NOT WHAT, BUT HOW? CUTTING HEIGHT

8.9.2020

COULD YOU SPARE A MINUTE TO READ ABOUT CUTTING HEIGHT?

Cutting height of grass has been popping up in various contexts in magazines and discussions on social media. Since we at Carbon Action are immersed in the subject, I thought I’d write a little update on the subject. What do we know about the effects of cutting height now, and what information will the study provide us with in the future?

Within Carbon Action, we have organized carbon farming training for hundreds of farmers, both for research facilities belonging to Carbon Action and for Valio dairy farmers. Participants seem to remember the importance of cutting height in storing carbon and its effects on growth. In particular, they remember this picture, which shows the measurements made at the Qvidja Farm in Parainen:

Carbon dioxide fluxes in the grasslands of Qvidja. After the first cutting in the summer of 2018 (left curve) the field was a source of carbon for a long time, (net exchange was positive, i.e. carbon flow from the ground to the atmosphere. In 2019 (right), when the grass was cut higher, the grass started to recover immediately after cutting. There is a significant difference in post-cutting photosynthesis between these years, and it is best seen in the dark blue area. Picture: Laura Heimsch, Finnish Meteorological Institute.

Feeling curious, I wanted to hear whether the direction of the carbon shown in the picture could already be verified scientifically. I decided to approach Laura Heimsch, who is involved in Carbon Action in many ways, and not only is she a carbon farmer herself, but she also does research in the Carbon Action research team.

Laura presenting research equipment on the Qvidja farm in Parainen, on a summer trip for Carbon Action carbon farmers in 2019. Also in the picture: Jenni Jääskeläinen, BSAG. Picture: Sanna Söderlund, BSAG

Putting her words carefully, like researchers often do, Laura said that no research has yet been published on the subject, but data is being collected, and that there are some things she can already say about the findings. In the following, I will list the information I received from Laura in italics, and explain it to the best of my ability.

CUT THE GRASS HIGHER

Why on earth should you cut the grass higher? Let’s start with the easiest fact that everyone involved with growing fodder already knows.

Cutting higher prevents soil from mixing with the fodder grass.

Check. Another well-known fact is that the fodder cut higher has less crop residue. But how does cutting height affect photosynthesis?

A low cutting height removes more assimilating leaf area than higher cutting heights. Carbon is therefore not transferred as efficiently from the atmosphere to the ecosystem.

In other words, photosynthesis is reduced or even stopped if the grass is cut down to a very short stubble. How about stress then? We all know stress is harmful to humans, but would it be appropriate to talk about plant stress as well?

When grass is cut below the optimal height, stress responses are activated in the plants, which can lead to problems later.

Exactly, excessive stress is harmful to all living organisms. Not only do plants experience stress when they are being cut, but stress can also affect their long-term wellbeing. In addition to avoiding stress, we need to look after the plant’s energy supplies.

Some grasses retain the carbohydrates needed for growth in the rhizome, others in the shoots, or at the base of the stem. Cutting close to these points continuously can lead to a depletion of the plant’s energy stores, affecting its lifespan. A low cutting height also impairs the survival of plants because nutrients are constantly used excessively.

Alright, so we all know that spending too much becomes expensive at some point. Fortunately, raising the cutting height can also make the grasslands more resilient. And even less self-destructive!

Cutting too low causes the grass to kill part of its rhizome, resulting in reduced water and nutrient uptake, followed by a slower recovery from cutting.

This in turn also affects the direction of the carbon flow: whether it is down in the ground or up into the air.

If the vegetative point is cut off regularly, the rhizome’s nutrient resources begin to deplete and the carbon flows away from the soil when it should flow from the atmosphere to the ground.

Circumstances also matter, meaning that long periods of drought can affect the vegetative points of the grass, and that’s where cutting height matters!

A drought or a long dormancy period can cause a higher vegetative point, which can lead to cutting below the vegetative point with low cutting height. The plant must start growing again from the base, and the nutrients, also carbon, are taken from the rhizome.

The vegetative point can thus fool you: I am not where you think I am, tee hee! One must be careful. And, as already stated, repeated low cutting makes it difficult for plants to survive because the plant is constantly overconsuming nutrients. As a result, weed problems can also develop.

A low cutting height causes some grass to die, for example, Timothy grass, which grows from its base: the grass will become mottled and the weeds will have room to grow.

Chemical weed control, on the other hand, has its downsides, and therefore dense grass would be the best repellent for weeds.

Pesticides impair the plants’ ability to absorb water and nutrients from the soil, and mineral fertilizers limit rhizome development.

So, raising the cutting height seems good, but what would be the optimal cutting height?

It would be optimal to cut about a third of the grass height, but max. 50%, especially on grasslands with lots of different types of grass, where the vegetative point varies with different species.

Sounds good, but in practice, it is not possible to raise the cutting height that much if the grass has been cultivated for fodder use. Laura thinks that if we could get to a situation where a maximum of 70% of the grass would be cut at once, that would already be a good thing.

I encourage farmers to boldly increase the cutting height on green fertilization fields, and it is also a good idea to raise the height to 15 cm for fodder grass. This is what I learned from the Carbon Action training experts.

RESEARCH IN QVIDJA

Chamber measurements are used to study the photosynthesis of grass, in other words, carbon sequestration, and the respiration of the field ecosystem, namely the release of carbon into the atmosphere. Picture: Sanna Söderlund, BSAG
  • The Finnish Meteorological Institute is researching carbon flows and carbon storage in arable land as part of the Carbon Action project in Qvidja.
  • The differences between 6 cm and 15 cm cutting height are being studied with chamber measurements.
  • Continuous flux measurements monitor forage grass carbon exchange for the third summer in a row. During the first summer, the cutting height was set to about 6 cm and the next year to 15 cm. Now, in the third year, the plan is to cut from a height of about 15 cm again. Grass diversity has also been increased over the past year.
  • The effect of low cutting on the grass in the summer of 2018 was visible. The field dried, and the grass turned completely brown. In June 2019, the amount of rain was as low as in 2018, but the grass did not dry out, instead, the field remained green throughout the summer. This was also clearly visible in carbon dioxide measurements and satellite data.
The cutting height can be increased for example with additional blocks. There are other ways too! Picture: Sanna Söderlund, BSAG

CONCLUSION

In summary, the benefits of raising the cutting height are that the grass’s chance of surviving improves, even in difficult conditions. The benefits to the yield due to higher cutting may not be seen in a year or two. However, when the grass is not cut repeatedly near the vegetative point, longer-lasting grass is obtained, and it remains denser and healthier, stores more carbon, and improves soil structure with its root system.

Raising the cutting height is easy with some devices, but others require more adjustment and learning, but it’s by no means impossible. Nor is it rocket science. If you do not reap your harvest yourself, you may want to ask your contractor how to raise the cutting height. Contractors will certainly be inventive if there is a demand. I have even heard that lifting the cutting height is quite normal in some regions of Finland!

This is, I believe, one of the most promising quick fixes for a multi-benefit carbon farming measure. This is a situation in which the farmer, the field, and the environment will benefit. We look forward to a scientific publication on the subject.

Text: Sanna Söderlund, training manager, farmer cooperation, Baltic Sea Action Group

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