Impact of volunteer GM maize on conventional crops is low

 
Scientific data on the role of maize volunteers on cross-pollination is limited. The most detailed studies have been conducted in Spain. The EU regulation on GM food and feed sets a threshold of 0.9 per cent incidental GM content in non-GM feed and food products. Above this threshold the products must be labelled as containing GM organisms (GMO). Volunteer plants are not planted deliberately by farmers. In the case of GM maize they usually grow from cobs or cob fragments that are left after harvesting and are particularly common in temperate regions.
To comply with the EU regulation it is important to understand the effect of GM volunteers on the yield of an otherwise conventional field.
The research took place in Girona, Spain, where both GM and conventional maize is grown. Twelve fields were researched in which GM maize had been grown in 2004 and conventional maize in 2005. The distribution of volunteer GM plants was recorded and classified over three years. The study also monitored the growth of the volunteers and the level of flowering and cob production. The research was supported by the EU SIGMEA and Co-Extra projects.
The density of volunteer plants ranged from residual (less than 30 per hectare) to extremely high (above 8000 per hectare and making up almost 10 per cent of total plants). The variation depended on several factors, such as climate and the preparation of the field before sowing. For example, furrows for irrigation eliminate a large number of volunteers.
The volunteer plants tended to be defective. They rarely produced cobs and those that were produced normally had no grains. Pollen dispersion appeared to be difficult because volunteers were much shorter than normal plants. When cross-fertilisation did occur it tended to be low.
On the basis of the number and fertility of the volunteer plants in the fields the study estimated the effect of the GM volunteers on the presence of GMO in the yield of a conventional maize crop grown in the field the following year. The percentage of GMO ranged from 0.016 to 0.16 per cent, depending on the field. This is well below the 0.9 per cent threshold established by EU legislation.
However, this contribution of volunteer plants to incidental GM levels should not be ignored, especially if the initial density of volunteer plants is above 1000 per hectare. This information is particularly valuable to growers who wish to know in advance the risk of incidental GMO from volunteers.
Maize volunteers are usually easily controlled by currently applied agricultural techniques and potential accidental presence may therefore be considered negligible.