Is Stocking Rate So Important?

by R. Dwayne Rice, Rangeland Management Specialist
Natural Resources Conservation Service (NRCS)
Lincoln, Kansas

Selecting the correct stocking rate for a pasture may be the most difficult, but also the most important, decision a manager can make. The problem would be greatly simplified if forage yields from pastures were the same year after year or fluctuated only between narrow limits. It is this unpredictability of forage yield that causes the difficulty. Hitting the magic 50 percent use of annual forage production is the goal, but what happens when the degree of use is consistently above 50 percent for 3-5 years? We know that plant responses to grazing are conditioned by past history, as well as environmental conditions; however, at the end of the season, it is stocking rate that will ultimately determine the potential forage production for several years into the future. This year’s stocking rate, the choice of the manager, will directly affect next year’s forage production, plant community, livestock production, and economic return from the pasture.

How much can the stocking rate affect forage production?
A number of research studies across the country have been conducted to measure the effects of stocking rate on forage production. Several of these studies have been conducted in the Kansas Flint Hills, where drought is not much of a concern, and near Hays, where precipitation is more variable. The results from these studies, regardless of where or when they were conducted, are remarkably similar. After 3-5 years, heavy grazing (the annual removal of 60 percent or more of the production of the primary forage species) resulted in less forage production in the pasture the following year compared to a moderately grazed pasture (the annual removal of 40-50 percent of the production of the primary forage species). Over a 25 year period, repeated heavy grazing resulted in a 20 percent decline in forage production while moderate grazing had no effect on forage production potential. It is important to note the difference between heavy grazing and moderate grazing is a mere 10 percent greater degree of use of the primary forage plants within the plant community. As would be expected due to selectivity of cattle, the most important productive and preferred forage species are the first to decline in production and number.

Why does this reduction in the primary forage species occur?
Heavy grazing affects more than just the defoliation of the primary forage plants. The effects of heavy grazing manifest themselves over time, reducing the amount of mulch and increasing the amount of bare ground exposed to direct sunlight. Ground cover, or mulch, has a moderating effect on soil moisture and temperature. Soil organisms are most active and efficient when the soil is moist and the temperature is between 86 and 95 degrees Fahrenheit. Soil exposed to direct sunlight is hotter than ambient air temperature and can reach levels lethal to these organisms. As the number of soil organisms is depleted or their activity slows, so does the availability of nutrients, the second most important limiting factor to forage production on rangelands.

Soil moisture, which is the most important limiting factor to forage production, is also affected by grazing intensity. Adequate mulch levels increase the rate of rainfall infiltration. One study conducted at Hays, Kansas, by J.L. Launchbaugh, showed infiltration rates of 0.73 and 1.19 inches per hour for heavy and moderate grazing systems, respectively. Run-off also increases as the amount of bare ground increases. The impact of a raindrop on bare soil looks like a micro-bomb, dislodging fine soil particles that seal the surface of the soil, resulting in less infiltration and moisture available to plants for forage production during the growing season. Mulch is a critical component in minimizing evaporation, moderating soil temperatures, and getting precipitation through the soil surface and into the soil profile where it is available for plant growth. Removing more than 50 percent of the annual production of the primary forage species reduces the amount of mulch available to protect the soil surface.

How does a reduction in forage quantity affect animal performance and the potential for economic return?
Both forage quality and quantity are factors in individual and per acre animal performance. Average individual animal gains of steers over a 7 year study at Hays, Kansas, by Launchbaugh, using high, moderate, and light stocking rates was 122, 188, and 217 pounds of gain per steer, respectively, during the summer grazing period. As would be expected, animal gain per acre was highest under the high stocking rate at 61 pounds of gain per acre while moderate stocking showed gains of 55 pounds per acre and light stocking produced 43 pounds of gain per acre. The bigger point to be made was the rate of gain was not sustainable: within 3-5 years there were significant changes in the plant community and the amount of bare ground increased resulting in less forage production and forage availability in the heavy stocked pastures. The heavily grazed pastures also had higher annual fluctuations in forage production during the study.

Another study, conducted by Jim Gerrish in Missouri, measured the nutrient quality of the available forage at heavy, moderate, and light stocking rates. Nutrient density per pound of forage consumed was highest in the heavily stocked pastures due to young plant material having a higher nutrient density than older plant material. However, the gains per individual animal were once again lowest in the heavily stocked pastures and highest in the lightly stocked pastures. Gerrish attributed the poor individual animal performance of the heavily stocked pastures to the lack of forage quantity rather than quality. The amount of forage available to the grazing animal was inadequate to meet their requirements for body maintenance and growth. Conversely, forage quantity was not limited in the lightly stocked pastures, and the animals through selective grazing could meet their requirements for maintenance and optimum gain. The moderately stocked pastures produced individual animal gains higher than the high stocking rate pastures and had higher gains per acre than the low stocking rate pastures. Gerrish concluded that moderate stocking rates produced the best combination of forage quality and quantity for individual animal performance and gains per acre, making the moderate stocking rate the best for sustained economic returns to the manager. more-

How do we know when pastures are being moderately grazed?
What gets measured gets managed, so begin by measuring how much is grown, how much is grazed, and how much is left. Stocking rates are unique to each individual pasture so there is not a standard average for a particular county or area. A couple of well-placed grazing exclusion cages, a yard stick, and a camera are all that are needed to determine the degree of use within a pasture. Designing and implementing an annual monitoring plan to measure and record forage production and degree of utilization within a specific pasture is crucial to maintaining adequate mulch levels, healthy belowground root systems and microbial populations, forage production, and animal performance. With measured information, managers can make timely adjustments in stocking rate decisions that will maintain productive native rangelands well into the future. If you would like assistance in monitoring your pastures, contact your local NRCS office.

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Last Modified: 12/24/2008