A MODEL FOR THE IMPACT OF HERBICIDE TOLERANCE ON THE PERFORMANCE OF OILSEED RAPE AS A VOLUNTEER WEED

The introduction of genetically modified, herbicide;tolerant, oilseed rape into the agricultural environment will have ramifications beyond weed control of the crop. Herbicide-tolerant rape will undoubtedly bec ome part of established volunteer weed populations that occur in many cereal rotations, but its longevity in these populations and its impac t as a weed and contaminant of future oilseed rape crops is uncertain, A life cycle model of volunteer oilseed rape was therefore constructe d, incorporating existing information an physiological processes such as emergence pattern, longevity of buried seed, death rates of various structures and flowering and seeding as functions of density. The mod el was designed to allow interaction with control factors such as harv esting efficiency, herbicide treatment, ploughing and the sequence of crops in the rotation. Many of the physiological parameters (including seed decay rates, fecundity at high density) are uncertain, simply th rough lack of information in the appropriate context. Other parameters such as harvesting efficiency and herbicide kill rates, are inherentl y variable in farming. Accordingly, a Monte-Carlo approach, in which t he model was run many times with different random realisations of para meter sets, was used to expose factors to which the seedbank was sensi tive. Sets of 1250 realisations were compared for each of two extreme conditions: where herbicide could be used according to current intensi ve farming practice and where it was not an option (representing total herbicide tolerance). Modelled seedbank numbers after 5 yr ranged fro m 10(-3) to 10(4) m(-2), realistic values found in arable soils. The u se of herbicide, together with efficient harvesting of seed, clearly h as an important suppressive effect on the oilseed rape seedbank, keepi ng it lower than 10(2) m(-2) (a typical sowing rate) after 5 yr in mor e than 80% of realisations. In the absence of herbicide, seedbanks wer e invariably greater, but their absolute value depended strongly on ha rvesting efficiency and the extent to which high density of plants sup pressed fecundity. Analysis of the time series from the simulations sh owed that the seedbank levels fluctuated by orders in magnitude from y ear to year in the absence of herbicide use. The sensitivity analysis of the life-cycle model led to the development of a simplified model f or the seedbank dynamics. The model shows that the essential features of the dynamics result from an interaction between density-dependent f ecundity and the perturbations due to management. Therefore prediction s of the effect of herbicide tolerance on seedbank dynamics are highly uncertain without knowledge of the density dependence of fecundity. F urthermore, the sensitivity to management practices suggests that seed bank levels will be substantially more difficult to control if the eff icacy of herbicide is compromised. It is concluded that the model and Monte-Carlo approach have many potential uses in exploring the effects of management, cultivar physiology and the nature of the transgenes.