H. van Keulen

On approaches and applications of the Wageningen crop models

Abstract Since the pioneering work of C.T. de Wit in the 1960s, the Wageningen group has built a tradition in developing and applying crop models. Rather than focusing on a few models, diversity is its trademark. Here we present an overview of the Wageningen crop and crop-soil modelling approaches along three criteria. The first criterion relates to the production situations the models are dealing with (i.e. potential, water and/or nutrient-limited, and actual production situations including pests, diseases and weeds). Second, models differ as a result of the objectives of model development, and hence required scale and degree of detail and comprehensiveness. Third, models have at least three potential application domains, i.e. research, education and support of learning and decision making processes. We describe both summary and more comprehensive modelling approaches for the major production situations. An overview of most of the Wageningen models is presented together with a more detailed description of LINTUL, SUCROS, ORYZA, WOFOST and INTERCOM. Illustrations for each of the three application domains are presented, i.e. plant type design, guiding experimental research, education, yield gap analysis, evaluation of manure policies, crop growth monitoring system and analysis and design of farming and regional land use systems. We discuss common issues of model verification, model validation, model validity and data requirements, and present information on software implementation, model and software documentation and distribution policy. Finally, we reflect upon the Wageningen modelling approaches and identify a number of key issues for future research. Major achievements of Wageningen modelling efforts include (1) a broad variety of approaches for modelling of systems at different scales and with different purposes; (2) their contribution to quantitative systems thinking in general, also for applications at higher hierarchical levels; (3) a strong linkage between crop modelling and higher education, both at undergraduate, graduate and post-doctoral level. To continually increase our understanding of crops and production systems a diversified approach must be cherished. At the same time we conclude that focus is required on a limited number of modules in a more integrated modelling framework for the benefit of analysing, evaluating and designing cropping systems. This review may be instrumental in the development of such an integrated framework.

The `School of de Wit' crop growth simulation models: A pedigree and historical overview

In this paper, a pedigree of the crop growth simulation models by the ‘School of de Wit’ is presented. The origins and philosophy of this school are traced from de Wits classical publication on modelling photosynthesis of leaf canopies in 1965. It is shown how changing research goals and priorities over the years have resulted in the evolution of a pedigree of models that are similar in philosophy but differ in level of complexity, the processes addressed and their functionality. In the beginning, modelling was motivated by the quest for scientific insight and the wish to quantify and integrate biophysical processes to explain the observed variation in crop growth. Later, the emphasis of, and funding for, agricultural research shifted towards putting acquired insights to practical and operational use. Model development became led by a demand for tactical and strategic decision support, yield forecasting, land zonation and explorative scenario studies. Modelling developments for different production situations are illustrated using the models the authors consider most important, i.e. BACROS, SUCROS, WOFOST, MACROS and LINTUL, but reference is also made to other models. Finally, comments are made about the usefulness and applicability of these models after nearly 30 years of development, and some future courses of action are suggested.

A comparison of the models AFRCWHEAT2, CERES-Wheat, Sirius, SUCROS2 and SWHEAT with measurements from wheat grown under drought

The predictions of five simulation models were compared with data from a winter sown wheat experiment performed in a mobile automatic rainshelter at Lincoln, New Zealand in 1991/1992, where observed grain yields ranged from 3.6 to 9.9 t ha−1. Four of the five models predicted the yield of the fully irrigated treatment to within 10%, and SWHEAT underestimated by more than 20%. The same four models also predicted the grain yield response to varying water supply with reasonable accuracy, but SWHEAT again underestimated the yield reduction with increasing drought. However, the performance of all the models in predicting both the time course and final amount of aboveground biomass, of leaf area index (LAI) and evapotranspiration, varied substantially. These variations were associated with their diverging assumptions about the effects of root distribution and soil dryness on the ability of the crops to extract water, the value of the ratio of water supply to water demand at which stress begins to reduce leaf area development, and photosynthetic, or light-use efficiency (LUE). All the models predicted, to varying degrees, that reductions in photosynthetic efficiency or LUE was an important contributor to reductions in the rate of biomass accumulation. In contrast, analysis of the experimental data indicated that this factor was a minor contributor to the reduction, and variation in light interception, associated with changes in LAI, was the major cause.

An evaluation of whole-farm nitrogen balances and related indices for efficient nitrogen use

Quantification of nitrogen (N) flows creates awareness among farmers, can help them to re-evaluate N management and may reduce nitrate loss to groundwater. Hence, whole-farm balances play a crucial role in legislation on N management in Netherlands. This paper reviews the strengths and weaknesses of whole-farm balances for assessment of the environmental impact of agriculture. The usefulness and reliability of a balance strongly depends on its completeness. The surplus per unit area indicates the environmental impact, provided that all relevant terms are included. However, the surplus per unit area, the surplus per unit output and the output per unit input, as derived from the balance, may not represent accurate indicators of the operational management skills of a farmer, as these estimates not only depend on the conversion of N within the farm, but also on the extent to which the farm relies on animal feed produced outside the farm and the extent to which processing of crops takes place outside the farm. Without additional information on the processes underlying the whole-farm level and N fluxes at spatial scales above the level of an individual farm, whole-farm balances do not reveal the nature and magnitude of losses, nor do they provide sufficient clues how to improve the efficiency of N use.

Some aspects of the role of organic matter in sustainable intensified arable farming systems in the West-African semi-arid-tropics (SAT.

