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No-till agriculture may not bring hoped-for boost in global crop yields, study finds
Direktsaat nur in trockenen Regionen von Vorteil
Davis, California, USA
October 22, 2014
No-till farming, such as used in this Illinois soybean field, shows promise in dry regions but causes lower yields in cold, moist areas like Northern Europe, a new study finds. (Paige Buck/USDA NRCS Illinois photo)
No-till farming, a key conservation agriculture strategy that avoids conventional plowing and otherwise disturbing the soil, may not bring a hoped-for boost in crop yields in much of the world, according to an extensive new meta-analysis by an international team led by the University of California, Davis.
As the core principle of conservation agriculture, no-till has been promoted worldwide in an effort to sustainably meet global food demand. But after examining results from 610 peer-reviewed studies, the researchers found that no-till often leads to yield declines compared to conventional tillage systems. It still shows promise for yield gains in dryland areas, however.
The landmark findings from their review are published online Oct. 22 in the journal Nature.
“The big challenge for agriculture is that we need to further increase yields but greatly reduce our environmental impacts,” said Cameron Pittelkow, who co-authored the study as a postdoctoral scholar at UC Davis and is now on the faculty of the University of Illinois. “The common assumption that no-till is going to play a large role in the sustainable intensification of agriculture doesn’t necessarily hold true, according to our research findings.”
About conservation agriculture
Conservation agriculture is currently practiced on 125 million hectares of land globally, an area nearly as big as the total U.S. cropland. Three key principles guide the concept: minimizing soil disturbance (also called no-till farming), protecting the soil with cover crops or leftover crop residue, and rotating the crops.
The goals of conservation agriculture are to improve long-term productivity, profits and food security, particularly under the threat of climate change. Because conservation agriculture avoids tillage, it is less time-consuming and can be more cost-effective than conventional farming methods.
In recent years, however, there has been some disagreement about the impact of no-till farming practices on yield.
New findings about yield
“This review was a tremendous undertaking and is probably the largest meta-analysis done in agriculture,” said co-author Bruce Linquist, a Cooperative Extension specialist at UC Davis.
After assessing more than 5,000 side-by-side observations, the researchers concluded that on average no-till negatively impacts yields at the global scale, yet several opportunities exist for more closely matching or even exceeding conventional tillage yields.
For example, yield reductions were minimized when the principles of crop rotation and residue retention were also practiced, highlighting the importance of implementing all three conservation agriculture principles as part of an integrated management system rather than no-till alone.
Moreover, when adopted in dry climates in combination with the other two principles of conservation agriculture, no-till farming performed significantly better than conventional tillage, likely due to the higher retention of soil moisture.
Dryland ecosystems are home to 38 percent of the world’s population, and millions of acres of land in arid regions of sub-Saharan Africa and South Asia have been identified as suitable for sustainable intensification. Yet, the authors also caution that practicing no-till in dryland areas without the implementation of the other two principles of conservation agriculture decreases yields.
In regions with moist climates and sufficient precipitation, no-till farming actually resulted in yields that were on average 6 to 9 percent lower than with conventional tillage methods.
“No one has ever stated that there would be a significant decline like this,” said Chris van Kessel, a professor of plant sciences at UC Davis and co-author of the study. “Our findings suggest that broad implementation of conservation agriculture may not be warranted in all areas, particularly where residue retention and crop rotation practices are hard to implement.”
Other co-authors are Xinqiang Liang of Zhejiang University, China; Mark E. Lundy of the UC Davis Department of Plant Sciences; Kees Jan van Groenigen and Natasja van Gestel, both of Northern Arizona University; Johan Six and Juhwan Lee, both of the Swiss Federal Institute of Technology, Switzerland; and Rodney T. Venterea of the U.S. Dept. of Agriculture-Agricultural Research Service.
