Herbicide-tolerant varieties (HTVs) are crop varieties that have been bred to be intentionally tolerant to herbicides. Their use may have consequences on the diversity of weed flora, i.e. the wild plants found among crops and targeted by herbicide treatments, but also on any non-target flora in the margins around fields. To measure these effects, scientists from ANSES's Plant Health Laboratory and INRAE's Agroecology joint research unit compared weed diversity and farming practices by looking at HTV sunflower crops on the one hand, and traditional varieties grown according to conventional or organic farming on the other. Their study was recently published in the journal Weed Research. In particular, they examined the effects of these cultivation methods on ragweed, a plant whose presence is problematic not only for crop yields but also for health, as it causes allergies in humans.
The study showed that HTV crops had the lowest diversity of weed species within the fields, while organic crops with traditional varieties had the most diversity.
What are HTVs?
Herbicide-tolerant varieties (HTVs) are plant varieties that have been bred for their genetic ability to resist herbicides applied to crops. This resistance may have been acquired naturally or through mutagenesis. When growing traditional varieties of sunflowers, chemical control of weeds of the Asteraceae family (which includes ragweed) is complicated by the fact that the sunflower also belongs to the Asteraceae family and is therefore susceptible to the same herbicides. Before HTVs were authorised, chemical weed control of sunflower crops relied solely on pre-emergence control (i.e. treatment before the crop emerged). Since 2010, France has authorised the use of certain sunflower varieties made tolerant to two herbicidal active substances, imazamox and tribenuron-methyl. In 2017, 27% of all land planted with sunflowers was sown with HTVs. These allow post-emergence weedkillers to be used, supposedly leading to better control of Asteraceae without any impact on the crop.
However, as these chemicals are also used in other crops, their use in HTV sunflowers increases the risk of weed resistance, especially in ragweed.
Wider use of herbicides
The difference in weed diversity within fields does not seem to be directly related to the varieties used but is rather due to different farming practices, explains Guillaume Fried, project leader in the Entomology and Invasive Plants Unit of the Plant Health Laboratory: "Farmers growing HTVs apply more herbicides to their crops. On average, they use 1.5 full doses of treatment per year, whereas those growing traditional varieties in conventional agriculture use 1.25. It may seem like a small difference, but it is enough to explain the reduction in biodiversity". Although weed management is necessary for agricultural production, losing too much diversity can be a concern: "Weeds provide useful services, for example they encourage beneficial insects," points out the scientist. These include insects that help to control crop pests.
The cultivation method does not affect the field margins
While farming practices influence plant biodiversity within the fields themselves, this seems to be less true of the margins. Plant diversity here is determined by other factors, such as soil, climate and landscape. For example, the presence of hedgerows or grassland nearby will promote plant diversity in the field margins. "However, the study should be repeated in five or ten years' time to see whether there is any as-yet undetected cumulative effect from growing HTVs," continues Guillaume Fried. The scientist conducted an earlier study on 500 fields throughout France, which showed that herbicides had a negative effect, even in the field margins.
The importance of preventive measures against ragweed
The scientists also found that there was as much or even more ragweed in HTV crops as in traditional crops. "We conducted the study in three départements: Isère, where ragweed is common, Cher, where it is moderately present, and Côte d'Or, where there is little ragweed. Our surveys did not show any apparent effectiveness of HTVs in controlling ragweed, whose abundance depended more on the geographical area or the frequency of sunflower planting in the crop rotation than on the type of variety used." This could be explained by the fact that farmers growing traditional varieties in conventional or organic farming diversify their rotations more: they do not generally grow the same species from one rotation to the next, whereas those with HTVs tend to grow mainly sunflower, a plant that favours the establishment of ragweed.