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1 Introduction

Debate

The advances in biotechnology have produced transgenic crops with tolerances towards particularly the non-selective herbicides glyphosate and glufosinate. Transgenic herbicide tolerant crops have caused public debate in especially the putative uncertainties about long-term consequences of growing herbicide tolerant oilseed rape.

Crop rotations

In Denmark, the studies about transgenic herbicide tolerant crops has focused on two crops: beets (Beta vulgaris L.) and oilseed rape (Brassica napus L.). Sugarbeet rotations occupy approx. 10% of the Danish arable (land whereas oilseed rape (winter and spring varieties) occur in crop rotations on approx. 36% of the arable land (Madsen et al., 1997).

Sugarbeet

The currently used weed control strategy in sugarbeet involves a mixture of herbicides (phenmedipham, ethofumesate, metamitron, chloridazon etc.) to control dicotyledonous weeds. The current weed control strategy normally requires several applications with high doses (2000–7000 g a.i. ha^-1) of various herbicides which is reflected in a high treatment frequency of 1.94-2.69 year^-1 (Miljøstyrelsen, 1996; 1997). Introduction of herbicide tolerant varieties of sugarbeet, which will allow for use of more potent herbicides, are therefore likely to reduce herbicide use in the short-term.

Oilseed rape

Only few herbicides for dicotyledonous (dicot) weed control are presently available in oilseed rape. Propyzamide, which only controls a few dicot weed species, is used in winter varieties and napropamide and clopyralid (selective mainly against Asteraceae species) are used in spring varieties. This leaves, in particular, winter oilseed rape prone to dicot weed infestations. Growers are presently experiencing oilseed rape fields severely infested with Sinapis arvensis L., Raphanus raphanistrum L. and Capsella bursa-pastoris (L.) Medicus (Kristensen, 1997). Therefore, the introduction of herbicide tolerant oilseed rape may be of benefit to Danish farmers in the short-term.

Concerns

Both Beta vulgaris and Brassica napus hybridize within the species as well as with wild relatives. For sugarbeet these problems seem to be restricted to succeeding sugarbeet fields, but oilseed rape can, in particular, be a problematic volunteer in succeeding crops (Lutman, 1993). Furthermore, the transgenic oilseed rape can pollinate other conventional oilseed rape fields, or oilseed rape can hybridize with the related weed species Brassica campestris L. (Jørgensen & Andersen, 1994; Mikkelsen et al., 1996). The questions are: Will volunteers and hybrids with wild relatives become a more severe agronomic or environmental problem when the herbicide tolerant crop is grown? And will the amount of herbicide used in a rotation with transgenic herbicide tolerant crops decrease or increase compared with the same rotation with non-transgenic crops? These questions can only be addressed experimentally by costly perennial experiments.

Objective