Optimal weed management in crop rotations: incorporating economics is crucial

Although the effects of crop rotation sequence and length on weed population dynamics have been studied, it is not clear whether or not the best strategy, from a weed population dynamics point of view, is also the economic optimal strategy. It is also not clear which biological and economic parameters are most important in determining this optimal strategy. We use a density-dependent periodic matrix model, integrated with an economic model, to study the effect of rotation length and weed control on the average annual net profit (ANP). The system of the weed Persicaria maculosa in carrots (crop A) and spring wheat (crop B) is used as an example case study. The bio-economic model shows that income is roughly constant with rotation length, apart from the shortest rotation sequence. These results are in agreement with a previous model study, which was restricted to biological dynamics. However, where the purely biological model suggests that weeding effort should be focussed on spring wheat years in the rotation, our bio-economic model shows that this is not a viable economic strategy. In fact, the mean ANP over a rotation is mainly determined by the ANP in carrot years (where the gross margin is high) and depends on the balance between the increase in seed population density in carrot years and its decrease in wheat years, in combination with weeding costs. The model can easily be extended to incorporate other damaging organisms, making the model broadly suitable for analysing a range of weed management strategies from an economic perspective