About UC Davis
UC Davis is a global community of individuals united to better humanity and our natural world while seeking solutions to some of our most pressing challenges. Located near the California state capital, UC Davis has more than 34,000 students, and the full-time equivalent of 4,100 faculty and other academics and 17,400 staff. The campus has an annual research budget of over $750 million, a comprehensive health system and about two dozen specialized research centers. The university offers interdisciplinary graduate study and 99 undergraduate majors in four colleges and six professional schools.
Zurich, Switzerland
October 23, 2014
Scientists took a closer look at a much-discussed agricultural method, conservation agriculture, and examined its productivity. In a large-scale overview study, they paint a different picture of this less tillage intense farming method: high yields can only be achieved with the method in dry regions.
Conservation agriculture is practised even in subsistence agriculture in developing countries, like here in Malawi – albeit by hand. (Photo: Johan Six / ETH Zurich)
While Swiss farmers tend to till their fields before planting a new crop, a large proportion of their South American colleagues have stopped this practice. Instead, these farmers practise no-till farming: they leave the residues and stubbles from the previous season in the fields and use a special machine to drill a hole through the residue layer into the soil to plant the new crop. No-till farming is part of a farming approach known as conservation agriculture. Because it halts tilling, conservation agriculture is less time-consuming and can be more cost-effective than conventional farming methods. There has been some disagreement about the impact on yields, however. Some earlier studies showed that conservation agriculture resulted in lower yields.
An international team of researchers led by the University of California Davis and including ETH scientists has now examined the yields achieved through conservation agriculture in a major overview study. The scientists analysed data from several hundred field trials reported in the scientific literature.
Increased yield in dry regions
The team concluded that in moist climate regions with sufficient precipitation, no-till actually results in lower yields. Yields in such regions are six to nine percent lower on average than with conventional farming methods. The situation is different in dry regions: if the entire range of conservation agriculture measures is implemented, average yield gains of seven percent were found.
In addition to stop with tilling, the conservation agriculture measures include leaving the residues from the previous crop in the field as well as planting different agricultural crops sequentially in a field, i.e. crop rotation rather than monoculture.
Less erosion, greater soil moisture content
One of the greatest benefits of conservation agriculture is that it lowers susceptibility to erosion. This is especially important in regions with extremely heavy rainfalls. Heavy rainfalls affect ploughed fields without plants much more severely than fields in which the residues from the prior season is still in the ground. Soil in which the residues from previous crop seasons remain can also store more water. In no-till farming, farmers use more efficiently the available water, especially during dry periods.
But there are also drawbacks: tilling also ploughs weeds into the ground. No-till farming skips this step, which means more herbicides are needed. Moreover, pests such as fungi and snails are more plentiful and in some cases have to be treated with chemical pesticides.
Disadvantage to feeding crop stubble to animals
As the research found, the three aforementioned components are key to the productivity of conservation agriculture. “In many places in Africa, farmers let livestock graze livestock on the residues left after harvest,” says Johan Six, Professor for Sustainable Agroecosystems, member of the World Food System Centre at ETH Zurich and co-author of the study. If residues are not maintained in the field, the yield of conservation agriculture is similar to the level of conventional farming methods in dry areas.
“The crop residues are particularly important to ensure that the soil moisture is not lost through evaporation, but stays in the soil. We attribute the higher yields of conservation agriculture in dry regions to this reduced soil evaporation effect”, says Six. Where vegetation residue is not left in fields in dry regions, he explains, an important advantage of conservation agriculture is nullified.
If farmers not only fail to leave crop residues but also do not rotate crops, the yield for no-till farming decreases by ten percent or more in both dry and moist regions compared to conventional methods. “This is largely due to the fact that the pressure from plant diseases and pests is much greater in monocultures than in crop-rotated fields,” says Juhwan Lee, post-doc in Six's group.
