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The massive use of herbicides in agriculture is a question. But could we do without it? To find out, INRA has been experimenting for 12 years in Burgundy. The research engineer Nicolas Munier-Jolain draws up a rather positive assessment of these crop protection systems. Using some levers, the amount of herbicides used was divided by 3!

The plants weeds , more commonly called weeds are not appreciated in the agriculture. They can compete with cultivated plants (for space, access to light , etc.), favor the arrival of parasites or depreciate crops (for example, by changing the  taste of production ). As a result, they have long been eliminated with herbicides .

The use of these  phytosanitary products  is the subject of growing controversy. Because they contain chemicals such as glyphosate or aminotriazole, actively participate in the pollution of  waters  surface and groundwater. However, they can be bioaccumulated and may have effects on the  health of organisms. In the face of increasing tensions caused by the use of herbicides, the National Institute for Agricultural Research ( INRA ) wanted to know if it was possible to reduce or even eliminate their use in agriculture .

Four crop protection systems were launched 12 years ago to test alternative farming practices and their combined effects. Nicolas Munier-Jolain, a research engineer at UMR Agroecology  (Dijon), returns for Futura-Sciences on the results of the last 10 years. Reducing the use of herbicides would be  “possible” , but  “not easy to implement on  farms , because it requires profound changes within farms, but also in agricultural sectors and beyond . 

“We are lucky because we have a large number of levers of agronomy [Editor’s note: means of action] that help manage the  weed flora . But we also have bad luck because, used alone, each of them is not very effective compared to a herbicide. Our questions have been: can the combination of levers become effective enough to control weed flora with  fewer herbicides  ? Can we also optimize interactions between levers? ”  The researchers did not stop there. If the solution found is good, it must be used on a large scale. “We also asked ourselves about the economic, organizational or environmental consequences our actions may have. “

The experimental setup includes several prototypes corresponding to various combinations of management levers. The “Typical Integrated Protection (No. 4)” system combines all available levers. In particular, it has been the subject of crop diversification, plowing and mowing. Chemical herbicides were used as a last resort.

“Each weed has a fairly marked life cycle, with specific periods of seed emergence and seed dispersal. If we make  monoculture , the species adapted to the crop will be able to redevelop, produce their seeds and multiply each year. “   However, they will be disrupted, affecting their growth, if tillage dates, sowing and harvesting vary each year. This is exactly what happens with crop diversification. The different  plant varieties  cultivated from one year to the next require agricultural practices of their own.

Tillage to manage weeds

“The second lever is  tillage,  which is divided into two components. Tillage is a cultural technique whose function is to return the soil. In doing so, it buries the  seeds  of weeds in depth.   This practice also goes back to ancient seeds, but they lost viability during their stay in the ground . It will take fewer herbicides to treat the seeds that will germinate.

“False seedlings, that is to say, a superficial work of the field that contributes, among other things, to break the clumps or to refine the structure of the soil, stimulate the germination of weed seeds. “Made in intercropping, this practice allows to grow weeds before sowing, facilitating their destruction. Regularly repeating this practice makes it possible to gradually deplete the seed stock.

These few examples of levers are not exhaustive. System No. 4 was also subjected to mechanical weeding, precise selection of cultivated varieties and densification of  seedlings  on a given surface.

Many positive reports but …

All efforts in System No. 4 have yielded encouraging results. “Overall, we managed to control the  weed flora  from a technical point of view. […] The quantity of herbicides applied has been divided by 3 on average over 12 years. “ However, the reduction was more modest, on the order of 30%, in another system using neither plowing nor  weeding  mechanically, which favors the development of grasses that require an increased number of phytosanitary treatments.

Plowing or sowing requires greater use of motorized equipment, which could increase the  carbon footprint  of the systems tested. However, crops require less fertilizer (therefore less energy to manufacture them), mainly because of diversification with fixing legumes cultures of nitrogen from the air . In the end, the CO emissions balance  remains roughly the same regardless of the studied system, conventional or integrated protection. The levers used also limited pest  and disease attacks  : the amount of  pesticides spread was reduced. The ecotoxicological assessment of System No. 4 has therefore proved very good.

There remains the question of productivity. “The input- related costs are lower [Editor’s note: around 170 euros per hectare], whether for herbicides or  other pesticides . […] Unfortunately, these systems are less productive [Editor’s note: the loss of gross product would represent about 200 euros per ha]. There are two reasons to explain the decline in productivity.  For example, yields, such as  wheat , tend to decrease because crop cycles are shorter, especially following a deliberate decline in sowing dates. In addition, diversification crops seem to be less productive than cereals and  rapeseed of the reference system. These factors being known in advance, the decline in productivity was expected.

Are not alternative systems viable? “They are hard to promote to farmers right now. Being little greedy in pesticides , they can nevertheless become interesting under certain economic conditions. The current context of very high agricultural commodity prices is not conducive to integrated protection, as lower costs are more difficult to offset productivity declines. But there are public supports in the form of agri-environmental measures that can offset the differential locally. The orders of magnitudeGross losses and agri-environmental measures overlap, allowing low herbicide use systems to become economically attractive to the farmer. The major challenge for agriculture is to find ways of diversifying the economic viability of systems with few pesticides. Farmers must organize themselves for this purpose, with the help of storage agencies, since diversification is obviously dependent on the local organization of markets. “

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