In this paper the role of organic manure in intensified farming systems in the SAT of West-Africa is discussed. Different aspects are treated: its function as a source of plant nutrients, its effects on soil physical and on soil chemical properties. It is concluded that often the major effect is through increased nutrient supply, but that in combination with chemical fertilizer — particularly nitrogen — it serves to counteract the negative effects of these fertilizers, particularly acidification and the increased removal of nutrients other than the one applied in the fertilizer.Insufficient organic material appears to be available to realize the required production increase and prevent the negative effects of nitrogen fertilizers. However, application of chemical fertilizer alone can lead to sustainable production systems provided export and losses of all nutrient elements are sufficiently compensated and acidification is avoided by using the correct type of nitrogen fertilizer, possibly in combination with liming.

Land Evaluation: From Intuition to Quantification

According to Stewart (1968), land evaluation is “the assessment of the suitability of land for man’s use in agriculture, forestry, engineering, hydrology, regional planning, recreation, etc.” Many disciplines have contributions to make to land evaluation in its widest sense. The present review focuses on the role of soil science in land evaluation.

Application of interactive multiple goal programming techniques for analysis and planning of regional agricultural development

This paper describes a method of investigating development possibilities for a region under various constraints and demands. Use is made of an interactive multiple goal linear programming technique. The method is illustrated with an example from a semi-arid zone in the Mediterranean Basin. It is concluded that the method can help to decide on feasible development pathways within a wide range of technical and socio-economic scenarios, and so explore the ‘margins for policy’. It enables communication between politics, planning and research and can therefore serve as a tool for more efficient development planning.

Agricultural development in the West African Sahelian region: a cure against land hunger?

Abstract Animal husbandry and arable farming in some West African countries are analysed with respect to the potential of the natural resources. It is demonstrated that low soil fertility is often the major constraint for production, both for food grains and for natural vegetation. The result is that the quality of the organic material used as forage is also low, which constrains animal production. Animal husbandry shows a trend of moving southwards and becoming more sedentary, which decreases production potential and leads to changing production targets. Arable farming without the use of external inputs increasingly exhausts the soil, with the risk of degradation. The integration of arable farming and animal husbandry often strengthens the former, but hardly improves animal production. It is argued that the only remedy for agriculture in the region is increased production per unit area by the application of external inputs. This will require a changing price policy to make it economically viable, and can only be achieved through solidarity of the international community in the area of agricultural trade policies.

Nitrogen (N) management in the ‘De Marke’ dairy farming system

In the sandy regions of The Netherlands, high losses of N from intensified dairy farms are threatening the environment. Therefore, government defined decreasing maximum levy-free N surplusses for the period 1998–2008. On most dairy farms, the current N surplus has to be reduced by half at least. Farmers fear that realizing these surplusses will be expensive, because it limits application of animal manure, which then has to be exported or additional land has to be bought. Moreover, farmers are worried about the impact on soil fertility. To explore the possibilities for reducing surplusses of average intensive farms by improved nutrient management, farming systems research is carried out at prototype farm ‘De Marke’. Results are compared with results of a commercial farm in the mid-1980s, the moment that systems research started and introduction of the milk quota system put a halt to further intensification. Results indicate that average intensive farms can realise a reduction in N surplus to a level below the defined final maximum, without the need to buy land or to export slurry. Inputs of N in purchased feeds and fertilisers decreased by 56 and 78%, respectively. Important factors are reduced feed intake per unit milk, as a result of a higher milk yield per cow, less young stock and judicious feeding, an improved utilization of ‘home-made’ manure and a considered balance between the grassland and maize area. Changed soil fertility status did not constrain crop production. Nitrate concentration in the upper groundwater decreased from 200 to 50 mg l-1, within a few years.

The role of livestock for sustainability in mixed farming: criteria and scenario studies under varying resource allocation

Abstract Cropping, when possible, tends to become more important than animal production because, in general, it can feed more people per area unit in terms of calories and protein. In such systems, the role of wasteland grazing as a source of energy for agriculture through animals for traction and dung is often taken over by the use of resources from fossil reserves. This changing role of animals in the sustainability of agriculture is addressed in this paper to discuss options and constraints for animal production in newly developing farming systems. Based on a brief literature review, this paper discusses how and in which way ruminant livestock has played or can continue to play a role in (newly developing forms of) sustainable agriculture. The role of livestock in different modes of agriculture ranging from expanded agriculture (EXPAGR), and high external inputs agriculture (HEIA) to low external inputs agriculture (LEIA), and new conservation agriculture (NCA) are elaborated. It is argued that even when fossil reserves based external inputs such as oil and fertilisers become more widely used, they should still be used with care to save money and finite resources as well as to avoid problems of waste disposal. However, in conditions with limited access to resources, it continues to be difficult to obtain inputs from fossil reserves. Under these conditions, the major options to increase system sustainability by reducing pollution problems and dependency on external resources are (a) to adjust ways and objectives of production systems to the access to resources, and (b) to achieve increased use and recycling of resources within the system itself. Definitions for sustainability are given and translated into four criteria, i.e. food production and degree of self-sufficiency in the short term based on energy, protein, clothing, shelter, etc.; food production and degree of self-sufficiency in the long term expressed in the form of soil organic matter (SOM) content; reduced dependence of external inputs (=nitrogen use); and aspects of resilience, stability and equity in crop–livestock systems. The results of scenario studies concerning use of grass and legume leys for livestock production illustrate options and trade-offs for different crop–livestock combinations in terms of these criteria for sustainability.