by Sam Schieffelin
Grazing capacity, also called carrying capacity, refers to the number of animals, measured in AU (animal units), that some area of land can sustain for a given length of time, usually a year.[1] There are several ways to determine this statistic, and all but the most technologically advanced follow the same basic principles. First, a measure of the field’s vegetation must be taken. This involves taking sample cuttings from the pasture (simulating grazing), weighing them, and calculating pounds per acre. This calculation is then compared to a table of values representing each type of animal and their AU equivalencies (for example, an adult horse is equivalent to 1.25 AU). Since an AU has a specific amount of vegetation it requires – 3% of its body weight or 30 pounds of vegetation per day – grazing capacity for any animal type can then be determined using the measured amount of vegetation that exists in the pasture.[2] This calculation can be difficult for areas of low vegetation such as America’s desert Southwest. Sources agree that the Sonoran Desert region, including Organ Pipe Cactus National Monument, has a grazing capacity of close to one single AU per square mile.[3]
It would be improper to talk about grazing and ranching in Organ Pipe Cactus National Monument without discussing the Gray family. As one of the first and only families to develop this area for ranching, the Grays found out firsthand how difficult raising cattle could be in this environment. Because of the extremely low grazing capacity they were forced to buy surrounding land parcels in excess of 500 square miles for grazing their herds of cattle just to sustain themselves – one family. When the monument was established in 1937 permits were given to allow grazing on the grounds. These were eventually nullified in the 1960s and the last cattle were removed from the monument in 1975.[4] The extremely low grazing capacity of the area inevitably resulted in overgrazing during the tenure of the Grays; something the vegetation in the monument is still slowly recovering from today.[5]
[1] University of Kentucky College of Agriculture, “Agripedia,” University of Kentucky, http://www.ca.uky.edu/agripedia/glossary/gcap.htm (accessed March 2012).
[2] Robert Fears, “How to Determine Livestock Grazing Capacity,” Cattlesoft, Inc., http://www.cattlemanagement.com/determine-livestock-grazing-capacity (accessed March 2012)
[3] National Parks Service, “The Desert Ranchers,” http://www.nps.gov/orpi/historyculture/ranching.htm (accessed March 2012); Bill Broyles, George Huey, Organ Pipe Cactus National Monument: Where Edges Meet (Tuscon, AZ: Southwest Parks and Monuments Association, 1996), 52.
[4] National Parks Service, “The Desert Ranchers,” http://www.nps.gov/orpi/historyculture/ranching.htm (accessed March 2012).
[5] Bill Broyles, George Huey, Organ Pipe Cactus National Monument: Where Edges Meet (Tuscon, AZ: Southwest Parks and Monuments Association, 1996), 52-53; National Parks Service, “The Desert Ranchers,” http://www.nps.gov/orpi/historyculture/ranching.htm (accessed March 2012).
National Border, National Park: A History of Organ Pipe Cactus National Monument 