Crop rotations called into question

Why it’s time to rethink the way we approach this agricultural practice: Mathias Cougnon & Associate Editor, Pieter De Frenne (Ghent University, Belgium) discuss the recent paper, Distantly related crops are not better rotation partners for tomato by by Ingerslew and Kaplan.

Crop rotations are central to common agricultural practice and growing related crops year after year on the same patch of land is generally avoided. Farmers usually never grow potatoes after potatoes, rapeseed after rapeseed, or cabbage after rapeseed; they are discouraged from doing so by agronomists from academic institutions and via policy incentives such as the EU Common Agricultural Policy. Fig. 1, for instance, illustrates recommended crop rotations in Switzerland. Combinations of rotation partners of the same species, genus and family are predominantly coloured in red, and thus best avoided.

Fig 1
Fig. 1 | Crop rotations as recommended in Switzerland. This table can be read as a correlation table: the crop planted in year 1 (“Vorfrucht”) is mentioned at the top, the follow-up crop cultivated in year 2 (“Folgekultur”) at the left. Green colours indicate well-suited crop rotations, red denote maladapted follow-up crops. Figure from Agroscope, the Swiss centre of excellence for agricultural research.

However, phylogenetically diverse crop rotations are not necessarily a good thing, as can be seen in the data from the M0401 field trial, run since 2004 on the experimental farm of Ghent University in Melle, central Belgium. The assumption that plants used in rotation are non-hosts for the focal crop’s pests is not met in this experiment. The aim of this specific trial was to test the long-term effects of compost amendments on the soil quality and crop yield. In this trial, four crops belonging to four different families are grown every year in the following rotation: potato followed by an Italian ryegrass catch crop, fodder beet, maize and Brussels sprout. Despite this diverse rotation, the yield of the fodder beets has plummeted since the start of the trial. In particular, the biomass produced by fodder beet in plots without extra compost is a third less than it was 14 years ago (see Fig. 2). The cause of this yield decline is the fungal root rot disease by Rhizoctonia solani strain (AG 2-2 IIIB). This strain of the fungus is causing growing problems in the sugar and fodder beet production throughout Europe. In rotations that include maize and ryegrass, both of which are also host crops of Rhizoctonia, more and more problems with Rhizoctonia root rot of beets are encountered. Breeding companies are now allocating major efforts towards the development of enhanced tolerance and resistance.

Fig 2
Fig. 2 | Fodder beet yield (beet biomass) grown in a rotation of potatoes-beets-maize-Brussels sprouts without (red) or with annual compost (green). Experiment conducted at the experimental farm of Ghent University in Melle, Belgium.

Nonetheless, phylogenetically related crop rotation partners, for instance with crops of the same genus or family, are successful in some circumstances. A good example are ley arable rotations that are practised on dairy farms in western Europe: e.g. three years of grass (mostly Lolium perenne) is followed by three years of arable crops e.g. Maize-Maize-Barley with undersown grass to establish the ley for the following three years. Although literally all the crops in this rotation belong to the Poaceae family, these rotations are very successful.

Kathryn Ingerslew & Ian Kaplan (Purdue University, Indiana, USA) now look at the long-standing question of crop rotation from a new angle. In a recent paper in Journal of Applied Ecology, using a plant-soil feedback framework, they show that distantly related crops are actually not better rotation partners. Their study species is tomato, which is grown in the soil in the USA (as opposed to hydroponics in much of Europe’s glasshouses).

To address the question of rotations, they grew tomatoes in soil that was preconditioned by 36 other plant species, including plants that are closely related to tomato (for example, potatoes, aubergine and tobacco) as well as taxonomically distant compared to tomato (for instance cereals but also cucumber and peas). They found that phylogenetic relatedness of the preconditioning plant is unable to predict the performance of tomato: Cucurbitaceae, and especially cucumbers, were the plants with the most negative feedbacks on tomato. On the other hand, tomato plants grown in soil in which first plants of the Asteraceae family had grown (e.g., thistle, lettuce and sunflower) were less susceptible to aboveground insect hornworm damage. Plants from the Fabaceae (beans, peas, clover), as well as corn and sunflower come out as ideal rotation partners for tomato. The result in no measurable negative feedback on tomato growth, suppression of the performance of the tomato insect pest investigated here, and (in the case of the Fabacea) they fix nitrogen from the air to make this available to the follow-up crop.

These findings illustrate that distantly related crops are indeed not always better rotation partners. The well-designed, novel experiment by Ingerslew & Kaplan calls for more coordinated efforts to develop similar research on other common crops. As the authors write, this is indeed much needed to develop better science-based criteria for crop rotations. Such future research should consider the crop’s functional life history traits, aboveground as well as belowground interactions between rotation partners, and rotation partner disease susceptibility. Rotations are the backbone of the globe’s agriculture, and deserve more attention from applied ecologists.

Read the full article, Distantly related crops are not better rotation partners for tomato in Journal of Applied Ecology.

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