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Bean crop using direct seeding under cover. Experiment in Ponta Grossa (Brazil). © INRA, INRA, Stéphane de Tourdonnet

Conservation agriculture: blazing a trail through research

Weed management: a key issue

Weed management is recognised as critical to no-till systems. Cover crops offer solutions, but only if they are well managed, providing enough cover to keep weeds in check without hindering the growth of primary crops.

By Pascale Mollier, translated by Inge Laino
Updated on 02/05/2014
Published on 11/08/2013
Common field thistle in cereal plots. © INRA, SIMONIN Gérard
Common field thistle in cereal plots © INRA, SIMONIN Gérard

No-till systems heavy on herbicides

Direct-seeding agricultural systems in North and South America that use soybean, cotton and maize varieties that are tolerant of glyphosate, a non-selective herbicide, have become dependent on the herbicide, requiring farmers to steadily increase its dosage (cf section 10, expert report on herbicide-tolerant varieties).

In France, agricultural systems that use simplified cultivation techniques (SCTs) also tend to use more herbicides than systems with tillage: +0.2 herbicides for common wheat, +0.3 for durum wheat, +0.6 for barley (data Crop Practices 2011). Several studies have shown that the absence of tillage changes the weed seed bank, whose highest concentration is in the first five centimetres of soil. This fosters the growth of plants that germinate quickly, especially grasses and perennials which reproduce via plant organs such as stolons and rhizomes (eg couch grass).

Potential margins of progress

Experiments carried out by INRA in Dijon since 2000 (1) have shown that the protective strategies included in SCT systems can reduce the need for herbicides by nearly 50%. However, this may come at the expense of economic performance, since crop rotations are prolonged and diversified, topsoil is repeatedly disturbed to boost the “false seedbed” effect (2), and varieties that are more resistant to pests are chosen over those that may be more productive.

According to Nicolas Munier-Jolain, “Direct seeding systems under crop cover can remedy certain drawbacks of simplified cultivation techniques: the absence of tillage can increase the rate of predation by organisms that feed on weed seeds and reduce germination, and crop cover can prevent the germination and growth of weeds. Despite this, these systems are not widely used in France and there is a lack of references to assess them (3). That is why we are teaming up with the Chamber of Agriculture of the Côte d’Or region of Burgundy in 2014 to implement a programme to evaluate, based on multiple criteria, systems that use direct seeding under crop cover.”  Fabrice Martin-Laurent, who specialises in the biodegradation of pesticides, will lead a study in parallel to determine if the higher microbial activity of untilled soil allows for a better breakdown of glyphosate (see inset).

Farmers are also experimenting with numerous innovative ways to cut back on the use of glyphosate, for example boosting effectiveness by reducing the volume and hardness of water, choosing better crop covers and destroying crop covers by scalping and rolling. (4)

(1) Chauvel B. et al. 2011. Integrated management of weed populations. Cah Agric, vol. 20- 3,194-203.

(2) The false seedbed technique consists of lightly working the soil to promote the germination of weeds that are then eliminated.

(3) Promoters of direct seeding in France cite several advantages: (i) absence of tillage prevents the germination of weeds, (ii) possibility of using frost-sensitive cover crops, such as niger, which die in freezing temperatures and therefore do not require the use of herbicides, (iii) mulch from previous cover crop residue promotes a healthy and balanced fauna (eg beetles that thrive on slugs and the seeds that produce weeds).

(4) See report on glyphosate, TCS n°62, March-May 2011.

Scientific contact(s):

Associated Division(s):
Environment and Agronomy, Plant Health and Environment, Plant Biology and Breeding
Associated Centre(s):

Glyphosate in water

Glyphosate is found in more than 30% of water tests, surface water and groundwater alike. One of its metabolites, AMPA (aminomethylphosphonic acid), is present in more than 60% of samples tested (1). In addition to its effect on plants, some studies show that the herbicide also has negative effects on micro-organisms in soil, fauna, flora, and human health. “We want to use the biodegradation of glyphosate as an indicator of the “filtering” function provided by the micro-flora of soil”, explains Fabrice Martin-Laurent. Measuring the potentially detrimental affects of glyphosate on soil microcosms incubated in labs will give scientists a fairly accurate idea of the purifying capacity of soils, and allow them to make comparisons between crop-cover systems and systems that use tillage.

(1) Source: General Commission on Sustainable Development - Observation and Statistics Department, Figures & Statistics, n° 436, July 2013.