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Solar energy fluctuations in forage

Dennis Hancock for Progressive Forage Published on 31 January 2018
Solar energy

Essentially everything we do as forage growers is to maximize the conversion of solar energy into digestible energy. We do not use photovoltaic cells. We use photosynthetic cells.

Forage growers have always sought to maximize photosynthesis, even though we may not think about it in those terms.

Whether we are selecting higher-yielding varieties, battling weeds that shade out our crop or managing diseases or insects that may damage the plant, we are fundamentally trying to maximize our photosynthetic power generators.

As a scientist, I have always been fascinated by the biochemical process of harnessing sunlight to capture carbon dioxide and converting solar energy into digestible forms of energy like sugars and other carbohydrates. Despite knowing many of the minute details, I continue to find myself in awe of the elegance of photosynthesis.

While I’ll spare you the lesson in biochemistry and plant physiology, it is instructive to consider how photosynthesis varies during the day and how these carbohydrates are used, transported to where they can be used or stored. Conveniently, there are parallels between the fate of energy captured by photovoltaic cells and the fate of energy captured by photosynthetic cells.

A small amount of the energy captured by solar panels can be used themselves, such as to power the motors and drives that keep the panel aligned with the sun, while the excess energy is transported away on the grid or down to a battery that can “store” the energy for later use.

Similarly, the plant cell where solar energy is captured can use some of that energy, but much of it is converted to sucrose and sent out to other parts of the plant or stored as starch.

Whether one is a manager of a solar panel array or a crop field, these diurnal patterns have management implications. For example, the rate of photosynthesis in many forage crops will be greatest from around mid-morning until around solar noon (Figure 1A).

The rate of photosynthesis in response to sunlight

After this mid-point in the day, photosynthesis occurs at an ever-slower rate until it stops as the daylight wanes in the early evening.

A fast rate of photosynthesis results in a surplus of sugar near the site of photosynthesis. The plant uses these sugars and carbohydrates for functions within the cell where they were photosynthesized, but it will also convert some to sucrose and starch. Sucrose, which you and I know as table sugar, is the carbohydrate currency in the plant. Sucrose is transported wherever the plant needs it.

Starch is carbohydrate storage. Sucrose is transported to roots, rhizomes and stolons where it is often stored as starch. But even in the photosynthetic cell, starch is created to store the glut of photosynthate until it can be converted into sucrose and transported out of the chloroplast.

Are these fine details important? Well, since it affects the amount of rapidly digestible energy available in the forage throughout the day, this can have implications for grazing and perhaps even harvest timing. Starch concentrations build throughout the day.

Photosynthesis occurs so rapidly the simple sugars cannot quickly be converted to sucrose and transported away fast enough. So the plant stacks all the extra sugar up as starch until nighttime, when it can catch up with the export of sucrose.

Since the plant’s respiration rate is highest during the daytime, there is a period of time when starch and simple sugars are at a high concentration and are being consumed at a relatively slow rate (Figure 1B). This means a forage crop’s digestible energy would be at a peak in the evening shortly after the sun has set.

Consider the implications of this if you are an animal grazing in a pasture. A cow grazing at the crack of dawn will likely have slightly fewer soluble carbohydrates in its breakfast than in its sunset feast. It’s the bovine equivalent of having ice cream for supper. (Oh, the irony.)

But if we have a horse that suffers from insulin resistance and laminitis issues, often collectively called equine metabolic syndrome, grazing forage high in sugar content in the early evening can be enough to exacerbate those issues.

One might also think cutting hay or silage crops within an hour of sunset would maximize the concentration of sugar and starch in the forage. If it were flash-frozen and freeze-dried immediately after cutting, this would be true. Such is impractical – unless your last name is Jetson and your first name is George.

Several research trials have been performed to determine whether forage quality is improved by cutting at sunset, but the results have been mixed. Just because the crop is cut at the peak of water-soluble (or non-fibrous) carbohydrate concentration does not mean one ends up with higher levels in the hay or silage. The problem is: Respiration does not stop when the plant is cut.

The plant is still alive, and respiration continues for a while after it has been cut. In fact, respiration does not approach zero until the plant has dried down to 40 to 50 percent moisture, and it may take a full day or more in humid environments for the forage to dry to this level.

Most forage agronomists in the humid areas of the U.S. contend the risks of the weatherman’s fibs (i.e., unanticipated rain damage) do not outweigh the benefits of waiting until evening to cut hay. In arid regions, where rapid drying conditions bring the crop moisture down quickly, cutting in the early evening may be beneficial.

Even in humid regions, cutting in the late afternoon and ensiling the wilted forage partway through the following day may result in slightly higher concentrations of fermentable carbohydrates. Sometimes, such a slight advantage can be enough when one is looking for an edge.  end mark

PHOTO: Fundamentally, the goal of forage crop production is to turn solar energy into digestible energy. The varying rate of photosynthesis can have implications for grazing and perhaps even harvest timing. Photo provided by Dennis Hancock.

Dennis Hancock
  • Dennis Hancock

  • Associate Professor and Extension Forage Agronomist
  • University of Georgia
  • Email Dennis Hancock

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