Feral and volunteer crop populations in the arable environment

Not all the seed from crop plants is harvested; many seeds are lost, either falling to the ground within the field or dispersed by machinery, birds, etc. to end up beyond the field margins. In some cases the seeds survive in the seedbank giving rise to volunteer populations within subsequent crops or feral populations outside of the cropped area.  The persistence and spread of volunteer and feral populations can lead to significant weed problems while providing a bridge for the dispersal and escape of traits present in cultivated populations.

Volunteer oilseed rape

In the cultivation of oilseed rape (Brassica napus L.), large numbers of seed are shed and fall to the ground before and during harvest. Given the right environmental conditions a proportion of these seeds will become dormant and enter the seedbank, emerging later if subjected to appropriate germination triggers. This has led to the presence of volunteer weed populations within arable fields and to persistent seed bank populations.

Volunteer plants can emerge in large numbers and as a single plant can produce 10,000 seeds or more, impurity of yield may be substantial. Despite this, volunteers have not generally posed a major problem. Their presence in cereal crops has been readily managed by the application of selective herbicides and their contamination of oilseed rape crops has been tolerated as oil quality and general varietal type of crop and volunteer are similar. However, this situation has changed with the introduction of varieties with unique oil characteristics and with the potential commercial licensing of genetically modified (GM) varieties. Under such conditions, the mixing of varieties resulting from the presence of volunteers will have economic and environmental implications.

The question is whether volunteer oilseed rape can persist for long enough at sufficient density to cause a significant problem. To understand and predict the impurity caused by volunteers in the seed bank, crop and harvested yield, a population dynamic model was developed in which the within-field dynamics of individual oilseed rape cohorts were simulated.

Analysis of the model showed that volunteer populations invariably became extinct in the absence of further cropping; but without careful control the populations could persist for many years and that the rapid suppression (within three years) of populations to levels set, for example by GM labelling legislation, would require stringent control of volunteer populations.

Feral alfalfa

Medicago sativa (alfalfa) is an important forage crop in North America which is also found along road sides and in other natural and semi-natural habitats. However, little information is available on the establishment capabilities ofPhotograph of a feral alfalfa population alfalfa in non-cultivated areas and the potential of these founding populations to become feral. Preliminary results reveal that weedy alfalfa populations are reproductively successful in road verges and that they can effectively establish self-perpetuating feral populations.

In collaboration with Muthu Bagavathiannan of the University of Manitoba and Rene Van Acker of the University of Guelph a model of feral alfalfa populations has been developed. This is a stage structured matrix population model in which the dynamics are determined by the transitions between the life-cycle stages.

Analysis of the model will help us to understand whether feral alfalfa populations could become invasive weeds in the road-side habitats and help to identifying appropriate strategies for its control.

Contact: Graham Begg

Papers and reports

Bagavathiannan M.V., Gulden R.H., Begg G.S., Van Acker R.C. 2009. The Demography of Feral Alfalfa. (Medicago sativa L.) Populations Occurring in Southern Manitoba, Canada. Joint WSSA/SWSS Annual Meeting. February 9-13, 2009.

Bagavathiannan, M.V., Van Acker, R.C. 2008. Crop ferality: Implications for novel trait confinement. Agriculture, Ecosystems and Environment 127, 1-6.

Begg, G.S., Hockaday, S., McNicol, J.W., Askew, M., Squire, G.R. 2006. Modelling the persistence of volunteer oilseed rape (Brassica napus). Ecological Modelling 198, 195-207.

Squire, G.R., Begg, G.S., et al. 2005. Final report of the DEFRA project: Monitoring the occurrence of GM oilseed rape volunteers in subsequent oilseed rape crops, CPEC23.

Squire, G.R., Begg, G.S., Askew, M. 2003. The potential for oilseed rape feral (volunteer) weeds to cause impurities in later oilseed rape crops. Final report of the DEFRA project: Consequences for Agriculture of the Introduction of Genetically Modified Crops, RG0114.

Begg, G.S., Young, M.W., Hawes, C., McNicol, J.W., Squire, G.R. 2003. Modelling feral (volunteer) oilseed rape in relation to thresholds of impurity. Proceedings of the 1st European Conference on the Co-Existence of Genetically Modified Crops with Conventional and Organic Crops, 169-171.