Same number of cows, same land = hidden overstocking. 

If you're running 100 head on 1,000 acres today—the same as you did 20 years ago, you might be putting more grazing pressure on your land. Why? Because those 100 cows likely weigh 1,400 lb each now versus 1,200 lb each back then. That's like adding 20 more cows to your pasture without adding any more grass. 

How Much Does a Cow Eat? 

On average, a grazing cow eats 2.2 to 2.3% of her body weight in dry matter (DM) every day on average. For example, a 1,200-lb cow eats about 26-28 lb of dry matter per day (1,200 × 2.3% = 27.6 lb). This varies based on: 

• Growth stage: A 2-year-old first-calf heifer (still growing) eats more as a percent of her total body weight than a mature cow. 
• Production status: A lactating cow eats significantly more than a dry cow. 
• Forage quality: Poor-quality mature grass = less intake; high-quality spring grass = more intake 

Why Calculate on a Dry Matter Basis? 

A cow eats a similar amount of forage dry matter whether she's eating lush spring grass or dry hay of similar quality. Only the water content changes—not her dry matter intake. Fresh spring grass contains 70-80% water (only 20-30% actual nutrients), whereas hay contains only 10-15% water (85-90% nutrients), meaning 10 lb of fresh spring grass only contains 2-3 lb of dry matter while 10 lb of hay contains 8.5-9 lb of dry matter. 

Animal Units (AU) and Animal Unit Months (AUM)  

• 1 Animal Unit (AU) = 1,000-lb cow with a calf under 3 months old 
• 1 Animal Unit Month (AUM) = the forage one AU consumes in one month = 780 lb of air-dried forage 

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.  

Converting Your Herd to Animal Units 

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 guessing 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. 

Example: 

• 1,200-lb cow + 400-lb calf (4 months old) = 1,600 total lb ÷ 1,000 = 1.60 AU 

Determining Available Forage 

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.  

Example Calculation: 

• Range produces 1,250 lb of forage per acre 
• Available for cattle: 1,250 × 25% = 313 lb per acre 
• AUM per acre: 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. 

Calculating Your Stocking Rate: A Real-World Example 

The Situation: 

• 2,560 acres divided into eight 320-acre pastures 
• Rotational grazing from June 1 to October 31 (5 months) 
• Average 0.35 AUM/acre available forage 
• 1,200-lb cows calving in March/April 
• Calves average 550 lb at November weaning 
• Bulls: 1,800 lb, 60-day breeding season, 1:25 ratio 

Step 1: Calculate total available forage  

• 2,560 acres × 0.35 AUM/acre = 896 total AUMs available 

Step 2: Calculate demand per cow-calf pair over 5 months 

Step 3: Calculate bull requirements  

• 1,800 lb ÷ 1,000 = 1.8 AU × 2 months = 3.6 AUM per bull 
• With 1 bull per 25 cows: 3.6 AUM ÷ 25 cows = 0.14 AUM per cow for bull 

Total demand per cow-calf pair: 7.95 + 0.14 = 8.09 AUM 

Step 4: Determine stocking rate  

• 896 available AUM ÷ 8.09 AUM per pair = 111 cow-calf pairs 

Result: Stock 111 pairs on 2,560 acres = 23 acres per pair for the 5-month grazing season 

Key Takeaways for Producers 

1. Monitor cow weights—bigger cows eat more  
2. Use actual data, not estimates—weigh cows for realistic averages 
3. Account for calf growth—older calves increase forage demands 
4. Plan for bulls—they add to forage requirements 
5. Conservative stocking protects range—overgrazing costs more in the long run 
6. Keep detailed records—track actual grazing demand against these calculations 

Questions to Ask Yourself 

• Have cows gotten bigger over the years? 
• Am I running the same number of head on the same acres as 10-20 years ago? 
• Is range condition improving, maintaining, or declining? 
• Do grazing records match my actual forage production? 

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.  

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.