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Not all cows are equal; some eat more

Jack Arterburn for Progressive Cattleman Published on 24 May 2019
Cows grazing in a pasture

On most ranches, average cow size has increased significantly over the past three decades as a result of genetic selection. These changes do not come without consequences to forage intake.

If the per-head counting method has been used to plan and track grazing, stocking rates may have unknowingly increased over time caused by increased forage intake of larger cows. Just as a lineman on a football team will eat more than the punter, larger cows will typically consume more forage than smaller cows. If the increase in forage demand is not accounted for, pastures may be getting overgrazed, which results in decreased long-term forage production and reduced plant community resilience to disasters such as drought.

Moving from a method of counting the number of head per acre to calculating animal units is a simple process and can help the manager accurately assess forage demand. The animal unit (AU) is a standardized unit for calculating forage demand and forage supply. For cattle, the standard animal is a theoretical 1,000-pound beef animal.

Animal unit equivalent (AUE) is used to adjust for cattle weighing more or less than 1,000 pounds by adjusting 0.1 AU for every 100 pounds of animal. For example, a cow weighing 1,200 pounds would equal 1.2 AUEs. A steer weighing 700 pounds would equal 0.7 AUEs.

Cattle consume an estimated 2.6 percent of their bodyweight in air-dried forage daily. Therefore, if the 1,000-pound standard animal grazes for one day, that animal will consume approximately 26 pounds of air-dried forage daily. This is equivalent to one animal unit day (AUD). A 1,200-pound dry cow will likely consume over 31 pounds of air-dried forage per day, and a 700-pound steer will consume around 18 pounds of air-dried forage per day.

Forage supply and demand are often displayed in animal unit months (AUMs), which is the amount of forage an animal will consume in one month, equal to 30 AUDs. AUMs are calculated by multiplying the 26 pounds of air-dried forage consumed daily by 30 days to get 780 pounds of air-dried forage consumed each month by a 1,000-pound beef animal. To adjust for animals not weighing 1,000 pounds, multiply the AUE by 780 pounds to calculate the animal’s monthly intake.

Using air-dried forage (90 percent dry matter, 10 percent moisture) can be helpful to visualize what this amount of forage looks like by visualizing the same amount of hay. It also makes comparing grazing and feeding hay easier. The bottom line: Knowing what cattle weigh and understanding their daily requirement is approximately 2.6 percent of their bodyweight in air-dried forage helps the grazing manager accurately calculate how much forage they are consuming.

Forage supply can also be calculated using AUs. We know 780 pounds of air-dried forage is the estimated requirement to feed a 1,000-pound beef animal for one month (1 AUM = 1,000-pound animal grazing for one month = 780 pounds of air-dried forage consumed per month). If a range site produces 2,000 pounds of air-dried forage annually, and a 25 percent grazing harvested is planned, 500 pounds of air-dried forage are available per acre. In a 200-acre pasture, there are 100,000 pounds of air-dried forage available (500 pounds multiplied by 200 acres) for the entire growing season.

If we are planning to graze 1,200-pound cows for one month, we can calculate how many cows the pasture can carry by dividing the forage supply (100,000 pounds) by the forage demand of each cow. Cows weighing 1,200 pounds equal 1.2 AU. Multiplying 1.2 AU by 780 pounds of air-dried forage consumed by a standard AU results in 936 pounds of air-dried forage consumed each month by one 1,200-pound cow. The 100,000 pounds of forage supply divided by 936 pounds of forage demand yields grazing for approximately 107 cows for one month.

To simplify calculating and tracking forage supply and demand in each pasture, the University of Nebraska – Lincoln developed the Grazing and Hay Record Spreadsheet. This spreadsheet is a Microsoft Excel-based template used for entering basic grazing records of individual pastures to calculate available AUMs per acre and AUDs per acre based on forage supply and forage demand over a given period of time.

For an explanation on how to use it, visit online (Grazing and Hay Record Spreadsheet).

The user enters the number of acres, the forage supply (AUMs per acre) for each pasture and the forage demand by livestock class, number of head and their average AUE for the planned grazing period. Do not forget to account for forage demand of bulls and calves once they reach 3 months old.

Summary reports are included with the spreadsheet and allow the grazing manager to track pasture use over time, noting season of pasture use, total forage utilized, as well as hay usage and the stocking rate for the entire ranch.

Accounting for forage demand by cattle is critically important to knowing forage utilization on range and pasture. A 1,400-pound cow with a January-born calf is going to consume significantly more forage in a June-through-October grazing season than a 1,100-pound cow with a May-born calf.

Head counts and the number of days grazed are important pieces of information, but they are not enough. Truly calculating forage consumed based on AUEs will help the grazing manager correctly document pasture use and provide accurate information that can help with effective range and pasture management.  end mark

PHOTO: To adjust for animals under 1,000 pounds, multiply the AUE by 780 pounds to calculate monthly intake. Photo by David Cooper.

Jack Arterburn is a member of the University of Nebraska Beef Team.

Jack Arterburn
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