L. Bastiaans

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.

Can simulation models help design rice cultivars that are more competitive against weeds

Differences in competitive ability between rice cultivars IR8 and Mahsuri, grown in well-fertilised irrigated conditions, were analysed by means of a mechanistic simulation model (INTERCOM) for crop-weed interaction. The analysis revealed that the greater competitive ability of Mahsuri was due mainly to a greater relative leaf area growth rate early in the season and larger maximum plant height. Comparison of experimental results and model outcomes indicated, however, that the present model is not entirely able to quantify the trade-off between competitive and yielding ability. The extent to which traits advantageous for competition may lead to increased lodging or a larger number of unproductive tillers is not quantified by the present model. This implies that use of INTERCOM for design of more competitive rice cultivars will require continuous interaction between modelling and experimentation. With those limitations in mind, the model was used to quantify the potential contribution of various attributes to competitive ability. A sensitivity analysis demonstrated that competition for light is mainly determined by morphological characteristics of which early relative leaf area growth rate, early relative height growth rate and maximum plant height were found to be the most important. The ability of the model to identify key traits with respect to competitive ability makes it a useful tool for designing rice ideotypes.

Cover crop residue management for optimizing weed control

Although residue management seems a key factor in residue-mediated weed suppression, very few studies have systematically compared the influence of different residue management strategies on the establishment of crop and weed species. We evaluated the effect of several methods of pre-treatment and placement of winter rye (Secale cereale L.) and winter oilseed rape (Brassica napus L.) residue on seedling emergence under field conditions. For both species two cultivars, differing in allelochemical content, were used. Residues incorporated in the upper soil layer exerted a large inhibitory effect on the establishment of the relatively early emerging lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.) seedlings, whereas the inhibitory effect on the slightly later emerging Stellaria media L. seedlings was variable, and often a stimulatory effect on the very late emerging Chenopodium album L. seedlings was observed. Differences between cover crop cultivars were minor. For winter oilseed rape residue, pre-treatment strongly affected the time-course of residue-mediated effects. Finely ground residues were only inhibitory to seedling establishment during the first two to three weeks, whereas cut residues became inhibitory after this period. For winter rye, residue placement was most important. Residue incorporation gave variable results, whereas placement of winter rye residue on top of the soil inhibited the emergence of all receptor species. In conclusion, the optimal residue management strategy for weed suppression depends both on the cover crop species used and the target weed species.

Design of weed management systems with a reduced reliance on herbicides poses new challenges and prerequisites for modeling crop–weed interactions

Abstract Ecophysiological models for interplant competition were first developed in the early 1980s to obtain a better understanding of the harmful effect of weeds on crop productivity. The models were developed from simulation models of monoculture crops; each competing species was represented by its own growth model, which were then linked by additional routines to account for the distribution of resources over competing species. Initially the models were used for the construction of more robust damage relationships to support rational decision making on the use of herbicides. At present, apart from a need to reduce the use of herbicides, the design of weed management systems with a reduced reliance on herbicides is advocated. As a result, the weed problem should be envisaged in a different perspective. Rather than focusing only on detrimental effects in current crops, main emphasis should shift towards the management of weed populations and weed management itself should become an integrated component of crop management. For the development of these alternative management systems, specific improvements with respect to prevention, alternative control technology and decision making seem promising. The new challenges for modeling crop–weed interactions and prerequisites for crop–weed competition models that follow from these developments are discussed.

Characterization of host tolerance to Striga hermonthica

SummaryOne of the most promising control options against the parasitic weed Striga hermonthica is the use of crop varieties that combine resistance with high levels of tolerance. The aim of this study was to clarify the relation between Striga infestation level, Striga infection level and relative yield loss of sorghum and to use this insight for exploring the options for a proper screening procedure for tolerance. In three pot experiments, conducted in Mali (2003) and The Netherlands (2003, 2004), four sorghum genotypes were exposed to a range of Striga infestation levels, ranging from 0.0625 to 16 seeds cm−3. Observations included regular Striga emergence counts and sorghum grain yield at maturity.There were significant genotype, infestation and genotype × infestation effects on sorghum yield. The relation between infestation level and infection level was density dependent. Furthermore, the relation between Striga infection level and relative yield loss was non-linear, though for the most resistant genotype Framida only the linear part of the relation was obtained, as even at high infestation levels only moderate infection levels were achieved. The results suggest that for resistant genotypes, tolerance can best be quantified as a reduced relative yield loss per aboveground Striga plant, whereas for less resistant genotypes the maximum relative yield loss can best be used. Whether both expressions of tolerance are interrelated could not be resolved. Complications of screening for tolerance under field conditions are discussed.

