Flora in field margins: benefits and risks that depend on agricultural practices

This study showed how the use of plant protection products and fertilisers reduces the positive effects of field margin vegetation and increases the proportion of plants that are potentially harmful to agriculture.  “This was the first time that such a research project was undertaken at national level. Over a five-year period, we studied 458 plots spread out across mainland France and Corsica”, explains Léa Genty, a postdoctoral fellow in the Entomology and Botany Unit of ANSES’s Plant Health Laboratory and the main author of the study. The project focused on three main types of crops: cereals, grapevines and market garden crops. 

Multiple benefits as well as risks to agriculture

Vegetation growing in the margins of fields provides multiple benefits, of which the scientists involved in the project studied three types. The supply of food resources for pollinating insects was assessed based on the diversity of plant species and the relative abundance and morphological diversity of flowering plants. The role of field margins in reducing soil erosion was also studied, by measuring the proportion of plants present all year round, particularly in autumn and winter when rainfall is highest. Lastly, the scientists studied the conservation of less common plants in agricultural areas, which may benefit from field margins. These plants include rare, non-harmful weeds (i.e. plants capable of growing among crops), found in less than 10% of agricultural plots, as well as grassland species, which are more similar to the plants observed in natural environments. However, field margins can also harbour potentially problematic weed species, which may compete with cultivated plants in plots, leading to reduced production. Therefore, the scientists also measured the frequency of occurrence of these species in the field margins surrounding each crop. 

The use of herbicides has the greatest negative impact on the benefits to be reaped

The results show that for all crops, the use of herbicides to control weeds within the adjacent plot is the factor with the greatest negative impact, even though field margin vegetation is not directly targeted by these treatments. Not only does this use reduce all the benefits provided, but it also increases the proportion of problematic weeds for crops within field margins. Similarly, the use of fertilisers increases the frequency of problematic weeds in these margins, while reducing the occurrence of species associated with more natural environments. 

“We might have expected the use of herbicides to reduce the number of problematic plants, but  actually these results weren’t all that surprising”, explains Genty. “Indirect exposure of field margins to herbicide and fertiliser practices in the adjacent plot fosters the establishment of herbicide-tolerant weeds that benefit from nutrient inputs. These plants are more competitive and pose a greater risk to agriculture”. 

The use of fungicides and insecticides is also detrimental to the rarer plant species found along field margins, as well as to floral diversity.

Practices that should be planned at both plot and municipal levels

Agricultural practices within municipalities are also important for preserving the benefits of field margins. For example, if the surrounding landscape consists of numerous agricultural plots where large amounts of herbicides and fertilisers are used, field margin vegetation is less diverse and contains a higher proportion of problematic weeds. These effects are not offset by less intensive practices at plot level. 

Lastly, the width of field margins is also decisive. The wider they are, the more the plant community is diverse and resembles that in natural environments. Increasing the width of field margins could therefore help to reconcile agricultural production with the preservation of wilder ecosystem services. 

“When farmers are encouraged to adopt practices that support field margin vegetation – especially through reduced pesticide use – one of their concerns is that these measures may be detrimental to their crops. However, our study showed that instead, these practices are not only beneficial for all the ecological services in question, but they also reduce the risks to agriculture associated with field margins”, concludes Genty.

In practical terms, field margins that are mown less frequently, are sufficiently wide and are less exposed to treatments, foster beneficial vegetation that limits weeds while supporting key agricultural services such as soil protection and pollinator presence.