Crop Models for Greenhouse Production Systems

Although the physiological principles involved in the growth of greenhouse and field crops are not basically different, the development of models for greenhouse crops to some extent has followed its own way. This is mainly due to the specific characteristics of the crops and of the greenhouse production systems involved. Many important greenhouse crops are multi-harvest crops, where the balance between vegetative and generative growth is an aspect of major concern to growers. Moreover, most products have a high water content and they are sold fresh. In food crops, taste is a valuable crop property. In ornamentals, shape and colour are important characteristics that put certain demands on the output of models. More generally, quality issues (e.g., shelf or vase life) often have to be approached in a different way than with field crops. Last but not least, the huge number of species is problematic for crop modellers in horticulture. Modern greenhouse production systems provide the grower with a highly advanced, but expensive system for controlling the aerial and root environments of the crop. Through this control system growers are able to control the production process in great detail. The high added value obtained in greenhouses and the high quality requirements go together with a great deal of human interference in the production process: either directly, by pruning and training, or indirectly, by using various organisms for pest control and pollination. Crop management and the interaction of pests and diseases with the crop are both aspects that make special demands on crop models. It is a major challenge for greenhouse growers to make the best possible use of the available options to achieve high productivity at the moment when products are required in the market. In addition, this must be accomplished while reducing the environmental impact by emissions of CO2, nutrients, and biocides and at minimum cost. To optimise greenhouse production systems, crop models are needed. But, they also have to be integrated into more complex models of the nursery as a whole to address planning, scheduling, and logistics. For policy makers, there is a need for models at regional or national scale that help them to decide on measures related to environmental issues or economic development. Models on product quality and integration of models from different disciplines to simulate nurseries and whole product chains are some of the important and challenging developments in greenhouse simulation over the last few years. Also, the implementation of models into practice is a hot issue generating many new and complex research questions. INTRODUCTION The facilities to grow crops in an enclosure, including the technology to control the environment, may be designated as greenhouse production systems. Greenhouse production systems are presently among the most sophisticated crop production systems (Challa et al., 1994). The methods used for heating, cooling, watering, lighting, screening, and fertilizing the crop in the most advanced production systems enable the grower to Proc. 4 IS on Cropmodels Eds. J.H. Lieth & L.R. Oki Acta Hort. 593, ISHS 2002 48 influence the environment of the crop in great detail. In this way, crop growth and development can be regulated with greater control than with field crops. The intensive involvement of the grower in the daily production process and the refined control possibilities give rise to a major knowledge requirement in terms of the number of processes and the time-scale of the controls (Challa, 1997). Further complications arise from the increasing role of alternative objectives in addition to the primary production objectives that are considered in traditional agriculture. These include the role of the production chain, the importance of energy saving, and other environmental issues. These developments all lead to an increasing need for effective and comprehensive information. Part of the information requirement of growers could be satisfied by crop growth models. Such models could provide detailed evaluations of alternatives, support decisions, and improve the performance of control systems by providing on-line estimations of relevant processes. Furthermore, they enable the grower to answer questions regarding complex phenomena that involve many inputs and outputs and conflicting objectives (Challa et al., 1994). Greenhouse crop models can be used as such, but they are particularly helpful when they are integrated into models of greenhouse production systems where economic and environmental processes are also integrated. Greenhouse crop models could also play an important role at a regional or national scale, where policy makers are dealing with problems of, for example, emissions of nutrients and biocides, economic and social development of a country, and other problems at a high level of aggregation (Challa and Pluimers, 1997). In the present overview, greenhouse crop models are discussed against the background of the particular situation of greenhouse horticulture as opposed to models developed for agriculture in open, uncontrolled environments. GREENHOUSE HORTICULTURE Modern, intensive greenhouse horticulture is characterised by high inputs and high outputs per m. Total investment requirements may amount up to about € 100 per m, depending on the type of crop and cultivation system. Investments are particularly high with potted plants where movable benches, sub-irrigation facilities, movable sun screens, supplementary lighting, and intensive mechanisation and automation are common. Modern, intensive greenhouse horticulture is, in fact, a high-tech, semi-industrial, purely economic activity. Unlike agriculture, there is no other justification, such as landscape or natural values, for greenhouse horticulture. The production process is closely controlled since the product specifications and delivery times are determined by the market. Greenhouse horticulture is taking place within an essentially closed environment. This closed environment isolates the crop from influences from the outside world. It reduces the influence of the outside weather and diminishes the pressure from pests, diseases, and weeds and allows effective forms of biological control. Compared to field agriculture, horticulture is characterised by a large diversity in crops and varieties. With ornamentals, there is a fast turn-over of species and varieties grown in greenhouses. This diversity is a major handicap in the advance of horticultural science because a huge database of crop specific knowledge has to be built and maintained. But, the worldwide research capacity is much smaller than in agriculture. Intensive greenhouse production of horticultural crops is a year-round activity with a constant supply to the market, efficient utilisation of expensive production facilities, and a continuous labour requirement. The continuous production, however, also gives rise to knowledge requirements with respect to production and quality control in highly variable environmental conditions. For example, extremely poor radiation conditions prevail in mid-winter at high latitudes and high radiation levels exist in the summer in the tropics and subtropics. Within the context described here, knowledge plays a key-role: the advanced