Determine How Many Acres of Pasture are Required For Your Cattle

When you want to graze livestock, it's particularly important to know just how much land needs to be made available for your herd.

This article is particularly useful if you have a certain group of cattle and are looking for a particular piece of land to graze them on. It will focus on relatively simple calculations associated with continuous grazing livestock on a large piece of land, with an example to help you understand the formulations further.

Steps

  1. Calculate how many animal units you have. In order to do this, you first need to know just how many animals you actually have, plus the average weight of the herd. For example, you may have 100 head of 1400 lb mature cows. The next thing you need to know is what an "animal unit" is.
    • Animal units (AU) are the basis of calculating stocking rates and pasture space for a group of animals. The standard single (1) AU is equivalent to one cow-calf pair weighing a collective 1000 lb. Some sources may confuse you with "with or without a calf" or as just a cow, but you should not worry about those technicalities so much as that 1 AU = one 1000 lb cow (or cow-calf pair).
    • To calculate the number of animal units you have, multiply the number of head by the animal unit equivalent of your cows.
      • By the example above: 100 cows x (1400 lb [weight of your cows] / 1000 [weight of one animal unit]) = 100 head x 1.4 AUE = 140 AUs
      • An "animal unit equivalent" is the adjustment that is calculated from the standard animal unit based on the average weight of your livestock. AUEs can be used for any weight of grazing animal being used on the pasture, from sheep to bulls to horses to deer. The calculation simply involves dividing the average weight of your animals by the weight of the standard animal unit.
  2. Calculate the estimated stocking rate for your area. The stocking rate for your location is determined by the amount of forage you have in terms of pounds per acre or kilograms per hectare, the utilization rate expected by the animals based on pasture condition, and how much of that forage is utilized by a standard animal unit for one month.
    • The amount of forage you have in your pastures can be eye-balled, though if you have not learned how to properly eye-ball how much forage you have, you can be far more in accurate than if you clipped various samples around the pasture, and weighed then after they are dried down.
      • To get an accurate clipping measurement, you need to use a 1 ft by 1 ft quadrat (square) so that you can calculate the amount of forage (in pounds) per acre (where 1 acre = 43,560 square feet) after its dry matter weight has been assessed.
    • Utilization rate is a percentage of how much a cow should remove compared with the total amount of forage that is actually there.
      • For most pastures that are in decent condition (with 6 to 8 inches of forage or more) the utilization rate should not exceed 55%. Poorer pastures with low amounts of herbage should have a utilization rate of less than 30%. Excellent pastures can have a high utilization rate only if the animals are in there for a short amount of time; otherwise it is best to stick with a 50 to 55% utilization rate to ensure plenty of plant material is left behind.
    • One AU is estimated to consume between 600 to 900 lb of forage in one month. To make things simpler, and as an average used by all stockmen to calculate their stocking rate for their land, the best estimate is that one animal unit will consume 800 pounds of forage in one month.
      • This is what makes an Animal Unit Month (AUM).
    • To calculate out stocking rate, let's assume your pastures are producing 1600 pounds of forage per acre. With a 50% utilization rate, and assuming one AUM consumes 800 pounds of forage a month, that will give you a stocking rate of 1.0 AUM/acre.
      • The formula is: Stocking Rate = (Forage production x [Utilization rate/100])/800 lb/month
        • Per the example: Stocking rate = (1600 lb/acre x (50%/100))/800 lb/month = 1.0 AUM/acre
  3. Determine how long your cows will be grazing on a piece of land for. Typically this is measured by months, not days. For most North American areas (not including the tropics) a typical grazing season is between 4 to 6 months. More southern grassland areas, like in California and down into Florida and Texas, are more flexible with being able to graze for 12 months a year. Thus, let's assume you live an area where you are only wanting to graze these 100 cows (see step 1 above) for 4 months.
    • Only use days if you are splitting up the pasture into multiple paddocks to rotationally graze your cattle in. This requires more extensive calculations, and an entirely different how-to article to perform.
  4. Put it all together to find how just how much space you need for your herd. Once you have figured out how many head of cattle you have, their weights and corresponding animal units, your stocking rate, and how long you intend to graze, you can plug them into a formula to get the amount of land needed for your herd.
    • The formula is: Required Pasture = (Number of Animal Units [Step 1] x Stocking rate [Step 2]) / Time to graze [Step 3]
      • Using the example created: Required Pasture = (140 AUs x 1.0 AUM/acre) / 4 months = 560 acres needed.
  5. Find out if you can get the required space for your livestock. You may need to recalculate if you cannot get the amount of acreage for your herd. Recalculation may involve reducing the number of cows, or the amount of time they spend in the pasture.
    • It's highly recommended to not change the weights of your cows if you've already determined their weights, because you could short-change both yourself and the person you may be leasing the land from to use to feed your cows for the summer. If you reduce the weight of your cows from 1400 pounds to 1200 pounds, you may be inviting serious overgrazing by libelling about your cows' actual average weights. This could land you into trouble and potential for being not allowed to lease from that person again.
    • Also do not change stocking rate unless you're absolutely sure that it needs to be changed. Always remember, the shorter the grasses look and the thinner the sward (collective term for lots of forage plants), the lower the stocking rate will be.
    • If you reduce the number of cows you have or the time allowed to graze, you can reduce the amount of land needed to raise your animals on. For instance, 90 head of 1400 lb cows can be grazed on 504 acres of land, whereas if you decided to graze your 100 head of 1400 lb cows for only 2 months, you could leave your cows on 280 acres for two months.
  6. Turn your cows out to pasture and let them graze for the allotted time. Keep an eye on the pasture and the cows. If you find they are not getting enough, you may need to pull them out sooner than expected.
    • These calculations are merely a guide and an estimation. They are not exact, nor ever will be. It's up to you, as manager, to pay attention to the resource (forage) the cows need in order to both keep your cows happy, and not cause significant damage to the land.