Too much moisture in moist regions
Why does conservation agriculture lead to lower yields compared to conventional farming in moist and cold regions such as northern Europe? Six surmises that the soils there retain too much moisture, the fields are more difficult to farm with the heavy machinery early in the growing season and therefore the growing season is shortened, to a degree that it becomes too short to achieve high yields.
Conservation agriculture is primarily practised in North and South America. The largest tracts of land in absolute terms that are farmed according to these principles are in the US. A fifth of all agricultural land there is farmed this way. In Brazil more than fifty percent of agricultural land employs no-till farming. In Europe, the method is less widespread; it is most widely used in Spain, where it is used on roughly a tenth of agricultural lands.
Literature reference
Pittelkow CM, Liang X, Linquist BA, van Groenigen KJ, Lee J, Lundy ME, van Gestel N, Six J, Venterea RT, van Kessel C: Productivity limits and potential of the principles of conservation agriculture. Nature, online publication 22 October 2014, doi: 10.1038/nature13809
Direktsaat nur in trockenen Regionen von Vorteil
Wissenschaftler nahmen eine vieldiskutierte landwirtschaftliche Methode, die «Conservation Agriculture», genau unter die Lupe und untersuchten deren Produktivität. In einer grossangelegten Übersichtsstudie zeichnen sie ein differenziertes Bild dieser bodenschonenden Anbaumethode: Hohe Erträge lassen sich damit nur in trockenen Regionen erzielen.
Während Schweizer Bauern ihre Äcker in der Regel pflügen, bevor sie eine neue Feldfrucht säen, verzichtet ein grosser Teil ihrer südamerikanischen Kollegen darauf. Stattdessen pflegen diese Landwirte die sogenannte Direktsaat: Sie lassen die Stoppeln der Vorkultur auf dem Feld stehen, ziehen mit einer speziellen Maschine Schlitze in den Boden, in welche die Maschine im selben Arbeitsschritt die nachfolgende Frucht sät. Die Direktsaat ist Teil einer Anbaumethode, die auf Englisch als «Conservation Agriculture» bezeichnet wird. Weil damit auf das Pflügen verzichtet werden kann, gilt die Conservation Agriculture gegenüber der herkömmlichen Anbaupraxis als zeit- und kosteneffizienter. Umstritten ist allerdings, wie es auf der Ertragsseite aussieht. Einige frühere Studien zeigten, dass die Conservation Agriculture zu geringeren Erträgen pro Fläche führt.
Ein internationales Team von Forschern unter der Leitung der University of California in Davis und mit Beteiligung von ETH-Wissenschaftlern untersuchte nun die Erträge der Conservation Agriculture in einer grossangelegten Übersichtsstudie. Die Wissenschaftler analysierten dazu die Daten von mehreren hundert Feldversuchen anhand der wissenschaftlichen Literatur.
Ertragssteigerung in trockenen Regionen
Sie kommen zum Schluss, dass die Conservation Agriculture in feuchten Klimaregionen, wo genügend Niederschläge fallen, tatsächlich zu tieferen Erträgen führt. Die Erträge sind gegenüber der konventionellen Landwirtschaft durchschnittlich sechs bis neun Prozent tiefer. Anders sieht es in trockenen Gegenden aus: Werden alle Massnahmen der Conservation Agriculture konsequent umgesetzt, kann sogar mit einer Ertragssteigerung von durchschnittlich sieben Prozent gerechnet werden.
Zu diesen Massnahmen gehören neben dem Verzicht auf das Pflügen auch das Stehenlassen der Stoppeln der Vorkultur sowie das Bebauen eines Feldes mit unterschiedlichen Nutzpflanzen in zeitlicher Abfolge, also Fruchtfolge statt Monokultur.
