[Simulation modules for diagnosing production constraints] | Modelos de simulacion para diagnostico de limitaciones

The processes of growth and development determine a plant stage. From the standpoint of simulation, it is important to separate these processes because they are seriously effected by different climatic factors. Development in this context refers to a moment in which certain critical actions take place in the crop (for example, beginning of tillering, end of vegetative stage, flowering, beginning of grain filling, etc.). Growth, on the other hand, refers to the change in weight, volume, height or area of a part or the plant as a whole. The potential biomass yield of a crop is the result of the rate of accumulation of the weight during the total growth period. The rate of biomass production of any crop is fundamentally determined by the quantity of solar energy that the crop can capture and use. This is valid for a considerable wider range of temperatures. On the other hand, the temperature determines the duration of growth. This is the reason why the highest yield potential of the crops are expressed in the environments (locations and/or years) where the temperatures are sufficiently low to maximize the length of their growth cycle. In the environments with high temperatures, such as in the tropics, the only way to achieve the total biomass production similar to that in the cooler regions is to combine the yields of two or more crops in a sequence in such a manner that the total growth periods of the two regions are equal. It is to say that the rate of potential biomass production is more or less constant over the regions and times within a given range of temperatures that permit growth. On the other hand, the duration of a growth stage is much more variable. Therefore, it is critical that a model be capable of simulating the duration of the growth periods in order to predict the production potentials. The duration of different growth periods is fundamentally determined by the sum of average daily temperatures (also called thermal sum, and expressed in degree days). The application of the thermal sum to simulate the growth duration also requires attention to other important factors such as: the base temperature, the maximum temperature above which the development ceases or starts to decline, effect of the day length or photoperiod over the duration of the growth stages in sensitive cultivars and the cold requirements of other cultivars to complete certain development phases. The DSSAT models are based on these principals to predict the duration of different growth stages in different species simulated. For each of the development stages considered, DSSAT models estimate daily potential growth taking into account the daily data of temperatures, rainfall and solar radiation. They estimate the daily dry matter production potential per plant and per unit area. On the other hand, the models calculate the availability of water and N available in the soil that will be necessary to sustain this daily production potential. For this reason, the models also use the same daily climatic information and the soil characteristics where simulation is taking place to calculate the balances of water, N and phosphorous in the soil on a daily basis. The results of these water and nutrient balances are utilized to convert the daily potential growth to achievable growth. In case the water or nutrient limitations do not exist, the real growth is equal to the potential growth for the day it is being simulated. Otherwise, the potential growth is reduced depending on the most limiting factor (water and/or N). To construct the balances of water and nutrients, conceptual models are used that control the balances using mathematical formulas derived from the experimental results. For example, in calculating the daily N balance, the models consider: the nature of the stubble of the previous crop, the rate of the mineralization of the organic matter, the immobilization of inorganic N, the transformations of the fertilizer N, possible washing of nitrates, loss of ammonia due to volatilization, etc. This N balance on the other hand, is influenced by the daily soil temperature, the availability of water and the absorption of the N by the crop, etc. In the present study, DSSAT-3 models were utilized to determine the yield potential of two types of wheat cultivars calibrated and validated for the Uruguayan conditions. Later, utilizing the climatic data of the past 30 years, the models were used to quantify the reduction in yield potential caused by different factors. The factors included were: (a) dates of seeding, (b) water availability, (c) N availability (from soil and from fertilizer), (d) tillage operation and root exploration, (e) diseases and insects in different growth stages, (f) types of cultivars with different phenology, sensitivity to photoperiod, cold requirement, harvest index and variations in the yield components.