Herbicide application pattern: In HR sugar beet cultivation, only the broad-spectrum herbicide glyphosate is applied, usually first at the post-emergence stage and second until 60 – 70% canopy closure. This makes timing more flexible and simplifies weed control (Champion et al., 2003*). In conventional agriculture, usually three to four (up to six) herbicide applications are done, with glyphosate often applied at the pre-seeding or pre-emergent stage to clear fields and postharvest for volunteer control (Märländer, 2005; Schütte and Mertens, 2010), while other herbicides are applied during crop development. With HR sugar beet cultivation, the aim is also to reduce the active ingredient (ai) amount and the number of herbicides. For the case of GMHR oilseed rape in the US and Canada, this holds true only for the first years of cultivation (Champion et al., 2003*; Benbrook, 2009*).
Similarities between sugar beet and oilseed rape rotations (break crops in cereal-dominated rotations typically grown one year in every three, four or five years) and biology (hybridisability, volunteer occurrence, wild relatives) allow changes in agricultural practice to be postulated after years of continued HR technology: a) weeds may become herbicide-tolerant through selection pressure and adaptation, especially if different HR crops resistant to glyphosate are cultivated in the same rotation (Beckie et al., 2006*; Owen and Zelaya, 2005*), b) the composition of weed communities and their seed bank will change (Heard et al., 2003a*; Owen and Zelaya, 2005*), and consequently, c) ai rates, application frequencies, and numbers of ai may increase again, particularly in low-disturbance seeding systems (Senior and Dale, 2002).
Gene flow and volunteers: Not eradicating sugar beet bolters and groundkeepers before flowering may lead to gene flow of the HR trait and to seed dispersal, although at rates far below those encountered with oilseed rape, for example, which regularly flowers and produces seeds. HR volunteers may occur in subsequent rotations when seeds of bolters and groundkeepers fall to the ground (Bartsch et al., 2003; Keller et al., 1999). HR weedy relatives or interspecific hybrids (Arnaud et al., 2003; Frese et al., 2001) may evolve due to pollen-mediated gene flow from HR bolters and HR groundkeepers. HR volunteers may also evolve in non-HR sugar beet fields due to pollen-mediated gene flow from flowering HR bolters and HR groundkeepers to neighbouring sugar beet fields, and also due to neighbouring volunteers from sugar beet seed banks (Desplanque et al., 2002; Viard et al., 2002). When neighbouring sugar beets with other HR traits are cultivated, multiple HR may develop in weed beet; selective advantage then maintains this, as evident in North America with GMHR oilseed rape (Beckie et al., 2006*; Orson, 2002*).
In sugar beet seed production areas, seed purity standards include minimum distances between fields to avoid cross-pollination (Märländer et al., 2010). Nonetheless, outcrossing into wild relatives may occur, also because the amount of GMHR sugar beet pollen is likely to be higher than that of neighbouring wild relatives.
Tillage and rotation system: GMHR sugar beet facilitates the use of enhanced crop cover and no-tillage or reduced-tillage. This, in turn, minimizes weed pressure and soil compaction, prevents soil erosion and promotes soil bioactivity (Agronomy Guide, 2010*; Thorbek and Bilde, 2004*). If HR sugar beet weeds and volunteers in the follow crops develop as a result of flowering bolters and/or groundkeepers, the necessary control may trigger more intensive tillage and/or may require wider rotations or crops with other HR traits. Since seeds may persist for years in the soil, returning to a conventional sugar beet in the crop rotation may become difficult due to HR volunteers and their admixture in the harvest.
Coexistence: Avoiding GM material presence in non-GM crop production practice may require changes in GMHR sugar beet cultivation (European Commission, 2003). Normal farming practice involves preventing flowering and thus reducing vertical gene flow to neighbouring fields. Depending on various factors, flowering bolters and/or groundkeepers can sometimes be encountered; they act as pollen donators or acceptors, producing HR seeds. Despite the potentially low rate of gene flow to avoid contamination of non-GMHR sugar beet fields and potential HR seed production, this calls for coexistence measures such as isolating fields of GM sugar beet, introducing isolation distances, and sowing and harvesting at a modified time schedule, preferably using other varieties (Schiemann, 2003*). In sugar beet seed production areas, flowering is necessary; to guarantee seed purity standards, seed companies have introduced various temporal and spatial isolation measures.