Weniger Erosion, mehr Bodenfeuchtigkeit
Ein grosser Vorteil der Conservation Agriculture ist, dass ein Acker damit weniger anfällig ist für Erosion. Diese ist insbesondere in Regionen mit Starkniederschlägen ein Problem. Starkniederschläge greifen einen umgepflügten Acker ohne Pflanzenbewuchs viel stärker an als einen, auf dem die Stoppeln der Vorkultur stehen gelassen wurden. Böden, auf denen die Pflanzenreste stehen gelassen werden, können ausserdem mehr Feuchtigkeit speichern. Bei Direktsaat müssen Bauern während Trockenperioden weniger bewässern.
Allerdings gibt es auch Nachteile: Beim Pflügen wird Unkraut untergepflügt. Wird darauf verzichtet, müssen die Landwirte mehr Herbizide einsetzen. Zudem sind bei der Direktsaat Schädlinge wie Schnecken häufiger und müssen unter Umständen mit chemischen Mitteln bekämpft werden.
Nachteil bei Verfütterung der Vegetationsreste an Tiere
Wie die Analyse der Forschenden ergeben hat, sind die drei genannten Komponenten zentral für die Produktivität der Conservation Agriculture. «In Afrika lassen Bauern vielerorts ihre Nutztiere auf die Äcker, damit sie die Erntereste fressen können», sagt Johan Six, Professor für nachhaltige Agrarökosysteme, Mitglied des World Food System Center der ETH Zürich und Mitautor der Studie. Dies senkt in trockenen Regionen den Ertrag von Conservation Agriculture auf das Niveau von konventionellen Anbaumethoden.
«Die Vegetationsreste sind besonders wichtig, damit die Feuchtigkeit nicht verdunstet, sondern im Boden bleibt. Den höheren Ertrag der Conservation Agriculture in trockenen Regionen führen wir auf diesen Verdunstungsschutz zurück», so Six. Wo die Vegetationsreste in trockenen Gebieten nicht stehengelassen würden, falle ein wichtiger Vorteil der Conservation Agriculture weg.
Verzichten Bauern neben dem Stehenlassen der Reste auch auf den Fruchtwechsel, sinkt der Ertrag bei Direktsaat sowohl in trockenen als auch in feuchten Regionen gegenüber konventionellen Anbaumethoden um zehn Prozent oder mehr. «Zu einem Grossteil hat das damit zu tun, dass der Druck von Pflanzenkrankheiten und Schädlingen in Monokulturen viel grösser ist als in Fruchtfolgen», sagt Juhwan Lee, Postdoc in der Gruppe von Six.
Zuviel Feuchte in feuchten Regionen
Warum führt die Conservation Agriculture in feuchten und trockenen Regionen, wie beispielsweise im nördlichen Europa, zu tieferen Erträgen als die konventionelle Landwirtschaft? Six erklärt sich das vor allem damit, dass dort der Boden zu viel Feuchtigkeit speichert, die Felder weniger gut mit Maschinen bestellt werden können und deshalb die Wachstumsperiode zu kurz ist, um hohe Erträge zu erzielen.
Conservation Agriculture wird vor allem in Nord- und Südamerika betrieben. Die absolut gesehen grössten Landwirtschaftsflächen, die nach deren Prinzipien angebaut werden, liegen in den USA. Ein Fünftel der Ackerfläche wird dort so bestellt. In Brasilien ist es mehr als die Hälfte. In Europa ist die Direktsaat weniger stark verbreitet, am stärksten noch in Spanien, dort auf rund einem Zehntel der Landwirtschaftsfläche.
Literaturhinweis
Pittelkow CM, Liang X, Linquist BA, van Groenigen KJ, Lee J, Lundy ME, van Gestel N, Six J, Venterea RT, van Kessel C: Productivity limits and potential of the principles of conservation agriculture. Nature, Online-Publikation vom 22. Oktober 2014, doi: 10.1038/nature13809 [http://dx.doi.org/10.1038/nature13809]
More news from:
. University of California, Davis
. ETH Zurich (Swiss Federal Institute of Technology Zürich)
Website: http://www.ucdavis.edu Published: October 23, 2014 |
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