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Diversifying crop rotations in arable farming is considered as an option to regulate weeds, pests and diseases, and therefore to decrease the need for pesticides. However, the potential for reducing the reliance on pesticide through crop diversification had, so far, not been quantified at the cropping systems scale. In this study, we analysed this relationship from 1285 cropping systems described from 795 arable farms of the French DEPHY network. Crop diversity was assessed in each cropping system through the number of crops grown in the pluriannual crop sequence. The reliance on pesticides was measured with the Treatment Frequency Index (TFI), averaged over the same crop sequence. The relationship between TFI and the number of crops displayed a bell-shape curve, with maximum pesticide use for intermediate number of crops. This relationship was strongly driven by the nature of crops grown which varies with crop diversity. Indeed, monocultures were always based on maize, a crop with a rather limited reliance on pesticides in France. Two-crop rotations were often based on maize and wheat (a crop with an intermediate-high reliance). Three-crop rotations introduced oilseed rape (a crop with a higher reliance) while four-crop sequences introduced sugar beet (a crop with a high reliance) and potato (the crop with the highest reliance). On the other hand, the proportion of crops with low to very low reliance on pesticides (such as oat, buckwheat, and temporary grasslands) increased in cropping systems with higher diversity. Since the reliance on pesticides varies drastically across crops (examples of average TFI: 13.5 for potato, 5.5 for sugar beet, 4.8 for oilseed rape, 3.3 for soft wheat, 1.8 for sunflower and maize, 0.6 for alfalfa and 0.4 for buckwheat), the nature of crops grown is a major factor explaining 37% of TFI variance over the whole data set.
We removed the effect of the nature of crop grown by computing the difference between the actual cropping system TFI and the ‘predicted’ system TFI (weighted average of mean crop TFIs with weight being the proportion of each crop in the cropping system). This reveals that increasing crop diversity significantly decreased pesticide use by -0.09 TFI per additional grown crop (Figure 1). Finally, crop diversity accounted for only 1% of total TFI variance. The remaining variance can be explained by many other factors not considered in this analysis, but known to drive the level of pesticide use (Lechenet et al., 2016) such as climate, soil type, pest pressure, cultivars, sowing dates, fertilisation, mechanical weeding, and strategy for decision to apply treatments.
The significant effect of crop diversity on pesticide use was lower than expected, particularly when compared to previous quantifications of the regulation of pests through crop diversity (Ratnadass et al., 2012; Kremen & Miles, 2012; Larsen & Noack, 2021). Indeed, pest regulation due to diversification can be significant, but not strong enough to impact pesticide use if the regulation does not decrease pest pressure below the thresholds applied by farmers when making the decision of treatments. Analysing pesticide use at the crop level, Guinet et al. (2023) also found significant but quite low effects of cropping system diversity, consistently with our results. However, the number of crops is probably not the best indicator of diversity regarding the impact on pest regulation and pesticide use, so further investigation is planned to compare indicators integrating the functional crop diversity.
Figure 1. Difference between observed and predicted cropping system TFI as a function of crop number.
References
Lechenet et al. (2016). Profiling farming management strategies with contrasting pesticide use in France. Agricultural Systems, 149, 40–53.
Ratnadass et al. (2012). Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: A review. Agronomy for Sustainable Development, 32(1), 273–303
Kremen & Miles (2012). Ecosystem services in biologically diversified versus conventional farming systems: benefits, externalities, and trade-offs. Ecology and Society, 17, 342
Larsen & Noack (2021). Impact of local and landscape complexity on the stability of field-level pest control. Nature Sustainability, 4, 120–128.
| Keywords | cropping system; diversification; pesticide; rotation |
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