Carrying capacity: You gotta know if it can hold ’em

Overgrazing can really hurt productivity, so it pays to know how many cows you can put on a pasture

Struggling with finding the right amount of cattle to graze on a pasture or not sure how many cattle a new rented pasture will hold?

The Beef Cattle Research Council now has a calculator to help you with that.

There are four basic principles of range management: Balancing forage supply and demand; managing for uniform distribution of livestock; avoiding grazing during periods of peak growth or sensitive times for the plants; and providing adequate rest during the growing season.

“Balancing the forage supply and demand is the first and foremost principle,” Tara Mulhern Davidson, extension assistant with the research council, said in a recent webinar.

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Putting more animals on a pasture than it can support will cost you, she said.

“Getting a better handle on carrying capacity and avoiding overloading your pasture will ensure your pasture is productive for the long term.”

There are a host of things that affect carrying capacity, including soil type, topography, the types of forages, rainfall, and your fertility program to name a few.

But it comes down to two factors — how much forage a pasture can produce and how much the livestock on it are eating.

The forage component of the equation is measured in animal unit months (AUMs), which is the amount a 1,000-pound cow will eat in a month.

“It works out to be about 780 pounds of forage,” said Mulhern Davidson. “It’s another way to count how much forage is actually there.”

The animal component is measured in animal unit equivalent (AUE).

“Not all animals are equal and they don’t all eat the same amount,” she said.

Cow-calf pairs are going to eat more than yearlings, so the animal unit equivalent takes this into account.

“A 1,300-pound cow with a calf at her side is equal to 1.3 AUE, where a 1,500-pound cow with a calf at her side is equal to 1.5 AUE. If you’re running steers or heifers, their AUE is 0.80 to 0.85.”

The AUE part of the equation is straightforward: The bigger the cows, the fewer the pasture can support.

However, when it comes to the forage part, the calculator offers you a choice: Easy but less accurate — or a more precise method that will require you to get out some scissors and head to the pasture.

Method No. 1 uses provincial or regional estimates on forage (both tame and native).

“Within the tool there are different estimates of how much forage there is based on rainfall, evapotranspiration, and pasture condition,” said Mulhern Davidson.

This method can be done right from a computer with no special equipment required.

The second, more accurate, method starts with a sampling frame, which should be 20 inches (50 centimetres) square. You can build a frame from lightweight PVC pipe. Alternatively you can use a hoop (Hula or homemade) that is 22.5 inches (56.4 centimetres) in diameter.

You can use regional estimates with the Carrying Capacity Calculator, but collecting actual samples will provide a more accurate assessment for your pastures. The homemade hoop in this photo is made from tubing and electrical tape.
photo: beefresearch.ca

Take the frame and get 10 samples across the pasture at a time when forage production is around its peak. Picking representative areas is best.

“If you have any low, highly productive areas, you want to get frames from there,” said Mulhern Davidson. “If you have any high areas, with poorer soils and lower production, you’ll want to get some frames from there, to make sure you are representing the whole pasture.”

To sample, the forage within the frame is clipped with scissors (or hand-held sheep shears) at a height that a cow would physically graze.

“Collect the samples, one sample per frame, and place them in paper bags,” she said. “Allow the samples to air-dry for a few days. Those samples will be weighed with a kitchen scale, and you can input your results into the calculator.”

The Method No. 2 calculator will allow you to enter the 10 samples separately (for both the total weight of the sample and the weight of the green matter).

The calculator isn’t meant to be the last word on pasture management, but it does put some hard numbers on what your pastures are capable of.

“It’s really hard to fit pasture management into a box,” said Mulhern Davidson. “Range management is both an art and a science.”

The calculator can be found at ‘Decision Making Tools’ at beefresearch.ca.

About the author

Contributor

Jill Burkhardt

Jill Burkhardt, her husband, Kelly, and their two children, own and operate a mixed farm near Gwynne, Alberta. Originally hailing from Montana, she has a degree in Range Management from Montana State University. Jill’s agricultural passions are cattle and range management but she enjoys writing and learning more about all aspects of farming.

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