Tips

  • Monitor forage and your animals throughout the season. Forage production is the greatest influence on stocking rates, and is prone to change throughout the season.
  • Remember most calculations are on a dry matter basis, unless otherwise specified. Dry Matter means when a sample of forage is, essentially, cooked until all the water is removed, and it is weighed as "dry matter." As-fed refers to forage that is fed as it is without going through the oven to dry it out.
  • To get a more accurate stocking rate calculation for your farm or ranch, it's highly recommended to take clippings and make dry matter measurements yourself. Take several samples over the pasture, and use a microwave oven (most economical and practical, just never use the one in the kitchen) to dry them out to get dry matter weights per square foot of sampled area.
    • Make sure you get more than one sample (at least 10 in different areas of the pasture or paddock is best) to calculate your average forage mass and forage yield.
    • Also, remember to weigh the container that holds the forage samples first in order to calculate just the weight of the sampled forage only, not the forage and container together. If you don't do this you could skew the results.
  • If you must and to save yourself time and more tedious work (because clipping, drying and weighing forage samples is certainly that), contact a local agricultural extension specialist to get an estimated stocking rate for your area.
  • There are two additional ways to obtain the animal unit equivalencies of your stock. One is using metabolic body weight calculations. The second is using divisible daily intake.
    • Metabolic body weight is a mathematical approach to equate animals based on their body surface area than true body weight–generally accepted and calculated as the animal's weight in pounds (or kilograms) to the 0.75 power (BW^0.75).
    • The MBW of one AU is: (1000 lb^0.75 =) 177.8 lb. If you had a herd of ewes that averaged 200 lb per ewe, then her AUE would be calculated as follows: (200 lb)^0.75 = 53.2 lb, thus (53.2/177.8) = 0.29 AUE.
    • Comparing daily intake levels is less mathematically complex to understand than using MBW calculations. Under grazing conditions, a 1000 lb cow-calf pair is expected to consume a rate of 2.5% of their body weight per day in dry matter forage. This means that one animal unit will consume (1000 lb x 0.025 =) 25 pounds of dry matter forage per day.
      • In order to get an AUE for the ewe above or even the article's 1400 lb cows, assume that both female species consume 2.5% of their body weight in DM forage per day. Thus, for the big cow, that means (1400 x 0.025 =) 35 pounds of DM forage per day, or for the ewe, (200 x 0.025 =) 5 lb of DM forage per day. To obtain their AUEs, simply divide their daily intake by the daily intake of the standard AU, as follows: For the cow: (35 lb/d) ÷ (25 lb/d) = 1.4 AUE; for the ewe, (5 lb/d) ÷ (25 lb/d) = 0.2 AUE.
  • Use a scale or weight-tape to determine the weight of your cattle more accurately. Some producers prefer to "eye-ball" their cattle, but this may not be the most accurate way, especially if you are not sure how to tell how much an animal weighs just by looking at it.

Warnings

  • Overgrazing is a real threat, and can be detrimental not only to the landscape, but to your animals too. Allowing your animals too long on a piece of land can reduce the amount of forage available next year without adequate rest and recovery.
  • Pasture conditions change all the time, just like environmental conditions do. They may change for the worse or for the better, so be prepared to de-stock (or re-stock) if need-be to keep pastures in good shape.
    • If feel you have to pull all your cows out of a pasture, then do so because it will be easier on the land when you come back to it in a better year.
  • Stocking rate calculations and calculations like that made above are mere estimates and guidelines, they are by no means accurate nor never changing. Take these with a grain of salt and make sure you monitor your cows and the forage that they are eating regularly.

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