Take Half, Leave Half: Grass Response to Grazing
By Rick L. Peterson, State Rangeland Management Specialist, Natural Resources Conservation Service
The old adage of “take half, leave half” is still alive and well, but how does it really apply? Many factors influence the growth of plants, and those that have the greatest effect on annual production on rangeland ecosystems, such as precipitation, temperature, topography and soil texture, depth, and fertility are all outside of the land manger’s control.
The only aspect that we can control on native rangeland is the amount of leaf area removed by grazers, and conversely, the leaf area remaining after the grazers have moved on.
We all know that, for plants to grow, they must have adequate leaf area to collect the solar energy needed to fuel their physiological processes. A grass plant, in general, produces twice the amount of leaf area it will need to complete growth, reproduce and remain healthy and vigorous, hence the idea of taking half for grazing but leaving half for the plant. Still, sustainable use of forage resources requires a little more forethought and planning.
Timing of grazing has a tremendous effect on the grasses’ ability to re-grow. Grasses have the greatest tolerance for grazing early in the growing season while still in the vegetative state. When grazed early, grasses can take advantage of available soil moisture, ramping up vegetative growth to replace what was lost. But before long, the grass plants’ stems or tillers will begin to elongate. As their growing points are elevated, the potential for damage by grazing increases. If the growing point is removed, the plant tiller will essentially stop growing, and any additional tiller growth must arise from dormant buds located at the base of the plant crown.
The effect of grazing with regard to the location of growing points is illustrated in the photograph.
The wheatgrass tiller on the left in the photo was grazed above the growing point and continued to grow after the grazing event. The tiller on the right was grazed below the growing point; all post-grazing re-growth has stopped and any additional growth must arise from axillary buds at the base of the stem below the soil surface. It would take time and extra energy for this plant to recover from grazing.
As you know, all grasses are not created equal; some are much more resistant to grazing than others. Take, for example, blue grama and bluebunch wheatgrass. Both are classified as bunchgrasses, having a growth habit in which tillers emerge from growing points along the stem at or near the soil surface. However, bluebunch elevates its growing points, making them susceptible to removal by grazing. Blue grama’s growing points remain close to the ground and are protected from grazing. Plants with this advantage have greater resistance to grazing pressure and tend to increase in a grazing system. Bluebunch wheatgrass, being more susceptible to damage and therefore less resistant to grazing pressure, will decrease. Over time, a shift in species composition of the plant community can result.
Another reason to follow the “take half, leave half” rule is the effect of grazing on root growth. To remain healthy and vigorous, a grass plant must replace between 20 and 50 percent of its root mass annually. Light to moderate grazing (50 percent or less) has a moderate to no adverse effect on new root growth. Once grazing levels reach 60 percent by weight, root growth is decreased by half. This can last from a few days to a couple weeks. When leaf removal reaches 80 percent, all root growth stops. If plants are over-utilized year after year, their ability to regenerate root material will be compromised; they’ll lose vigor and will be unable to hold their place in the plant community.
Healthy plant communities in general are closed, where all of the available space is occupied both above ground and below. These communities are resistant to the invasion of weeds and other undesirable plants species. If a plant community is continually over grazed it will become open and weedy species will invade. Further decline may cause the plant community to cross a threshold, becoming dominated by weedy species. From an ecological standpoint, a threshold is very difficult to re-cross. Once a plant community has crossed over to a new steady state, it is unlikely to go back to what it was without significant inputs of resources and energy invested over a long period of time.
There are three key concepts to grazing management that will help to maintain adequate leaf area and preserve the health and vigor of your forage resources. They are: intensity (the amount of leaf area removed), frequency (the number of times a plant is grazed) and timing (the season of use). The duration or length of time a given number of animals grazes an area greatly influences grazing intensity and frequency.
Season-long grazing systems can be very efficient. However, these tend to create scenarios where a pasture can be under-stocked yet overgrazed. This happens because the intensity and frequency of grazing is targeted to a few grass species. Over time, those preferred species can be greatly reduced, or even removed, from the plant community. The optimal grazing system involves a rotation among several pastures; this allows the utilization of available forage and provides adequate time for re-growth of leaf material before the next grazing event. A built-in deferment period during the growing season will give plants a chance to recover, reproduce and remain healthy and competitive.
Maintaining strong, healthy plant communities on our rangelands becomes even more important as cheatgrass increases across the landscape. Cheatgrass is most likely already a component in the plant communities on your ranch. This is something we all have to recognize. The key now is to manage our forage resources in a way that ensures their continued productivity and ability to resist conversion to cheatgrass dominated communities. Remember: take half, but leave half.