Literature on the conversion to organic farming is scattered. However, both the conversion of farmers to organic farming and of consumers to organic food are the driving forces for the development of the organic sector. In this review, we combine agricultural and social scientists’ viewpoints for a critical appraisal of literature on conversion to organic food and farming. First, a brief historical retrospective enables us to refer the scientific production to the institutional and economic context over the past decades. Secondly, we review the methods used to analyse conversion in agricultural and social sciences, and show that emphasis is most often laid upon the effects of conversion and the motivations to convert, on the basis of comparative approaches with so-called conventional agriculture. Therefore, the literature minimises the importance of transitional aspects and trajectories, and rarely approaches conversion as a longer process than its legal duration and from a wider point of view. Thirdly, we examine the paradigms of input efficiency and system redesign, which frame discussions about transitions in agriculture, beyond organics, and therefore helps shed light on sustainability issues. We suggest that analysing conversion and more generally transitions in agriculture as multidimensional issues, involving both production and social practices, entails interdisciplinary approaches and the redefinition of some central research topics.

https://hal.archives-ouvertes.fr/hal-00886434/document

Conversion to organic farming: a multidimensional research object at the crossroads of agricultural and social sciences. A review

Weed management is a primary concern of organic farmers. Crop rotation is an important potential management approach for regulating weed seed populations in the soil of organic farming systems. This research was conducted to determine the effect of three organic crop rotations on the weed seedbank during the first 6 yr of a long-term cropping systems experiment at Beltsville, MD. The rotations consisted of (i) a 2-yr corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation, (ii) a 3-yr corn-soybean-wheat (Triticum aestivum L.) rotation, and (iii) a 4-yr corn-soybean-wheat-red clover (Trifolium pratense L.)/orchardgrass (Dactylis glomerata L.) hay rotation. Weed seed populations were determined by a greenhouse emergence assay using soil samples taken in the early spring of each year. The seedbanks of smooth pigweed (Amaranthus hybridus L.) and common lambsquarters (Chenopodium album L.) preceding corn were usually lower following the hay years of the 4-yr rotation or the wheat year of the 3-yr rotation than following the soybean year of the 2-yr rotation. However, annual grass seedbanks preceding corn tended to be higher following the hay years of the 4-yr rotation than following the wheat year of the 3-yr rotation or the soybean year of the 2-yr rotation. Seedbanks in the 3- and 4-yr rotations were similar to those of the 2-yr corn-soybean rotation (higher smooth pigweed and common lambsquarters and lower annual grass) when these longer rotations began with a corn-soybean sequence than with other sequences. Sequences beginning with hay had lower smooth pigweed and common lambsquarters seedbank populations than all other sequences. The seedbank in spring significantly predicted weed abundance at maturity in corn in at least 2 of 4 yr for all species. Results show that longer rotations with more phenologically diverse crops can reduce seedbank populations and abundance of important annual broadleaf weed species in organic production systems.

Weed Seedbank Dynamics in Three Organic Farming Crop Rotations

Summary

All of the arable land of a farm in southern Germany was investigated during an 8-year period, starting 2 years before and ending 6 years after the change to organic farming. The first 3 years after the conversion, the total seed number in the soil increased from 4050 to 17 320 m−2. From the fourth to the sixth year, it dropped back to 10 220 m−2. The number of seeds increased particularly at sites with a low crop cover and a high density of weed plants at the soil surface. The increase predominantly occurring on the more fertile soils may have been caused by the rotation commencing with less competitive crops. Crops which increased the seedbank by 30–40% were winter cereals, sunflowers and lupins. Potatoes and sown fallow caused no significant change and grass–clover mixtures even reduced the number of seeds by 39%. Among 44 species occurring frequently enough for statistical analysis, 31 increased and only 3 decreased. The change of management particularly increased summer annual, perennial and dicotyledonous weeds. This can be attributed to both operations which are characteristic of organic farming (e.g. replacing herbicide applications by mechanical weed control) and to general modifications of the management practice which may also occur in non-organic farming systems (increasing the percentage of broad leaved and spring sown crops in crop rotation). The present study confirms investigations into the aboveground vegetation that indicate that arable organic farming favours plant species diversity and provides evidence that the conversion need not encourage the dominance of a few noxious weeds.

