Amanda Grev, Forage Specialist
University of Maryland Extension
As we move into the traditionally driest, hottest days of summer, we can expect growth rates of cool-season grass pastures slow dramatically and pasture productivity to decline. However, there are management practices that producers can implement to maximize plant growth during these hot, dry spells.
It takes grass to grow grass.
The key to having productive pastures is optimizing plant photosynthesis. Think of your pasture as a solar panel where green, growing leaves are energy producers. To maximize production, livestock need to be rotated off of a pasture in a timely fashion to ensure an effective “solar panel” or leaf area is left in the paddock following grazing. Most cool-season forages need at least 3 to 4 inches of post-grazing residual to effectively take advantage of photosynthesis for regrowth. In addition to providing a photosynthetic base for plant regrowth, the leaf material that remains after a grazing bout also shades the soil surface, keeping soil temperatures cooler and helping to reduce soil moisture loss.
Removing leaf matter affects the roots as well, as those roots rely on the leaves to supply energy from photosynthesis. The amount of live growth occurring below ground is roughly equivalent to the amount of live growth occurring above ground, and research has shown that the amount of above ground forage mass removed impacts root health. Up to 50 percent of the plant can be removed with little to no impact on root growth. With greater than 50 percent removal, root growth slows dramatically, and removing 70 percent or more of the above ground forage mass stops root growth completely. This is where the old rule of thumb “take half, leave half” comes into play. Leaving half of the leaf area on the plant has minimal impacts to the plant root system, enabling the plant to continue to absorb nutrients and moisture and recover quicker following grazing. If the take half, leave half rule is violated and pastures are grazed too low, plant root growth stops and root reserves are used to regrow leaf tissue, diminishing the vigor of the plant root system and the overall productivity of the plant.
Provide a rest period.
One of the most common mistakes in grazing management is not providing a long enough recovery period for pastures after g razing. Pasture forages require a rest period in order to maintain vigorous production. When a plant is grazed, the loss of leaf material means the plant loses its energy-producing center. The plants’ response is to rebuild that center using stored energy reserves. If the plant is given rest following grazing, new leaves will develop and will replenish this energy supply. Without rest, the plant is not able to replenish its energy supply and will continue to use the remainder of its stored energy to produce new leaves. As energy supplies are depleted, the plant will be unable to maintain production and will eventually die, leading to weak stands, overgrazed pastures, and the invasion of weeds or other non-desirable forages.
Maintaining flexibility in your system will allow you to balance the length of the rest period with the plant growth rate and is fundamental to successful grazing management. How long recovery takes will depend on a number of things, including the plant species, grazing pressure, and the time of year. As we get hotter and drier, grass growth rates will slow down and the days of rest required may be much longer than that required during the spring when rapid growth is occurring. Regardless, the rest period must be long enough to allow the plants to recover and grow back to a practical grazing height before livestock are allowed to graze again; for most grasses, this height falls in the 8 to 10 inch range.
To accommodate for this longer rest period, the rotation speed between paddocks will have to slow down. The basic rule is: when pastures are growing fast, rotate fast; when pastures are growing slowly, rotate slowly. Remember that the goal of the rest is to allow young green leaves to maximize photosynthesis.
Don’t ignore seed heads.
A plant that is producing seed heads is undergoing reproductive growth and not putting energy into leafy growth or tiller production. Clipping seed heads from these grasses will allow the plant to return to leafy or vegetative growth, which will increase forage quality and result in more total forage being produced over the course of the season. Clipping will also serve the added benefit of helping to control weed populations.
Seed heads can also be an indication of uneven grazing patterns in your pasture. If selective grazing is occurring, some plants are likely being overgrazed while others not enough. If this is happening, consider adding more divisions or paddocks into your pasture system. This means you will be grazing your animals on smaller areas, increasing the stocking density. A greater stocking density will reduce the amount of selective grazing that occurs, increasing forage utilization and reducing the need for pasture clipping.
While we can’t control how hot or dry summer will get, we can strategically manage the grass we have to help keep summer paddocks productive and growing.