Do the math for proper pasture populations

Carrying capacity is the average number of animals a pasture can sustain over time. Stocking rate is the number of animal unit months (AUM) supplied by one acre of land.

It’s time to take stock when your pasture looks like a pool table.

Or rather, it’s time to take stock off.

Overgrazed pastures are all too common on the Prairies, so it’s important to calculate the carrying capacity of a field before turning out cattle or horses.

Barry Yaremcio, a beef and forage specialist with Alberta Agriculture, says producers make four common mistakes when calculating how many animals to put in a pasture.

“Number one, the actual size of the animal. When was the last time a farmer has actually weighted 10 percent or 15 percent of his cows and took an average cow weight? Most times they are underestimating weight by anyplace from 100 to 200 pounds.”

Yaremcio said that miscalculation has big ramifications. A 1,400 lb. cow will eat about 160 lb. of fresh grass per day. By underestimating average weight, the cow might be short by 22 to 25 lb. of needed forage intake each day.

The second common error is miscalculating how much feed is available to animals, which depends on grass and forage species, precipitation, soil type and other environmental factors.

Third on Yaremcio’s list is consumption.

“I might have 1,000 lb. of grass per acre, but how much are the animals actually going to consume? Is it 60 percent, 70 percent? How much will they lie on, how much will they manure on and how much will they actually waste and not eat?”

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Cattle eat the most attractive grass and forage first. The remaining fibrous and chewy grasses will not be as attractive and cattle will likely eat less. Producers have to take that into account when calculating their stocking rates.

Then there’s the major matter of pasture health, which can easily be jeopardized by overgrazing and inattention. Grazing in one year will affect grazing quality in the following year. Overgrazing damages plants and leaves pastures open to buckbrush and weed growth, as well as soil and wind erosion.

“There’s guys that just throw the cows into the pasture and say go, and they leave them there from the first of June to the end of October, and they graze the heck out of this land,” Yaremcio said.

“It looks like a pool table, and there’s nothing there to promote growth.”

He recommended rotational grazing, which can be reasonably easy using portable fencing that divides pastures into several paddocks.

Moving the cattle after they’ve eaten about 50 percent of available forage will leave the paddock in good condition for regrowth.

Grazing cattle also fertilizes the land. Studies show plants have higher nitrogen retention from fresh cattle manure than they do from manure hauled from elsewhere.

That’s part of the financial benefit, but Yaremcio also points to the costs of inadequate summer grazing in terms of cow condition.

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“Let’s say you’ve overgrazed your pasture, you didn’t wean your calves and that cow is going to be 100 pounds lighter than what she should be going into winter.

“Comparing the thin cow to the average condition cow, you’re looking at 1,400 lb. more hay to keep the thin cow warm and get her back up to good condition by calving season.”

Horses require more careful pasture management than cattle, Yaremcio said.

They eat more than cattle and have a grazing style more damaging to plants. They clip grass off rather than pull it as cows do.

“Potential for injury and harm to those grass species is much higher with horse than with cows,” he said.

“You have to be a better manager, you have to be on top of when to rotate those horses … than you do with cows because they’ll do more damage in a shorter period of time.”

How much do they eat?

  • cow intake depends on animal size and feed quality
  • cows normally consume 1.4 to four percent of their body weight daily
  • a lactating cow will eat 40 to 60 percent more than a dry cow
  • older cows eat more than younger cows
  • feed intake can increase up to 30 percent in colder temperatures and decrease by that amount in hot and humid conditions
  • snow and mud can decrease feed intake by up to 15 percent
  • nutrient deficiencies can decrease intake by 10 to 20 percent
  • growth promoting implants increase feed intake
  • cattle graze about 12 hours per day; horses graze 14 to 17 hours per day
  • a mature horse will eat about 22 pounds of dry matter daily and require 1.5 times as much pasture as cattle

Source: Alberta Agriculture

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