In the first article in this series, Turn Sunshine Into $ by Understanding How Plants Grow, we talked about how we can work with our animals to graze when plants have the most nutritional value during the day. Now let’s talk about how we work with our animals to make sure we’re catching all the sunlight we can so we get as much forage as possible. Let’s manage for our “Light Interceptors.”
We all know that there is more solar energy when the sun is higher in the sky and the days are longer, and there is less when the sun is lower in the sky and days are shorter. We also know that sunlight determines the daily potential photosynthesis for a plant. We can think of the leaves of the plant as solar collectors, gathering energy so the plant can grow, and the roots as the “batteries” where it stores energy for future use. (Note that cool season grasses are a bit different. They store very little energy in their roots, but the length of the roots is an important indicator to the health of the plant. We’ll cover cool season grasses in greater depth down the road.)
We can manage how much energy we’re collecting and storing in our plants and roots by how long or short we graze in relationship to where the sun is in the sky. Removing leaves means that the plant will draw on it’s batteries to rebound. The shorter you graze, the more of your batteries, or root reserves you use. And just like your recharging any battery, the less energy it has stored, the more time it needs to recover. You can see the effect in this picture.
There is a point when it doesn’t do much good to have more leaf area. As leaves grow beyond maximum light interception they shade one another, and older leaves become less efficient and die. As leaf area increases, further energy reserves in the roots return to the high level needed to sustain the next growth cycle. Beyond this point net growth is reduced and forage quality declines. That’s why it’s important for you to graze the forage. You’re helping it achieve it’s maximum growth and health.
As you’ve guessed already, when the sun is high in the sky, a tall, thick pasture intercepts more light than a short, thin pasture. The light that is not intercepted by the leaves is lost to the plant. If the sun hits the ground instead of the plants, it warms the ground which may increase plant respiration. On cold spring days this could be good, jumpstarting growth. On hot summer days this could be bad, stressing plants. So as you’re managing you’ll be considering the weather as well, looking at what’s happening that day, and what the forecast is for the upcoming weeks and months.
Let’s start with April. We’ve just passed the Vernal Equinox, and the sun is getting higher in the sky. Temperatures are warming and grass is beginning to grow. The graph here shows us that we get maximum sunlight collection with a canopy height of about 8 inches. How much of it should you graze? Well that’s where the “Art” part of the “Art and Science of Grazing Management” comes in. As the producer you’re going to decide based on what you need to feed your cattle, and how much rest you can give that pasture after you’ve grazed it. By the way, this light interception/growth curve is the same for June. That’s convenient for helping you plan in April what you might be doing in June.
Let’s move into May now. The sun is rising in the sky even higher now, and so the plants can be even taller to capture the most energy possible. In fact, they may be taller than the maximum you need because of what happens to pasture forages in May. They start shooting grass heads up, like raising flagpoles throughout the pasture. When those grass heads come out, the grass begins to “tiller” or send out more side shoots and stems, and raises leaves taller to grasp for all the sun it can get. Those taller leaves allow more light to penetrate through the canopy, so you might actually be losing some of the energy you’d like to capture and store. When the reproductive tillers are grazed down, the pasture is then mostly leaves, and the light is captured more evenly, not reaching as deeply into the canopy.
There’s a sort of trend there, with the light penetrating most deeply in the spring, and less deeply in fall. Here we’ve added the curves for July and August and September and October. Notice how the height of the canopy required to capture 90% of the suns rays decreases over the course of the grazing season. It turns out that the needs of the forage and the needs of the animals are well-suited to each other. Our job is to manage the time and timing in different pastures so that we’re keeping the solar collectors and their batteries in the best shape possible to do their jobs.
One more thing – Energy reserves cycle with growth
In many plants there is an energy reserve cycle during periods of growth after a harvest. Let’s use tall fescue and white clover as examples (Fig. 2). When the leaves of these plants are grazed off, new leaves start to grow. When most of the leaves are removed, energy for new growth comes from stored energy reserves. The site of stored energy reserves is species specific. Tall fescue stores energy reserves in the lower leaf sheath and underground stems called rhizomes. White clover stores energy reserves in the above ground stem called a stolon.
Over a period of days the energy reserves decrease as new leaves grow. As leaf area increases, more sunlight is intercepted and photosynthesis increases, providing more energy for growth. At some point photosynthesis is great enough to produce more sugar than is needed for growth, resulting in sugar moving into storage and an increase in energy reserves in the plant.
So what should you do with all this information?
It sounds a little complex, but if you’ve got a rechargeable battery in anything you own, you have a good grasp of how all this works. Use that experience and all your other good experience with your grazing animals to make adjustments to your grazing management that will keep your solar collectors working and your batteries charging.
Next week – what this looks like under rotational grazing!