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A simulation study was conducted to examine the effect of pattern of herbicide use on development of resistance to two herbicides with different modes of action in finite weed populations. The effects of the size of the treatment area (analogous to initial weed population), germination fraction and degree of self-pollination in the weed were investigated. The results indicate that the probability of developing resistance to one or both herbicides decreases as the size of the area/initial population decreases. For treatment areas of 100 ha or less with an initial weed seedbank of 100 seeds m )2 and initial frequencies of the resistance genes of 10 )6 , development of resistance to both herbicides (double-resistance) is uncommon within 50 years for all types of weeds if both herbicides are used in all years (used in combination). If herbicides are used in alternate years (rotated) double-resistance almost always occurs in 100 ha areas but is uncommon in areas of 1 ha or less. The results suggest that adoption of practices that limit movement of weeds in conjunction with using herbicides in combination rather than in rotation can substantially delay development of herbicide resistance.

A model is described which simulates the growth of fibrous root systems. The root growth is specified in terms of growing time, numbers of axes, initiation times of axes, growth rates and branching characteristics of the roots, and characteristics governing the direction of root growth. The model generates a representation of the root system in which the locations of all branches and root tips are recorded in three-dimensional coordinates, and updates this representation in discrete time steps until the specified growing time is reached.Data are presented from a simulation of wheat root growth by the model. The simulated root system is represented pictorially and also graphically in the form of root length and root tip number profiles which are stratified by branching order class. The pictorial representations produced by the model are much more realistic than any which have been produced by past root growth models, and the graphical representations show trends in root length and root tip numbers which are the same as those commonly observed in real roots.

Little is known about root architectural attributes that aid the capture of nitrate from coarse-textured soil profiles of high leaching potential. In this study, a range of root architectures from the herringbone to the dichotomous structure were simulated, and their capacity to take up nitrate leaching through a sandy profile was recorded. All root systems had equal total volume at each point in time, and so were considered cost equivalent. These simulations showed that the root architecture likely to maximize nitrate capture from sandy soils (under the Mediterranean rainfall pattern experienced in Western Australia) is one that quickly produces a high density of roots in the top-soil early in the season, thereby reducing total nitrate leached with opening season rains, but also has vigorous taproot growth, enabling access to deep-stored water and leached nitrate later in the season. This is the first published, spatially explicit attempt to assess the ability of different root architectures equivalent in cost, to capture nitrate from a spatially and temporally heterogeneous soil environment.

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