The big variable is precipitation, which greatly affects accuracy of forage supply estimates.
For the purpose of grazing, forage supply is expressed as carrying capacity, which is the stocking rate sustainable over time per unit of area. Stocking rate is defined as the number of animals on a given amount of land over a certain period of time and is usually expressed as animal units per unit of land area. An animal unit (AU) is a 1,000-pound beef cow consuming an average of 2.6 percent of her bodyweight (26 pounds) daily throughout her annual production cycle.
Using these figures as a base, then an 800-pound cow is 0.8 AU and a 1,200-pound cow is 1.2 AUs. Because of the size difference and different amounts of consumption, it takes six 130-pound domestic ewe sheep to equal one AU, whereas eight 70-pound Angora nanny goats are one AU.
Check with your local USDA-NRCS office or state extension specialist for an animal unit equivalent chart. The one for Texas is found here: Animal unit equivalent chart.
Measuring forage production
“Measurement of the amount of forage produced is required for calculating a pasture’s carrying capacity. A method for measuring annual production is to clip and weigh standing forage at the end of the growing season in an ungrazed area,” says Daren Redfearn, with the University of Nebraska – Lincoln. “Measurement of standing forage at the end of the growing season is a conservative measurement because much of the production was lost to decomposition and insects.”
One way to ensure ungrazed forage is available at the end of the growing season is to fence small areas within a pasture. Minimum functional height of the exclosures is 4 feet, and the minimum size is 10 feet square; however, larger exclosures will facilitate more representative sampling.
For clipping forage, Bethany Johnson at the University of Nebraska – Lincoln recommends placement of a frame of known size directly on the soil surface in an ungrazed area. She uses a circular frame of 58.9 inches in circumference. Forage within the frame is clipped to ground level, and unpalatable and noxious plants are removed. The remaining clippings are put into a pre-weighed (in grams) empty bag.
More accurate results are obtained by drying the clipped forage, which is accomplished by laying the bag on the dashboard of a vehicle for a few days. After drying, the bag is weighed (in grams) and the weight of the bag is subtracted from the total weight. Net weight is multiplied by 50 for total forage in pounds per acre.
“The old rule of thumb, ‘take half, leave half,’ is often used in determining stocking rates,” says Redfearn. “This rule was formulated on the premise that 50 percent of the annual peak standing crop can be removed from the site without negatively affecting the plant community relative to species abundance. Of the 50 percent, it is assumed that one-half (25 percent) is actually consumed by livestock and the other half (25 percent) is laid on, trampled, eaten by insects and other animals, and decomposed. These assumptions lead to a harvest efficiency of 25 percent.”
Since harvest efficiency is estimated at 25 percent, calculated total pounds of forage per acre is divided by four to derive a rough estimate of pounds per acre of grazeable forage. Carrying capacities of introduced forages differ from rangeland vegetation due to the allowable percent of utilization. If adequate moisture and fertility are available, utilization maximums of 50 to 75 percent or higher are possible on introduced forages.
Examples of carrying capacity calculations are presented in Table 1.
Forage production used in the equations is an estimate based on forage yields during previous years. It is important to record annual forage production plus annual rainfall so records can be compared when making forage production estimates. Number of days planned for livestock grazing on a particular pasture is also a principle part of the equations.
Better forecasting in the future
Multiannual rainfall records and forage production history greatly improve ability to forecast forage availability during the next grazing season; however, unexpected timing and amounts of precipitation may cause an unplanned decrease or increase in forage production. In an attempt to better guard against these variabilities, and particularly the risk of drought, a forage risk assessment management system (FRAMS) is currently in the beta test phase of development.
The system, a web-based risk management tool, provides a way to monitor and assess performance of free-ranging animals and forage conditions in response to site-specific weather. FRAMS is a tool for determining least-cost feeding and for destocking decisions relative to market and weather risk.
The system is being developed by the Center for Natural Resource Information Technology (CNRIT) located at the Texas A&M University Blackland Research and Extension Center in Temple, Texas. CNRIT’s primary role is promoting technology in agriculture worldwide.
“Climate- and remote-sensing products are improving capabilities for assessing drought impacts on rangelands and can be used with simulation modeling to provide near real-time predictions of forage conditions. Incorporation of other relevant data streams into FRAMS will provide estimates of forage quality, livestock prices and futures, and feed and hay prices,” says Jay Angerer, director of CNRIT. “Early warning system outputs, along with short- and long-term forecasting information, can improve risk management decision making.”
“FRAMS utilizes National Oceanic and Atmospheric Agency [NOAA] weather data for forage modeling and the Nutritional Balance Analyzer to assess livestock nutritional status and performance,” says William Fox, one of the principal investigators at CNRIT. “The system will also allow ranchers to establish geo-referenced rain gauges online and assign a plant community to them.
Each site will automatically link to the NOAA 12-by-12- or 4-by-4-mile weather grid system where the recorded rainfall input by the rancher is integrated with the solar radiation and temperature data to drive a site-specific forage growth simulation model. This latter model computes forage deviation from normal and percentile ranking.”
Putting the right number of animals on a pasture is just one part of proper grazing management. Monitoring grazing pressure and adjusting stocking rates when necessary is also important. There are several ways to monitor forage condition, but one of the simplest is the grazing stick, obtained at most NRCS offices. The stick is used to measure grass height, which is converted to pounds of forage per acre using conversion tables printed on the stick.
Forage supply must meet or exceed livestock demand in order for the ranch to remain profitable, so maintaining the proper stocking rate is a key component of success. 
PHOTO: Different livestock species have different animal unit requirements for forage. Different animal weights also factor into the calculations even among one species. Photo by Robert Fears.
Robert Fears is a freelance writer based in Georgetown, Texas. Email Robert Fears.
