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Management

Plant health: Carrying capacity impacts plant viability

Determining the carrying capacity of summer grass can not only help a producer calculate forage availability, but it can also help them manage their range resources. 

“Good range management practices are important for the long-term sustainability of our range resources,” according to Karla Jenkins, University of Nebraska beef cattle extension specialist. 

Jenkins presented a recent webinar on “Calculating AUMs.” 

“The importance of good range management has become particularly obvious since last year’s drought,” she explained. “If we don’t keep a good handle on how we graze our pastures, it can cause long-term damage that may take a long time to heal, or may never heal.” 

Range records

Although Jenkins said producers can use estimates to get a good starting point on how heavily to graze, good annual production records are more important.

“Many factors can impact the actual carrying capacity of the range,” she explained. 

For example, the physiological state of the animal, such as whether it is lactating, can cause the animal to consume 20 percent more forage. Insect damage, hail, drought and fire can also impact range conditions. 

“It is hard to know if we are getting where we are going if you don’t know where we’ve been,” Jenkins continued. 

Producers should keep track of when and how long cattle were in each pasture, how many AUMs were used and the amount of rainfall received in the pasture. 

“Actual records are a much better estimate than a book value of what the value of that pasture will be over time,” she said. 

Taking photos is also a good way to remember what a pasture looked like and how it has changed over time. Producers should take photos of the same area with a distinct marker so they can see how the pasture has changed. This marker can be anything from a steel post to a landmark.

Starting point

One animal unit month (AUM) is equal to 780 pounds of air-dried forage. One animal unit day (AUD) is equal to 26 pounds of air-dried forage. One animal unit (AU) of beef is a 1,000-pound animal. Jenkins reminded producers that since not every animal weighs 1,000 pounds, they need an estimated weight of the animal to accurately determine AUMs – or an animal unit equivalent (AUE). For instance, a 1,200-pound cow is 1.2 AUEs, a 300-pound calf is 0.3 AUEs, and a 2,000-pound bull is two AUEs. 

“It is important to figure in the bull when calculating AUMs,” Jenkins said. “He will be consuming a considerable amount of forage during the time he is in the pasture.”

At some point, the calf will also need to be added to the amount of AUMs needed. Jenkins suggested that for a cow that calves at the end of March, the calf will need to be added to the AUMs in June. 

“By then, it will be consuming enough forage that it will need to be accounted for,” she explained. “As we go through the grazing season, we will need to account for the calf eating about one percent of his body weight while he is nursing the cow. That amount will slowly go up as his body weight increases.” 

For a 1,100-pound cow and 300-pound calf, Jenkins averages the average forage demand to 1.4 AUEs per pair from May through October.

To determine the number of AUMs needed for 100 head of cows, multiply the number of pairs by 1.4 AUEs per pair and again by six months. One hundred pairs and four bulls grazing for six months, for example, will require 856 AUMs.

Available AUMs

Once the producer has determined how many AUMs are needed, the next thing that needs to be considered is how many AUMs are available. 

Forage availability is estimated by vegetative zone, soil type, range site, plant species and rainfall. Differences in rainfall can cause differences in vegetation, Jenkins said. Using a vegetative zone the range is located in can help the producer determine how much carrying capacity the range has by providing a starting point. The producer will then have to evaluate the range on past management, rating the condition from excellent to poor. 

Jenkins said if carrying capacity is 0.30 AUM per acre, the number of estimated acres of forage needed can be determined by dividing total AUMs need by AUMs produced per acre. For the 100 pairs and four bulls used in the previous example, 856 AUMs divided by 0.3 AUM per acre require 2,853 acres of estimated forage. 

“Determine what your pasture is,” Jenkins encourages producers. “The goal is to not only have long term sustainability of a range, but we also want to have good utilization, so we need to know what we are working with.” 

Making measurements

A special measuring device called a hoop or a square can be used to provide an estimate of how much forage is in a pasture. Jenkins recommends taking samples in several different areas of the pasture because there will be several variations. 

Once the hoop is placed on the ground, everything should be clipped inside the hoop, placed in a bag and air dried and weighed on a gram scale. Depending upon the size of the hoop, a conversion factor will help the producer determine how much forage is in the pasture. 

Jenkins said once the amount of forage is estimated, it has to be converted for the take half and leave half formula. 

“It is important to take half and leave half because the plant has to regenerate its roots and be able to grow for the next year,” she explained. 

Typically, 50 percent of the plant is vigor, 25 percent is for insects and trampling and 25 percent is for livestock use, she said. 

Once the producer has determined how much forage is available in the pasture, he can determine how best to graze it based on whether it is predominantly a cool or warm season grass and how many cattle he has to utilize it.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..