Effects on weed and invertebrate abundance and diversity of herbicide management in genetically modified herbicide-tolerant winter-sown oilseed rape

Abstract

We evaluated the effects of the herbicide management associated with genetically modified herbicide-tolerant (GMHT) winter oilseed rape (WOSR) on weed and invertebrate abundance and diversity by testing the null hypotheses that there is no difference between the effects of herbicide management of GMHT WOSR and that of comparable conventional varieties. For total weeds, there were few treatment differences between GMHT and conventional cropping, but large and opposite treatment effects were observed for dicots and monocots. In the GMHT treatment, there were fewer dicots and monocots than in conventional crops. At harvest, dicot biomass and seed rain in the GMHT treatment were one-third of that in the conventional, while monocot biomass was threefold greater and monocot seed rain almost fivefold greater in the GMHT treatment than in the conventional. These differential effects persisted into the following two years of the rotation. Bees and Butterflies that forage and select for dicot weeds were less abundant in GMHT WORS management in July. Year totals for Collembola were greater under GMHT management. There were few other treatment effects on invertebrates, despite the marked effects of herbicide management on the weeds.

Figure 1

Timing of herbicide applications, percentage weed and crop cover and crop height against the number of days from sowing. Conventional (hatched boxes, dashed lines) and GMHT (open boxes, solid lines) WOSR crops. Boxes span the 25th to 75th percentiles; the line denotes the median; whiskers denote the 10th and 90th percentiles. Four successive treatments (herbicides T1, T2 and T3 and desiccants Des) were applied to conventional (n=63, 30, 6 and 41 sites for T1–3 and Des) and GHMT treatments (n=63, 18 and 41 sites for T1–2 and Des). Weed and crop cover and crop height assessed on conventional (n=44, 62, 12, 20, 59, 61, 59, 64, 43, 6) and GMHT (n=44, 63, 11, 20, 57, 62, 59, 62, 42, 6) sites. Assessments of crop height and weed cover were made infrequently over the winter (four sites between 85 and 168 days from sowing) and these data are not included.

Figure 2

Multiplicative treatment ratio, R (GMHT:conventional), for total weeds, dicots and monocots. Lines represent where the indicator is equivalent in both treatments (R=1), or where it is 50% higher (R=1.5) or 50% lower (R=0.67) in the GMHT than in the conventional.

Figure 3

Graphs of counts of total weeds, N, for dicots and monocots, for each of n sites, for samples of seedling, early spring, post-herbicide and final counts. The y-axis for each graph is the difference in count between the two treatments on a logarithmic scale: log(N_GM+1)−log(N_C+1), for which the mean value is the quantity d (table 2). The x-axis is the sum of the counts for the treatments on a logarithmic scale: log(N_GM+1)+log(N_C+1), a measure of the overall abundance per field. The equality line, R=1 (see fig. 6 of Firbank et al. 2003b), is shown for reference as the horizontal line y=0. The curve is a smooth spline fitted through the points with four d.f.