Development of arable weed seedbanks during the 6 years after the change from conventional to organic farming

Dynamics of weed populations in arable fields are influenced by environmental and soil characteristics and also by cropping system and management practices. Manipulation of cropping systems to improve weed management requires a better understanding of the spatial and temporal dynamics of weeds, seed losses and seed production. To assess the effect of different cropping systems and various crop rotations on the weed population and seed bank, we conducted a field study at the experimental farm in Khorasan Agricultural Research Center, Mashhad, Iran. The experimental design was a split plot with 3 crop rotations as the main plots, and five cropping systems consisting of high-input, medium-input, low-input, organic and integrated systems applied to the sub-plots. Our results show that weed seed densities in organic and integrated cropping systems, of about 5000-6000 seeds/m 2 were higher than conventional and high-input cropping systems showing about 2000 seeds/m 2 . Weed seed density in continuous winter wheat of approximately 6300 seeds/m 2 was higher than other rotations with about 5000 seeds/m 2 . Weed composition in the high-input system was 11 species with 66 plants/m 2 . Whereas in the low-input and organic systems, the weed populations were 15 and 13 species with 145 and 220 plants/m 2 , respectively. Changes in weed seed bank density and species composition often occur when crop management practices and crop rotations are altered. For example, continuous winter wheat fields showed more annual grass weeds, but broadleaf weeds were more abundant in sugar beet-winter wheat rotation. The weed population in continuous winter wheat plots comprised 90% grass and sedge weeds, while in sugar beet-wheat rotation, it was only 43% of total weed density. Broadleaf weeds were 55.2% in sugar beet-winter wheat but 9.4% of total weed density in continuous winter wheat. Different rotations that include crops with different life cycles such as winter wheat-maize and winter wheat-sugar beet could lead to additional benefits of reducing the weed seed bank. conventional agriculture / ecological farming / integrated crop management / low-input crop production / organic farming

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Effect of cropping systems and crop rotations on weeds

Agriculture provides the most essential service to mankind, as production of crops in sufficient amounts is necessary for food security and livelihood. This chapter examines the question of whether organic agriculture can produce enough food to meet future demand. This question relates to a moral imperative and any evaluation must therefore be based on objective scientific facts excluding ideological bias, political correctness, economic incentives or environmental opinions. The chapter begins by defining the conditions necessary for a stringent evaluation of crop yields and explains potential pitfalls. Yield data from national statistics, organic and conventional long-term experiments and comparative studies are then compiled and evaluated, followed by a discussion of the main factors behind low-yielding production. In a global perspective, the scientific literature shows that organic yields are between 25 and 50% lower than conventional yields, depending on whether the organic system has access to animal manure. The amount of manure available on organic farms is usually not sufficient to produce similar crop yields as in conventional systems and therefore green manures are commonly used. However, organic crop yields reported for rotations with green manure require correction for years without crop export from the field, which reduces average yield over the crop rotation. When organic yields are similar to those in conventional production, nutrient input through manure is usually higher than nutrient addition in conventional agriculture, but such high inputs are usually only possible through transfer of large amounts of manure from conventional to organic production. The main factors limiting organic yields are lower nutrient availability, poorer weed control and limited possibilities to improve the nutrient status of infertile soils. It is thus very likely that the rules that actually define organic agriculture, i.e. exclusive use of manures and untreated minerals, greatly limit the potential to increase yields. Our analysis of some yield-related statements repeatedly used by advocates of organic agriculture reached the following conclusions: Organic manure is a severely limited resource, unavailable in quantities sufficient for sustaining high crop yields; legumes are not a free and environmentally sound N source that can replace inorganic fertilisers throughout; and low native soil fertility cannot be overcome with local inputs and untreated minerals alone.

/pdf/can-organic-crop-production-feed-the-world-4aapj1y0fl.pdf

Can organic crop production feed the world

The modification of the weed seed bank in the top soil (0–20 cm) during a six-year period including a sequence of annual crops and a perennial grass-clover ley in the rotation, were analysed in an organic cropping system after conversion to organic farming. The seed bank of dicotyledonous annual species varied from a minimum of 7200 seeds m−2, the year following a three-year period with perennial ley, to a maximum of 17600 seeds m−2, the year following a three-year period with annual crops. The number of species in the seed bank was essentially constant at 18–21 during the rotation, but the number of emerged species in the field decreased from about 19–20 during the annual crop period to a minimum of eight in third year ley, indicating a reduced input of seeds to the seed bank during the ley period. Dicotyledonous perennial weed seeds in the soil constituted only 2–3% of the seed bank. The correspondence between the soil seed bank and emerged weed plants was not straightforward. For Capsella burs...

https://www.tandfonline.com/doi/pdf/10.1080/01448765.2001.9754910

H. Sjursen

Papers

Change of the Weed Seed Bank during the First Complete Six-Course Crop Rotation after Conversion from Conventional to Organic Farming