In the third episode of the Radio Carbone podcast, we look at the management of slugs and small rodents. In Conservation Agriculture, reduced tillage and the presence of surface residues can sometimes lead to their proliferation.
We provide an answer to Nicolas, a farmer in Eure-et-Loir (28) involved in the Soil Capital carbon programme, who has noticed significant damage to his oilseed rape and pea crops, as well as to his plant cover. He is wondering how to limit the pressure of these pests without having to intensify tillage or increase his consumption of plant protection products.
According to Benoît Chorro, agronomist and trainer at Icosystème, the management of slugs and small rodents must be multifactorial, combining levers at the level of the cropping system and technical itineraries.
To limit slug pressure, Benoit Chorro insists first and foremost on balancing the soil ration throughout the rotation. Straw cereals create a favourable context for slug development, as the straw constitutes a high-carbon environment that will encourage these pests to turn to living biomass (crop or plant cover) with a lower C/N ratio. Incorporating legumes or exchanging straw for manure are therefore effective ways of reducing the C/N ratio of residues and speeding up the transformation of carbon-rich organic matter.
From a technical point of view, Benoit Chorro starts by talking about straw and chaff management. When they are shredded, it's important to spread them out evenly and, if necessary, mulch them lightly to speed up the breakdown of the straw if the soil isn't able to assimilate it quickly enough. Mulching, like straw harrowing, is generally effective in controlling slugs (and small rodents), but also disturbs their natural predators (carabid beetles, staphylins), whose population may take several years to grow to a natural regulatory balance.
Benoit Chorro goes on to talk about changing sowing dates. As the early stages of crop development are the most sensitive, sowing at a time that favours vegetative growth can reduce the damage caused. In autumn, bringing the sowing date forward a little enables cereals to reach less susceptible stages more quickly than when sown later in the season, when cold conditions slow growth. In spring, the tendency is to delay sowing to ensure a rapid start-up in warmer, more fertile conditions.
In oilseed rape, localised fertilisation can be applied at sowing to get the crop off to a fast start. Benoit Chorro also mentions planting companion plants (broad beans, clovers, vetches, vetch, nyger, camelina, etc.) with oilseed rape, which can in some cases divert some of the slugs to these service plants.
Without tillage, the only way to prevent an outbreak of rodents is to encourage their natural predators (birds of prey, foxes). In the plots, shredding the straw gives birds of prey better access to the soil, as high straw could injure them. Benoit Chorro advises setting up perches around the plots so that the birds of prey can rest and watch for prey.
For Benoît Chorro, the impact of slugs on crop emergence can therefore be limited without intensifying tillage, by combining a series of preventive measures: rebalancing the C/N ratio of residues in the rotation, setting up conditions conducive to rapid crop start-up (changing sowing dates, localised fertilisation) and inserting companion plants that can act as decoys (e.g. combined oilseed rape). To counter outbreaks of small rodents, alternative methods to tillage also exist, based on the establishment of conditions favorable to their natural predation (shredding of straw, installation of perches).
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