Crop simulation models: predicting the future of pulses
This post was featured in IYP2016.org.
From the past to the present, pulses benefit agricultural systems
Pulse crops have always been playing a beneficial and central role in crop rotations. Even the Romans and ancient Chinese already knew the benefit of using peas and soybean. When pulses are used as a ‘break crop’ for pests or diseases, wheat yield has been shown to increase by up to 1.2 tons per hectare  and the benefit even lasts for an additional wheat crop.
Pulse crops have the natural capacity to fix nitrogen gas from the atmosphere, thanks to an association with soil bacteria called rhizobium, something that other crops like rice, wheat or maize cannot do. This brings ‘free’ nitrogen fertilizer into the soil, where it fuels production of pulse grains that contain more than double the protein (over 25%) found in cereal grains (less that 8-10%). After harvest, pulse crops leave behind nitrogen-rich residues that are good for feeding cattle and for ‘feeding’ the soil and subsequent crops.
When pulses are part of a crop rotation, not only can they supply a major nitrogen benefit for themselves and other crops while interrupting pest and disease cycles, they are also very likely affecting the soil microflora beneficially. Saving money on nitrogen fertilizer, gaining a high-protein food and fodder source, reducing crop losses, and boosting soil fertility  are strong incentives for including pulses in farming systems. Given the large genetic variation in their ability to harvest atmospheric nitrogen, there is a significant opportunity to increase the favorable effects of pulses on farming systems  by breeding cultivars with high nitrogen-fixing capacity.
Climate change creates new challenges for pulse production
Despite the fossil fuel used in manufacturing nitrogen fertilizers, prices are often low making it possible for farmers to achieve high yields of cereal crops through significant nitrogen addition. Since they have to expend their own energy to acquire nitrogen from the atmosphere, pulses tend to have lower yield volumes than cereals and have often been relegated to marginal lands or secondary roles in crop rotations.
Climate change stands to worsen the picture for agriculture overall, including pulse crops. A 2 to 4 ºC warming of average surface air temperature is expected over the next 100 years. Crops will mature quicker if the weather is warmer, but will have less time to capture light energy, reducing yield. Breeding longer duration cultivars can help with this. Flowering and seed-setting are extremely sensitive to increases in temperature. Expected increases in the frequency of heat waves can badly decrease the number of pods and seeds on a pulse plant, reducing yield dramatically. For instance, chickpea yield decreases if exposed to temperatures above 30ºC during flowering. There is a lot of research, using large germplasm collections for different crops, to identify germplasm with tolerance to high temperatures and to breed improved cultivars .
‘Atmospheric drought’ is a third, more insidious effect in which the increased dryness of the atmosphere that comes with higher temperatures creates a higher evaporative demand from plants. Recent research has found that some crops have genetic traits  that enable plants to restrict water losses under atmospheric drought. Exciting new methods allow researchers to screen large germplasm collections quickly and precisely.
Crop simulation models can make the search faster and easier
The vagaries of climate related alteration of growing conditions make it difficult for producers and scientists to adapt. The climate conditions that put strains on crops are never the same, making the selection of improved cultivars difficult. Yet, these stress conditions do follow patterns, which can be classified to make changes more ‘predictable.’ For instance, seasonal weather may be dry in 8 years out of 10, or 83 years out of 100, indicating that a drought tolerant cultivar for that location would be needed at least 80% of the time. The tools used for doing this classification are called crop simulation models. With software that uses weather data and certain crop characteristics as input, these models can simulate crop yield in many different locations and years and also capture the spatial variation in soils, weather, farming practices, and other factors that influence yield.
As we learn more about the highly diverse farming conditions and climate variability that farmers deal with, scientists can find the best suited plants faster and easier by taking advantage of crop simulation models. For instance, scientists can test how plants with different genetic traits are likely to perform in different growing conditions and more easily search for plants that will do well in different places . Crop simulation is very handy for understanding what a crop needs in order to achieve higher yields in a given environment. This model-informed approach can also be used to anticipate how different pulse plants will do under different cultivation methods, in intercrops, and with different density or timing of planting.
Accelerating use of models
Crop simulation models exist for pulse crops, but their use in backstopping crop improvement remains marginal. This is, in part, due to the convention of breeding ‘mega-varieties,’ cultivars thought to be adaptable to many different farming contexts. Crop simulation brings a new paradigm of breeding for specific conditions, focused at smaller geographical scales, to deliver larger overall returns.
Looking ahead, modelling analysis can be embedded in the scope of breeding programs. Good modelling tools exist (e.g. Simple Simulation Modeling, SSS), but the community of scientists using these tools is still small and we need to do more to improve their use. Key frontiers include expanding the number of pulse species and cultivars modeled as well as developing the coefficients and datasets needed to validate models for different pulses in specific regions . More work is also needed to simulate the beneficial effects of pulse crops in farming systems such as the amount of residual nitrogen available to crops following pulses in a rotation and the nutritional (e.g. protein) benefits produced on a farm when pulses are grown.
Pulse crops have many benefits for farming systems, both from the angle of human and animal nutrition and of soil health and farming system sustainability. They face production challenges and these will get worse in future climate scenarios. Yet, the biological basis for increasing tolerance to production constraints is being much better understood and germplasm collections offer a treasure trove of solutions for breeding improved cultivars. There are also software tools such as crop simulation models that allow scientists to decipher the complexity of pulse production and simplify the choice of targeted interventions to maximize productivity and sustainability.
In Troia, Portugal this week, nearly four hundred scientists met at the Second International Legume Society conference to review progress in developing basic knowledge and innovative solutions, like crop simulation models, for achieving resilient, high-yield pulse crop production to contribute to global sustainability challenges.
About the author:
Dr. Vincent Vadez, Principal Scientist, CGIAR-ICRISAT
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- Sinclair TR et al. 2010. Assessment across the United States of the benefits of altered soybean drought traits. Agronomy Journal, 102:475-482
- Sinclair TR. 2014. Soybean production potential in Africa. Global Food Security, 3:31-40.
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