“Herbicides diminish the quality of the terroir”

Jean Cao is the consultant oenologist to the Luxembourg independent winegrowers. As a graduate of the French National Institute of Higher Education in Agricultural Sciences (SupAgro) in Montpellier and Bordeaux Sciences Agro, he has a clear understanding of issues resulting from herbicide use in vineyards.


Could you remind us what glyphosate is?

Jean Cao: It is a highly soluble systemic foliar weedkiller. It is absorbed through the foliage and spread by the sap throughout the plant, ultimately causing it to wither. It is highly effective and very low-cost. Ultimately, it could be said it was a victim of its own success. 

What do we know about its risk to humans?

That’s a tough question and still subject to debate. As the research currently stands, the manufacturers (including Bayer, which produces Roundup and many other agents after the chemical entered the public domain in 2000) have failed to prove that the chemical is harmless to humans. This uncertainty has brought about the current situation (editor’s note: the International Agency for Research on Cancer, a World Health Organization agency classified glyphosate as “probably carcinogenic” in 2015).

Luxembourg is set to prohibit the use of glyphosate on 1 January 2021, but is this the end of systemic herbicides?

No. People will continue to use weedkillers made with other chemicals, some of which are more harmful and more expensive. While the glyphosate ban is a good first step, more needs to be done if we are to end the use of herbicides. It is also important for this ban to be combined with a communication campaign for producers and consumers on the role of weeds (I don’t like this term [translator’s note: the French word for “weeds” translates literally as “bad herbs”]), which can be a positive one.

Glyphosate kills weeds, but does it also affect the vines?

Absolutely – it does not distinguish between plants or choose where it goes. The dosage and the time at which it is applied are the only means of ensuring it kills weeds rather than vines. While vines become more resistant as they develop, treatment errors later in development enable the chemical to migrate towards the base of the plant, causing more serious damage.

Why are herbicides used in vineyards?

It all depends on the circumstances, but there are several key reasons: avoiding competition for nutrients, limiting water stress so weeds do not absorb the water the vine needs, etc. Weeds can also provide a focus for the spread of fungal diseases such as mildew. For winemakers seeking to eliminate everything but the vine stocks on their parcels, glyphosate is very effective – by using it, they can save time and money. 

Glyphosate does not distinguish between plants or choose where it goes.

How can wine-growers manage without it?

Instead of chemical weeding, we need to manage agrosystems more effectively, which requires special equipment, and most importantly, new working methods. Various techniques exist. Managing plant cover – whether spontaneous or sown – is one. Special tillage (editor’s note: ploughing) is another. However, this requires an investment both financially and in terms of time and labour. This is not easy and it’s not sufficient just to buy new equipment. Not all winemakers can afford to do it as everything is interlinked – more work means more workers, training, machinery, diesel, etc. In such circumstances, an impact on the price of wine is inevitable. Customers need to be told that if they buy slightly more expensive wine, they are investing in the environment. Once again, communication plays a vital role.

What do you mean by “managing plant cover”? 

It means striking a balance between soil and plants so that the whole forms an ecosystem where all elements benefit. In order to manage plant cover, you can cut, flatten or remove weeds from among the vines. However, it’s not so easy to decide when to do this. A whole host of factors must be constantly borne in mind such as ensuring that vines are productive, protecting soils from erosion, and considering repercussions in terms of water stress, disease or the risk of frost (editor’s note: since moisture is retained by vegetation, the risk of vines being affected is higher if the rows are overgrown). 

What are the benefits for vines of managed plant cover combined with soil maintenance? 

There are multiple benefits. Weeds growing around vines structure the soil, attract insects that are vital to it, and generate organic matter providing vine stocks with nutrients. A fully-fledged agrosystem is the key to living soil and countless benefits.

What is the importance of soil structure?

This is a fundamental parameter. Soil structure is characterised by porosity, which allows air and water to circulate in the earth. Without porosity, the soil and roots do not function well. Green cover is beneficial as it attracts earthworms and microorganisms that aerate the substratum. Maintenance and fertilising methods also have a direct impact on levels of biological activity. However, these factors are difficult to manage. 

Luxembourgian vineyards are often sloping. How does this affect soil management?

That’s true and consequently plant cover is the best option, especially due to issues with runoff that may result from shallow ploughing. Windows for shallow ploughing are very small for those seeking to avoid land slippage at the bottom of slopes. By managing plant cover, it is possible to optimise costs (inputs, labour, etc.), preserve the soil’s physical, chemical and biological qualities in the long term, and avoid any negative impact on the environment.

The key to producing good wine is an excellent understanding of all the systems at work in a terroir.

Are glyphosate-free vineyards necessarily healthy vineyards?

They are once they are brought into balance. Herbicides kill plants, indirectly eliminating microfauna, microorganisms, etc. They significantly reduce living organic matter (animal, plant, fungal and microbial material), which includes all active biomass (roots, earthworms and soil microflora). Once chemical weedkiller use is discontinued, it is still possible to revive soil that is virtually dead, but it takes time. Vineyards are certified organic three years after chemical weedkiller use has ceased, but is this sufficient time to cleanse the soil? It is impossible to say with any certainty.

Is it possible to practise precision wine-growing aimed at producing outstanding wine without abandoning herbicides?

Those seeking to produce exceptional products must consider every little detail. Herbicides diminish the quality of the terroir. Admittedly, the impact is only small, but all these small factors ultimately make a difference. The key to producing good wine is an excellent understanding of all the systems at work in a terroir. Armed with this understanding, it should be possible to avoid using herbicides.

In your view, is the decision to ban glyphosate a good one for Luxembourgian wine-growing?  

I know that many winemakers have already stopped using it and I think this is great! Yes, it’s a good decision. It enables us to get back to basics – the most important factor for making good wine is the soil and we must look after it. Luxembourgian terroirs are exceptionally rich and have great potential. After all, let’s not forget that the vines determine the quality of wine.


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