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Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics.

During the last decades a lot of research have been made on the use of cover crops. Cover crops are grown for many purposes, but most of the resent interest have focused on their effects on nitrogen. Studies have been made on catch crops grown to catch N from the soil and prevent leaching losses to the environment and on legume green manure crops grown to improve the N supply for succeeding crops. Many of the experiments have been agronomic studies, where choise of plant species or management strategies have been tested to identify the optimal way to grow cover crops in a specific situation. Other experiments have aimed at gaining more basic understanding of the effects of catch crops or green manure crops on N dynamics. These studies include subjects as catch crop growth, root growth, N uptake and soil depletion, kill-date, N mineralisation and pre-emptive competition, and how these factors interact with soil, climatic conditions, and the main crops in the cropping system, both in the short term and in the longer term. Together, the results from these studies have given a more comprehensive understanding of the mechanisms by which a catch crop or a green manure affect N leaching losses and N supply for succeeding crops. The principles governing the effect of catch crops on N supply for succeeding crops have been found to differ basically from the effects N effects of added organic matter. This is mainly due to the fact that a catch crop do not add N to the soil, the N which is incorporated with the catch crop has first been taken from the soil.

In the review, we discuss this new knowledge of catch crops and green manures, and how it helps us to understand why the effects obtained by catch crops are so variable. We also discuss how it can be used to develop strategies which will improve the results we obtain from catch crops and green manures, and to make them more predictable.

Many studies have been made on other effects of cover crops, on soil borne diseases, pests, weeds, soil structure, erosion, soil biology, and other nutrients than N. Though there are many studies, they are scattered over a large number of themes, and research in cover crop effects in most of these themes can be said to be at a very early stage. However, many very interesting effects have been observed, an there seems to be a significant potential for development of cover cropping also for other objectives than improved N husbandry.

Catch crops and green manures as biological tools in nitrogen management in temperate zones

Summary Despite the serious threat which weeds offer to organic crop production, relatively little attention has so far been paid to research on weed management in organic agriculture, an issue that is often approached from a reductionist perspective. This paper aims to outline why and how this problem should instead be tackled from a system perspective. Compared with conventional agriculture, in organic agriculture the effects of cultural practices (e.g. fertilization and direct weed control) on crop:weed interactions usually manifest themselves more slowly. It follows that weed management should be tackled in an extended time domain and needs deep integration with the other cultural practices, aiming to optimize the whole cropping system rather than weed control per se. In this respect, cover crop management is an important issue because of its implications for soil, nutrient, pest and weed management. It is stressed that direct (physical) weed control can only be successful where preventive and cultural weed management is applied to reduce weed emergence (e.g. through appropriate choice of crop sequence, tillage, smother/cover crops) and improve crop competitive ability (e.g. through appropriate choice of crop genotype, sowing/planting pattern and fertilization strategy). Two examples of system-oriented weed management systems designed for organic agriculture are illustrated as well as future perspectives and problems.

Weed management in organic agriculture: are we addressing the right issues?

Simulation of spring barley yield in different climatic zones of Northern and Central Europe: A comparison of nine crop models

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

A field experiment is being conducted which focuses on crop rotations for cereal production in organic farming. The objective of the experiment is to explore the possibilities for both short-term and long-term increases in organic cereal production through manipulation of crop-rotation design on different soil types. This paper describes the design of a rather complex experiment, and later papers will describe and discuss the results. Three factors are included in the experiment in a factorial design with two replicates: (1) fraction of grass-clover and pulses in the rotation (crop rotation); (2) catch crop (with or without catch crop or bi-cropped clover); and (3) manure (with or without animal manure applied as slurry). All fields in all rotations are represented in each year. The experimental factors are defined to allow management to be adjusted for optimization of the individual treatment combinations. This makes the systems more realistic and the results more applicable in practical farming. The exp...

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Design of an Organic Farming Crop-Rotation Experiment

Effects on weeds of management in newly converted organic crop rotations in Denmark

The possibilities for increasing total grain yield in organic cereal production through manipulation of crop rotation design were investigated in a field experiment on different soil types in Denmark from 1997 to 2000. Three experimental factors were included in the experiment in a factorial design: 1) proportion of grass-clover and pulses in the rotation, 2) catch crop (with and without), and 3) manure (with and without). Three four-course rotations were compared. Two of the rotations had one year of grass-clover as a green manure crop, either followed by spring wheat or by winter wheat. The grass-clover was replaced by winter cereals in the third rotation. Animal manure was applied as slurry in rates corresponding to 40% of the nitrogen (N) demand of the cereal crops.
	Rotational grain yields of the cereal and pulse crops were calculated by summing yields for each plot over the four years in the rotation. The rotational yields were affected by all experimental factors (rotation, manure and catch crop). However, the largest effects on both dry matter and N yields were caused by differences between sites caused by differences in soils, climate and cropping history. The rotation without a green manure crop produced the greatest total yield. Dry matter and N yields in this rotation were about 10% higher than in the rotation with a grass-clover ley in one year of four. Therefore, the yield benefits from the grass-clover ley could not compensate for the yield reduction as a result of leaving 25% of the rotation out of production. There were no differences in dry matter and N yields in grains between the rotations, where either spring or winter cereals followed the grass-clover ley. The N use efficiency for ammonium-N in the applied manure corresponded to that obtained from N in commercial fertilizer. There were only very small yield benefits from the use of catch crops. However, this may change over time as fertility builds up in the system with catch crops.

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Ilse Ankær Rasmussen

Papers

Design of an Organic Farming Crop-Rotation Experiment

Effects on weeds of management in newly converted organic crop rotations in Denmark

Whole-rotation dry matter and nitrogen grain yields from the first course of an organic farming crop rotation experiment

Agronomic considerations and dilemmas in the Danish crop rotation experiment.

Design of the Danish crop rotation experiment.