Mowing grassland preserves valuable habitats, but conventional mulching mowers kill numerous animals and over-fertilize some biotopes. There are more gentle alternatives.
The idea sounds quite practical: why bother collecting and removing mown grass if you don’t want to use it as animal feed? You could also just leave it there. So-called mulching devices work according to this principle, which shred the plant material and distribute it over the surface so that it rots directly there. This not only saves work, but also fertilizer, since the nutrients are returned to the soil. And the annoying question “Where to put the grass clippings?” is no longer necessary.
Such devices are used, for example, on wasteland, in public parks and on roadsides as well as in some private gardens. However, many conservationists and ecologists are critical of this development – for two reasons. “Both the mulching itself and the nutrient boost it triggers can damage biodiversity,” says Holger Pfeffer from the Leibniz Center for Agricultural Landscape Research (ZALF) in Müncheberg, Brandenburg.
One of the sticking points is the technology used. Basically, there are two different ways to mow grassland or to shorten the vegetation. One is to cut the plants with sharp knives, such as a scythe or what is known as a bar mower. The latter usually has two rails with triangular blades that move against each other when mowing.
The other form of mowers chops off vegetation with rotating blades. This type includes rotary mowers, for example, which farmers often use to mow meadows. These devices are more efficient and robust than bar mowers and can reach a higher speed. That is why they have become increasingly popular in agriculture in recent decades. The mulching devices also usually work according to this principle and shred the clippings at the same time. And that’s where the problem lies. Because rotary mowers kill a particularly large number of animals at work.
Watch out, knives!
Rainer Oppermann knows reports from the 1960s, according to which meadows resembled a battlefield after the use of a rotary mower. “From today’s perspective, that sounds exaggerated,” says the head of the Institute for Agroecology and Biodiversity in Mannheim. But is it? To find out, he and his team began investigating the ecological consequences of mowing back in the 1990s.
The reason at the time was concern for the white storks, whose populations in Germany were declining in many places despite all conservation efforts. Could this be due to the widespread use of rotary mowers, which killed many amphibians and deprived the birds of their food base? “We first investigated this suspicion in the Federsee nature reserve in Baden-Württemberg,” Oppermann recalls. Experiments with dummy frogs on mowed meadows actually indicated that there might be something to the suspicion. The fate of the real amphibians, however, remained unclear: “There were simply so few that we could not create any statistically sound studies on the consequences of mowing.”
So he and his team drove to north-east Poland for new experiments, where in some meadows there are still between 300 and 1,200 amphibians per hectare. There, the researchers mowed areas measuring 40 by 50 meters with various devices and then used pitchforks to search the grass for amphibians.
Bar mowers proved to be by far the most amphibian-friendly devices. With a cutting height of seven centimetres, they killed or injured around ten percent of the frogs living on an area. In contrast, the rotary mower actually left a trail of devastation: the average loss rate for adult and juvenile animals was 27 percent, and in some areas even more than 40 percent. Only very young frogs, smaller than one inch, were spared more often.
The differences in the number of victims are due to the different effective areas of the devices. With the bar mower, the animals only risk their life and health if they get caught directly between the five to ten centimeter long blades. In the case of rotary mowers, whose rotating blades often cover a width of more than one meter, the danger zone is significantly larger. And surprisingly, the slower the tractor drives, the easier it is to catch the amphibians. “That’s because frogs crouch as long as possible and only jump away at the last moment,” says Rainer Oppermann. If the mower is moving slowly, they have a lot of time to hop into the mower deck.
In view of such loss rates, the use of vo n rotary mowers actually have significant consequences for the already threatened amphibian populations. According to Oppermann, the effects could be even greater than the numbers suggest. “You have to remember that common and moor frogs need three years to reach sexual maturity,” says the biologist. “So they have to survive several mowing dates before they can start a family.” And with each of these, their chances of survival decrease. The researcher therefore advocates using only bar mowers in amphibian habitats.
Alternatives exist
There are now also special “eco-mowers” for mowing roadsides, which are advertised as being particularly insect-friendly. The researchers also tested such a model, which comes from the same company as the conventional mulching head, for comparison purposes. This device does not mow deeper than ten centimeters above the ground and, thanks to specially designed blades, has a smaller attack surface than a conventional rotary mower. In addition, the shape of the blades and the largely closed underside of the eco-mower are intended to prevent insects from being torn into the mower by a dangerous suction. And finally, the material is not mulched, but sucked in from above and transported away by a sophisticated air duct.
In fact, this technique apparently saves insect lives, as the studies by Johannes Steidle and his colleagues show. In spiders, cicadas, bugs, butterflies and insect larvae, there was no longer any evidence of any loss. In the case of hymenoptera there were at least 15 percent fewer losses and in the case of dipterans 25 percent fewer losses. “In our view, investments in innovative technology therefore have great potential to effectively reduce insect decline in grassland,” says Steidle. This could be an important component in the fight against insect mortality, especially on extensively maintained roadsides that are only mowed once or twice a year.
Refuges for six-legged friends
However, animal-friendly mowing does not only have technical aspects. The »Brandenburg Insect Protection Catalogue,« which Holger Pfeffer and his team from ZALF developed together with the Senckenberg German Entomological Institute and the Eberswalde University for Sustainable Development, lists a number of other components. “It is also important, for example, not to completely mow large areas,” says the researcher. If strips of old grass remain and some areas are mowed later than others, the grassland dwellers can survive there and reclaim the mowed areas later.
On the grass Simply doing without it completely is not an option. After all, many valuable habitats, from nutrient-poor grassland to orchid meadows, were only created through agricultural use. In the past, such areas were grazed or mowed, although they did not bring particularly good yields. Mowers or nibbling animals kept these landscapes open and pushed back budding bushes and trees. A specialized flora and fauna has adapted to these conditions.
However, many traditional forms of use are no longer economical. Meadows become fallow land, where the woods quickly regain the upper hand. “But that devalues these living spaces,” emphasizes Holger Pfeffer. Where previously there was a biotope rich in flowers and insects, all-world species are now spreading. “This problem has been smoldering for a long time,” says the researcher. In many places, species-rich grassland simply can no longer be optimally cared for. “Then we came up with the idea of simply mulching instead of doing nothing at all.” However, for many areas that is not a good solution.
Additional fertilizer
On the one hand, the ground is covered and shaded by the shredded grass. “That promotes strong-growing plants,” says Rainer Oppermann. Dandelions or types of grass such as meadow fescue, cocksfoot or featherweed have no problems with this. On the other hand, more delicate and sensitive plants such as tormentil, spring gentian or various types of carnations on nutrient-poor grassland do not even get through the mulch layer.
Above all, however, the rotting of the material on the surfaces leads to an accumulation of nutrients. And that poses problems for the ecosystems there. Because many of the most species-rich and valuable grassland communities are adapted to nutrient-poor conditions. An additional fertilizer boost is not helpful here. This can lead to more competitive species displacing the specialized hunger artists. “So it is best to use bar mowers on such areas and remove the mowing material from the area,” says Holger Pfeffer. It can then be fed to livestock, composted or used in a biogas plant to generate energy. “So that this can be implemented, the funding policy is required,” emphasizes the expert. In Brandenburg, for example, there are programs that specifically support such measures. Farmers can apply for an investment subsidy to purchase the necessary mowing technology. There is then a subsidy so that it can be used on areas that are valuable for nature conservation.
However, according to Holger Pfeffer, the municipalities also have a responsibility to provide more carefully mown grassland habitats. The researcher sees great potential here: “The municipal area can become just as important for insect protection as agriculture.”