschi et al. (2006), the stabilization of maize yield with low water deficiency or high relative evapotranspiration, which characterize a low risk situation, it occurs next to a relative evapotranspiration (ETr ETc -1 ) similar or highest than 0.7 and, when lower than this value, it explains almost 80% of the maize grain yield variations.The absence of linearity noticed when the ratio ETr ETc -1 was higher than 0.8 can be related to the maintenance of constant value of ky in 1.15.This parameterization may represent a performance limitation of the model as it does not permit the introduction of ky values for the different maize phonological phases.According toDoorenbos and Kassam (1979), the sensibility of the maize culture to the water supply is rated as medium/high, with ky values between 1.00 to 1.15 because the ky factor > 1.0 indicates that the intensity of grain yield decrease is proportionally highest in relation to the increase of water deficiency.Although the results point out to highest frequency of low risk of water deficiency, the occurrence of relative evapotranspiration lower than 0.7 indicates the possibility of water deficiency which may set high grain yield decreases (Figure1).Massignam (2005) found that the lower values of ETr ETc -1 , when matching periods of high atmospheric evaporative demand and large rain variability, set the highest grain yield decreases in the Catarinense Midwest region.In experimental conditions, Cancela et al. (2006) warn about the difficulty to adjust the mean losses of maize relative yield, estimated with the Isareg model, with the respective experimental data, once experimental results do not cover the demand conditions for the dry and the very dry years.However, as it can be seen in the Figure 2, the correlation between the grain yield decrease estimates based on data from the IBGE presented a satisfactory degree of dependence of approximately 70% of the annual variations of grain yield decrease estimated by the Isareg model.Thus, the Isareg model was able to estimate the grain yield decrease due to water deficiency in the maize harvest when compared to IBGE data, despite the limitations noticed when the relative evapotranspiration presented values highest than 0.8 indicating that the model was less efficient to estimate the grain yield decrease.The grain yield decrease of the maize culture due to water deficiency, estimated by the Isareg model, in the west of the State of Santa Catarina presented an average value of 25.1% between the different periods, maturation cycles and harvests in the period between 1990-2011, in the west of the State of Santa Catarina (Figure 3).

Figure 1 .Figure 2 .Figure 3 .
Figure 1.Maize grain yield decrease and the mean ratio of the real (ETr) and the culture evapotranspirations (ETc), in the period of 1990-2011, in the west of the State of Santa Catarina.The ETr ETc -1 values are averages of the 19 sowing time and of three maize maturation cycles in Cambisol soil under direct sowing.
, in the central region of Rio Grande do Sul.For Massignam (2005), the water deficiency did not interfere in the determination of the maize sowing period for Campos Novos, in the Catarinense Midwest.Considering the small variations of grain yield decrease, due to the sowing time and between the maturation cycles (Figure3), it can be inferred that the decision concerning the maize sowing period in the west of the State of Santa Catarina should not be limited only to the occurrence of periods with a highest probability of water deficiency, and the complexity of biotic and abiotic factors related to the different production systems must be considered, such as the characteristics of the farm, the maize cultivations, the farmer's technological level and the financial capital available.For the 19 sowing time assessed in each agricultural year (1987 to 2011), the grain yield decrease estimated through the Isareg model ranged between 0.6 a 61.2% with an average value of 25.1% (Figure4), in which in 8% of the years studied, there was a decrease above 40% in the maize grain yield.

Figure 4 .Figure 5 .
Figure 4. Average grain yield decrease estimated through the Isareg model for each agricultural year and for different maize sowing time, in the period between 1990-2011 in the west of the State of Santa Catarina.The productvity decrease was the average of the three maize maturation cycles.

Table 1 .
, for the Far West region counties.The estimate of the beginning days of each maize phenologic phase was defined based on degrees day.The crop coefficients (Kc) and the water Input parameters for Isareg model simulations for each agricultural year and for different maize sowing time, in the period between 1990-2011 in the west of the State of Santa Catarina.