This summer has certainly been hot and dry for many of us, which has reduced productivity of many cool-season perennial grass pastures. As we continue into the traditionally driest, hottest days of summer, there are management practices that can be implemented to maximize plant growth and protect pasture stands during these hot, dry periods.
The key to having really 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 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. This becomes especially important during drought conditions as we try to conserve every bit of soil moisture.
The amount of residual left following grazing affects plant energy reserves as well, as most cool-season grasses store their energy reserves in the lower 3 to 4 inches of the stem base. Therefore, with a lower grazing height, plants not only have less leaf area remaining for photosynthesis, they also have less stored carbohydrate reserves left to provide energy for regrowth. On the contrary, a higher grazing height will leave more leaf area for photosynthesis and also more stored carbohydrate reserves.
Post-grazing residuals will also affect plant root growth, as plant roots rely on the leaves to supply energy from photosynthesis. In general, the amount of live root growth occurring below ground is roughly equivalent to the amount of live forage growth occurring above ground. Research has shown that the amount of above ground forage removed impacts root health. Up to 50 percent of above ground biomass can be removed with little to no impact on root growth. However, with greater than 50 percent removal, root growth slows dramatically, and removing 70 percent or more of above ground biomass can completely stop root growth. This is where the old rule of thumb “take half, leave half” comes into play. Leaving half of the leaf area on the plant lessens impacts to the plant root system, enabling the plant to continue to absorb nutrients and moisture and recover quicker following grazing. If pastures are grazed too low, plant root growth slows or stops, diminishing the vigor of the plant root system and the overall productivity of the plant.
“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…”
One of the most common mistakes people make in grazing management is providing too short of a recovery period for pastures after grazing. Pasture forages require a rest period in order to maintain productivity and longevity. 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 that 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 start to die out, 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, the amount of grazing pressure, and the time of year. As we get hotter and drier, grass growth rates 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 cool-season grasses, this will be at or above 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 grow and maximize photosynthetic capacity.
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 can encourage the plant to go back into leafy or vegetative growth, which will increase forage quality and result in more total forage being produced over the course of the growing season. Clipping will also serve the added benefit of helping to control certain 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 are becoming too mature. 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.
It is important to recognize that clipping pastures does have a cost, both in time spent and in fuel burned completing the task. In reality, the need to mow or clip is usually site and time specific and will depend on several factors, including the amount of seed heads present, the buildup of thatch over time, the timing and intensity of grazing, and any weed pressure. Potential benefits of clipping must be weighed against the costs that are associated with mowing pastures to determine if it is a practical expense economically. In some cases, mowing will have a low return on investment, and you may be better off focusing on other things and reducing the time and money spent mowing.
When conditions are hot and dry and cool-season pastures are just not growing, consider other potential sources of forage that can be used. This could include grazing regrowth on hay fields, utilizing warm-season annual forages, grazing droughted crop fields, or feeding hay or baleage. In the long run, using these alternative sources now will help preserve pasture stands and allow you to grow more grass later in the year. If you are feeding hay, remember to compare the quality of the hay against the class of livestock you are feeding to make sure their nutritional needs are being met. Having your hay tested will help you determine if you need to provide any supplementation with a grain, supplement, or other byproduct feed.
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 protect pasture stand integrity.
This article appears on July 28, 2023, in Volume 4, Issue 2 of the Maryland Milk Moos newsletter.
Maryland Milk Moo's, July 28, 2023, Vol. 4, Issue 2
Maryland Milk Moos is a quarterly newsletter published by the University of Maryland Extension that focuses on dairy topics related to Nutrition and Production, Herd Management, and Forage Production. To subscribe to this newsletter, click the button below to enter your contact information.
