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How do you decide what pastures to use, and how many cows to put on a pasture?
From an interview with Kit Metzger (Flying M Ranch)
As a friend in the Forest Service says, "the only way to find out how many cattle you can run in a pasture is to put them in there."
There are two initialisms you should know: AUM, or "Animal Units per Month," and ADA, or "Animal Days per Acre." AUMs are general units for planning and recording. If you had 200 head of cattle in a pasture for a month during a season, that pasture has 200 AUMs. The next time the permit is up for discussion or you're starting the next graze-plan, you can say "we've done 200 AUMs for the last few years, but the land is really taking a beating, so let's decrease to 150 AUMs." To figure out an ADA, which is more specific, you go out to the pasture and make a square X by X paces, and determine if that will hold a cow a day, based on the forage and water availability.
And then, Kit says, "you do the math." Most of your planning comes from knowing the outcomes of previous years: how many ADA one pasture has over another, and what your AUMs were. If you're not sure, it doesn't hurt to go out and do some monitoring!
But another factor to consider is that while this one area might hold a cow a day for ten days, in what condition will it hold that cow? Is it good feed or poor? You also have to remember to save, say, some of that ground in case of drought later in the season, or if the elk move on or off that area (depending on the season).
You want to be conservative when it comes to deciding land use and herd management.
From Bob Prosser
(Bar T Bar Ranch)
We at Bar T Bar use a rotation system that is based on not using a pasture the same time each year. In the winter country we are gone during the summer growing season, so we are just basically allocating available forage for the winter. Since we are there in the spring growing season, we try to rest each pasture one spring out of three.
Determining how many cows or calves and how long they stay in a pasture is based on historical data adjusted for vegetative production above or below the average. The long term average is different for every pasture but ranges from 4 to 8 Animal Days per Acre.
Although this is based on data, it is still an art to schedule the whole season, and requires monitoring during the grazing period.
Let's say there are 3600 acres in a pasture and long-term average is 5 ADA, then there would be 18000 available. If production is 80% of normal then it would be 14,400 Animal Days. If the herd is 300 cows it would be (14400/300) or 48 days of grazing.
In the summer country, since we are there during the growing season, the forage base changes daily as the rains come. We use the same historical averages, but use more of a reverse(early or late) use between 2 pastures, or rotate between many pastures changing the order, and in some cases an Annual Deferment between 2 pastures.
The summer country rotations are very fluid as rains vary from year to year.