Analysing crop yield and plant quality in an intercropping system using an eco-physiological model for interplant competition

An eco-physiological model was used to improve understanding of interplant competition based on physiological, morphological and phenological processes. The model was parameterised based on characteristics of the plants in monocultures and its performance was evaluated for the crop mixtures using experimental data from different growing seasons. A light interception routine accounting for row-geometry was compared to a routine assuming a homogeneous horizontal leaf area distribution. The models simulated the light distribution among the species equally well. The production of the two crops in the mixture was accurately simulated using parameter values based on monoculture growth characteristics. Morphological characteristics of the species such as the relative growth rate of leaf area during early growth and specific leaf area largely determined the competitive strength of the species. Dry matter production of the species, particularly if grown in mixture, was highly sensitive to maximum plant height and radiation use efficiency. Celery was found to be a stronger competitor than leek and clear responses of quality characteristics to plant density in monoculture and mixtures were observed. The model was used to determine ranges of plant densities that enable the intercropping system to meet current quality standards of the component crops.

Evaluating the use of two contrasting legume species as relay intercrop in upland rice cropping systems

Abstract In the savanna zone of West Africa with a prolonged dry season, the successful growth of legumes as short-season fallow necessitates relay establishment of cover crops into the preceding food crop. Inappropriate choice of legume species and establishment dates may result in severe interspecific competition for light, water and nutrients and a subsequent yield loss in the main crop. In the current study, the highly competitive Cajanus cajan L. and the weakly competitive Stylosanthes hamata Taub. were relay-seeded into two upland rice cultivars with different competitive ability (WAB56-50, WAB450-24-3-2-P18-HB syn. V4 ) at different sowing dates, ranging from 0 to 84 days after rice sowing (DARS). Using a logistic function was adequate to describe rice grain yield, final rice biomass and legume biomass at 1 week after rice harvest as a function of legume sowing date. Rice biomass and grain yield were significantly ( P C. cajan was the most appropriate legume species of the two under study, to suit a relay intercropping system with upland rice, as it produced reasonable amounts of biomass at low levels of rice grain yield loss. This conclusion was confirmed by results on the after-effect of the legumes, where only the improved fallow by C. cajan gave a significant increase in the yield of the subsequent rice crop. Optimum sowing time of C. cajan was between 30 and 35 DARS. The effect of selecting a specific legume was found to surpass that of choosing either one of the two rice cultivars, for which contrasting results were obtained in 2 years of experimentation.

Intercropping System Optimization for Yield, Quality, and Weed Suppression Combining Mechanistic and Descriptive Models

concluded that the intercropping system needed to be optimized with respect to crop quality and weed supIntercropping leek (Allium porrum L.) with celery (Apium graveopression for successful implementation and suggested lens L.) is an option to reduce growth and reproductive potential of weeds while maintaining productivity. In this study, a combined applying ecophysiological simulation models to optimodeling approach is used to optimize a leek—celery intercropping mize the system. Earlier, Kropff and Van Laar (1993) system with respect to crop yield, product quality, and weed suppresadvocated the use of modeling to develop and optimize sion. An ecophysiological model for interplant competition was used weed management systems with respect to cost effecto simulate yield and product quality of the crops as well as biomass tiveness and minimization of environmental effects. and seed production of the weed Senecio vulgaris L. for a wide range Ecophysiological crop growth models can be very efof crop mixtures and weed infestations. The results of the simulations fective to evaluate and develop complex systems, such as were summarized using a descriptive hyperbolic yield–density model, multispecies plant communities (Kropff and Van Laar, which then allowed evaluation of the intercropping system in terms 1993). Based on physiological, morphological, and pheof productivity, product quality, and ability to suppress weeds. In a nological processes, such models provide insight into the weed-free mixture, the competitive ability of celery was six times higher than that of leek. With respect to late-emerging S. vulgaris, competitive relationships of the system. These models the relative competitive ability of leek was 5.4 times lower than that facilitate the exploration of complex systems without of celery. Replacing two leek plants of a leek monoculture by one extensive field experimentation to investigate all opcelery plant resulted in almost 20% biomass reduction of late-emergtions in a wide range of conditions. Empirical models ing S. vulgaris. Crop mixtures with a leek density of about 20 plants and regression techniques can help analyze the final m 2 and a leek/celery ratio of 2 proved to be the optimum intercropoutcome of competition trials and describe plant interping system, given the current price ratios. Compared with leek monoference in cropping systems. Approaches to describe culture, profitability was maintained, and late-season weed suppresinterplant relationships have been developed for a long sion was greatly increased, resulting in reduced weed seed production. time and have helped improve understanding of competitive effects between crops and between crops and weeds (De Wit, 1960; Kira et al., 1953; Shinozaki and R studies have addressed intercropping as an Kira, 1956). option for an integrated weed management, particThe current study attempts to combine a mechanistic ularly in farming systems with low external inputs (Capand descriptive modeling approach to optimize the sysorali et al., 1998; Itulya and Aguyoh, 1998; Liebman tem. A well-evaluated ecophysiological model, such as and Davis, 2000; Rana and Pal, 1999; Schoofs and Entz, INTERCOM (Kropff and Van Laar, 1993), provides 2000). Effects of crop diversification on weeds have the necessary insight into the processes and plant charbeen reviewed by Liebman and Dyck (1993), Liebman acteristics determining mutual competitive effects and and Ohno (1998), and Teasdale (1998). As an example allows generating a large number of data sets for a of functional biodiversity, intercropping leek (Allium wide range of densities and environments. Subsequent porrum L.) with celery (Apium graveolens L.) showed application of a descriptive model to the generated data various beneficial effects, such as the reduction of weeds sets can help summarize the results, calculate the RCA and pests and an improved resource capture, while cropof the system components, and describe yield and prodping practices were not hampered (Baumann et al., 2000, uct quality of the component crops in relation to plant 2001a). Celery improved weed suppression by the candensity and mixing ratios. The objective of this study opy by increasing its light interception. As a result, inwas to evaluate the use of combined modeling apcoming radiation was captured more efficiently by the proaches for analysis and design of a leek and celery intercrop canopy, and less radiation was available for intercropping system to optimize this system with regermination and growth of weeds. However, the strong spect to yield and quality while improving weed suprelative competitive ability (RCA) of celery in the interpression. cropping system resulted in a loss of leek quality because stem diameter was reduced to 20 mm (market criteMATERIALS AND METHODS rion) (Baumann et al., 2001a). The authors, therefore,

