The costs of human-induced evolution in an agricultural system

Abstract

Pesticides have underpinned significant improvements in global food security, albeit with associated environmental costs. Currently, the yield benefits of pesticides are threatened as overuse has led to wide-scale evolution of resistance. Yet despite this threat, there are no large-scale estimates of crop yield losses or economic costs due to resistance. Here, we combine national-scale density and resistance data for the weed Alopecurus myosuroides (black-grass) with crop yield maps and a new economic model to estimate that the annual cost of resistance in England is £0.4bn in lost gross profit (2014 prices), and annual wheat yield loss due to resistance is 0.8 million tonnes. A total loss of herbicide control against black-grass would cost £1bn and 3.4 million tonnes of lost wheat yield annually. Worldwide, there are 253 herbicide-resistant weeds, so the global impact of resistance could be enormous. Our research provides an urgent case for national-scale planning to combat further evolution of resistance, and an incentive for policies focused on increasing yields through more sustainable food-production systems rather than relying so heavily on herbicides.

Fig. 1. Estimating yield penalties using black-grass density and winter wheat yield data.

a, The average effect of black-grass density on the yield of winter wheat. Black points are model-estimated average yields, bars show 95% confidence intervals generated from 10,000 parametric bootstrap re-samples (some confidence intervals are narrow enough to be obscured by the point; all values and confidence intervals given in Supplementary Table 2). Grey points show observed yield for each 20 x 20 m plot from 17 fields over 4 years. See SI for individual field estimates across years. b, Average yield loss of winter wheat relative to the reference state, calculated based on yield estimates and bootstrap resamples. Reference state = low density (note the estimate for low density is fixed at 0). Percent reduction for subsequent density states as follows: medium 0 %; high 7.45 %; very high 25.60 % (Supplementary Table 2). The y-axis of (b) is reversed so that the direction of the effect of black-grass density is the same between (a) and (b). Further details in SI.

Fig. 2. Field-scale costs and yield loss due to resistant black-grass.

These estimates were generated by running empirical field management and black-grass density data (number of fields = 66) through BGRI-ECOMOD. a and b show yield loss due to resistant black-grass (YL_R, t ha^-1): a, average field-scale yield losses in winter wheat; b, maximum field-scale yield loss in winter wheat in the event of total loss of herbicide control. c – e show cost of resistance (C_R, £ ha^-1): average field-scale C_R for c, years in winter wheat crops and d, all years’ data, i.e. across a rotation; e, maximum field-scale C_R in the event of total loss of herbicide control. Fields are overlaid on a map of modelled density (square root) of Alopecurus myosuroides averaged over 2015-2017. This density map was generated by fitting a generalized additive model to the data reported in Hicks et al. (2018), with spatial covariates representing latitude and longitude.

Fig. 3. The relative contribution of herbicide costs, lost yield and operations costs to total costs in winter wheat crops.

Values are average per hectare costs estimated by running empirical field management and black-grass density data through BGRI-ECOMOD (number of fields = 66). a, Costs due to resistant black-grass plants and b, costs due to infestation. Herbicide costs consider only those herbicide applications targeting black-grass. (Error bars intentionally omitted as the purpose is to illustrate the contribution of component parts and, when data are presented in this way, error bars of individual components influence each other and are misleading).

Fig. 4. Annual impacts of herbicide resistant black-grass at regional and national scales.

a, Annual winter wheat yield losses due to resistance (YL_R). National YL_R given in million tonnes; regional figures in thousand tonnes. b, Annual economic cost of resistance (C_R) across all crops and c, in winter wheat crops. National C_R in billion GBP, regional C_R in million GBP. Figures in brackets are 95% confidence intervals. Regions are UK Government Office regions: EE East of England; SE South East; YH Yorkshire and the Humber; EM East Midlands; WM West Midlands. For each region, the mean per hectare C_R and YL_R at each black-grass density state were multiplied by the crop area estimated to have that density state. For full details of scaling-up process see Methods and SI.