Beware of Stocking Rate Creep
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Is your average cow size greater than it was ten or twenty years ago? As breed genetics and harvest weights change, the cows grazing pasture today tend to be larger than they were 10 or 20 years ago. Larger cows eat more, and if an operation is running the same number of cows today for the same amount of time on the same amount of rangeland as 10 or 20 years ago, the stocking rate has increased. But has the forage production increased to match the stocking rate?
How Much Does a Cow Eat?
The nutritive value of 1-lb of dormant winter grass cannot be directly compared with 1-lb of growing spring grass. To accurately compare these feeds, they must both be on a dry matter (DM) basis, or the weight of that 1-lb of grass after all the water has been removed. The amount of forage DM a grazing cow eats every day varies from 1.5 to 3.5% of her body weight. This percent consumed depends on her growth stage, production, and forage quality. A 2-year-old, first-calf heifer is still growing; a 6-year-old cow is not. Dry cows in mid-gestation on low quality mature forage may only eat 1.8% of their body weight on a DM basis while those same cows at peak lactation on excellent quality forage may eat 3.5% of their body weight on a DM basis. A standard number to calculate daily DM consumption of feed by a beef animal on forage is 2.2 to 2.3% of its body weight.
Stocking Rate, AUM, AU
The Society of Range Management defines stocking rate as the relationship between the number of animals and the grazing management unit utilized over a specified time.
The animal unit month (AUM) estimates how much forage one animal unit (AU) grazes/eats in one month. In beef production, 1 AU is often considered a 1,000-lb cow with a calf less than 3 months of age. Based on research, one AUM is estimated at 780 lb of air-dried forage (90% DM). Another way to say this is it takes 780 lb of air-dried forage for one month to feed a 1,000 lb cow with a nursing calf less than three months of age.
1 AUM = 780 lb air-dried forage
1 AU = 1,000 lb cow w/calf less than 3 months old
Your Cows = How Many AUs?
Many beef cows with a nursing calf less than 3 months of age do not equal 1 AU. Not everyone has a scale handy to weigh cows, but instead of “guesstimating” cow size, look at the weights of your cull cows sold at the sale barn. Those cows may not represent ideal cows in the herd, but they will get an estimate. If the average cow size is 1,200 lb, one cow = 1.20 AU. An AU does not count the calf if less than 3 months of age. For calves 3 months and older, add their weight to the AU calculation.
1,200 lb cow + 400-lb calf (4 months old) = 1.60 AU
How Much Forage can be Grazed?
On range and pasture, forage production available for a grazing animal can be expressed as AUM/acre. In continuous season grazing, 25% of total forage production is considered available for grazing. Another 25% is lost to trampling, wildlife, and forage-eating insects. The remaining 50% maintains plant health and vigor, protects the soil, and provides ground cover to capture and reduce evaporation. If a range site during peak standing forage is estimated to produce 1,250 pounds of air-dried forage (90% DM) per acre.
1,250 lb forage × 25% available to grazing cattle = 313 lb/acre
Remember: 1 AUM = 780 lb air-dried forage
313 ÷ 780 = 0.4 AUM/acre
Rangeland forage production can vary widely. Annual precipitation, range condition, soil type, slope, and growing season affect the amount of forage produced. If a range site is estimated to have 0.4 AUM/acre of forage that can be grazed, this means 2.5 acres during the growing season are needed to feed 1 AU (1,000 lb cow with a calf less than 3 months of age).
The NRCS office and Web Soil Survey can provide information to estimate available AUM/acre for a range site. “Truth test” these values to verify that current range condition compares with NRCS estimates. Historic grazing records can be a helpful tool when used with range assessment to determine a reasonable AUM stocking rate per acre.
Stocking Rate Calculation Example
Let’s say we have 2,560 acres of rangeland divided into eight, 320-acre pastures that will be used in a rotational grazing program from June 1 through Oct 31 (5 months). The pastures average 0.35 AUM/acre of forage available to be grazed. How many cows with calves can graze these pastures over the 5-month grazing period?
2,560 acres × 0.35 AUM/acre = 896 total AUMs available
To calculate the number of cow-calf pairs that can be grazed over the grazing season we need to know the total forage demand by the pairs and the bulls that will be turned out with them. To arrive at the forage demand, we calculate total weight that will be grazing per pair per month and divide that by 1,000 to calculate AUM demand per pair. These cows weigh 1,200 lb and calve in March and April. Calves average 550 lb at weaning November 1. Bulls (1,800 lb) will be with the cows for a 60-day breeding season at a ratio of one bull to 25 cows.
Calculate the bull demand and add to the cow-calf demand.
1,800 ÷ 1,000 = 1.8 AUM × 2 months (60-day breeding season) = 3.6 AUM for 1 bull
Divide total AUM available by the AUM needed for each pair for the 5-month growing season.
896 AUM ÷ 7.85 AUM/pair = 114 pair
This translates to a stocking rate of 1 cow-calf pair to 22.5 acres for the 5-month grazing season on this example rangeland.
For more information on understanding stocking rates and grazing management, please see the Extension Publication Integrating Management Objectives and Grazing Strategies on Semi-arid Rangeland.
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