Can photosynthesis-related parameters be used to establish the activity of acetolactate synthase-inhibiting herbicides on weeds?

Abstract The application of the acetolactate synthase (ALS)–inhibiting herbicide metsulfuron on greenhouse- and field-grown black nightshade and greenhouse-grown ladysthumb resulted in progressive inhibition of the level of carbon dioxide (CO2) fixation, the relative quantum efficiency of electron transport through photosystem I (ΦPSI) and II (ΦPSII), and the leaf chlorophyll content. Photosynthetic-related measurements, measured 2 to 4 d after treatment (DAT) at photon flux densities of 400 to 500 μmol m−2 s−1, provided valuable information before the visual symptoms that first appeared at 7 to 10 DAT with the herbicide. Measurements of the quantum efficiency for electron transport by photosystem II and the loss in leaf chlorophyll content appeared to be two of the most practical parameters to use when designing an early detection method to assess the toxicity of metsulfuron. The use of chlorophyll fluorescence would require a comparison of steady-state ΦPSII measurements for control and treated plants, which could be realized by either measuring in time (before/after application) or space (treated/untreated patch). Nomenclature: Metsulfuron; black nightshade, Solanum nigrum L. SOLNI; ladysthumb, Polygonum persicaria L. POLPE.

Inflorescence characteristics, seed composition, and allometric relationships predicting seed yields in the biomass crop Cynara cardunculus

Cynara (Cynara cardunculus) is a perennial C3 herb that has its potential as bioenergy crop. This paper aims (a) to derive empirical relationships to predict cynara seed yield per head and per unit area, avoiding laborious extraction of seeds from the complex structure of its inflorescences; (b) to determine the head‐weight distribution per unit area, the seed composition and the oil profile of cynara seeds; and (c) to estimate the range of cynara biomass, seed and oil yield in representative parts of Greece. We analyzed 16 field experiments, varying in crop age and environmental conditions in Greece. Seed yield per head (SYhead) can be accurately predicted as a linear function of dry head weight (Hw): SYhead=0.429·Hw−2.9 (r2=0.96; n=617). Based on this relationship, we developed a simple two‐parameter equation to predict seed yield per unit area (SY): SY=HN·(0.429·μ−2.9), where μ is the mean head weight (g head−1) and HN is the total number of heads per unit area, respectively. The models were tested against current and published data (n=180 for head‐level; n=35 for unit area‐level models), and proved to be valid under diverse management and environmental conditions. Attainable cynara seed yields ranged from 190 to 480 g m−2 yr−1, on dry soils and on aquic soils (shallow ground water level). This variation in seed yield was sufficiently explained by the analyses of head‐weight distribution per unit area (small, medium and large heads) and variability of seed/head weight ratio at head level. Seed oil concentration (average: 23%) and crude protein concentration (average: 18.7%) were rather invariant across different seed sizes (range: 26–56 mg seed−1) and growing environments.