Herd Math for the Multi-species Grazier
By Paul Dorrance
It never fails. Like a group of fishermen telling their “big one that got away” story, where the fish gets bigger and bigger with each telling, we graziers are no different. Instead of comparing the size of the fish, we rank ourselves by asking a very loaded question: “How many cows do you run per acre?” Then we wait, hoping that the other person’s number is lower than ours, so that we can pat ourselves on the back for obviously being a better forage and livestock manager. Maybe we round our number up. Maybe to the nearest 10 head. And heaven help you if you happen to graze livestock AND fish …
Another iteration of the question is guaranteed to surface whenever I speak a group of prospective graziers: “How many acres do I need per cow?” The internet is full of “official estimates” and it is a critical question to ask as you are planning your initial grazing enterprise, but I feel like I let both experienced and future graziers down when my answer is always the same: It depends.
What Is Carrying Capacity, And Why Do I Care?
Calculating the carrying capacity of our pastures provides important information, but it is important to note that we are really just creating an estimate. There are so many variables that go into the set of assumptions, including climate, growing zone, forage species, environmental pressures, livestock selection, management strategies, etc. Absolutely use this discussion to calibrate your own eye for forage production and animal performance, or track capacity trends on your own acreage relative to weather, management changes, etc. But avoid the pitfall of comparison, even (or especially) compared to other graziers in your local area.
It is also important to define what “carrying capacity” is. Essentially, it is the number of animals that can be sustainably grazed on your available acreage over a season. The critical piece of that definition is sustainably grazed. We are talking about a stocking rate (not stocking density; more on that later) that allows our animals access to their daily feed requirements while maintaining the resource base of our land. If you graze more animals than your land can support over a season, then your resource base will begin to deteriorate in the form of decreased plant species, erosion, increased weed pressures and decreased productivity.
Additionally, carrying capacity is measured over a long period of time, either a season or a year. If you are a contract grazier who purchases weaned calves in the spring and sells them in the fall, then you would calculate your carrying capacity over the entire time you have animals on your pastures. If you have a cow-calf operation, then your calculation would typically be based on the forage growing season in your area. This isn’t a short-term stocking density calculation, although that does affect things. This is a season-long estimate of your sustainable stocking rate.
Pasture Math – The Basics
There are four basic parts that go into a carrying capacity calculation (say that three times fast!): forage production, utilization rate, animal nutrition requirements and length of grazing season. Forage production is expressed in pounds/acre, and can typically be found with a quick Google search. Remember to find estimates at least specific to your state/climatic region and your forage species, and make sure that the number is expressed as “dry matter.” For my area of south-central Ohio, with a mixed species of grasses and legumes, I use a three tons/acre figure for my calculations. Your region and species mix will result in a different number. Alternatively, you can calculate your actual production value by clipping, drying and weighing your forage — but seriously, who has time for that?
Utilization rate encapsulates the idea that the cow isn’t going to eat every single piece of grass on your place. In fact she will eat some, leave some, trample some and poop on some. The concept of “take half, leave half” is included in this number, as is selectivity and waste. This also happens to be where the tenets of rotational and management-intensive grazing (MiG) really shine, as increased stocking density concentrates animal impact, reduces selectivity and allows for plant rest periods. Typical utilization rates for a continuous grazing scenario are 30-40%, but rotating pastures even just four times per season increases your utilization rate to 40-60%. Practicing MiG principles with 8 or more paddock divisions can yield rates closer to 70-90% utilization! This is critical to maximizing our carrying capacity, and often could be the difference between being red or black on our farm’s balance sheet. That being said, especially if you are just starting out, I would recommend starting with a conservative number here to avoid over-stocking and pasture damage. It is so much better to have too much grass and not enough animals at the end of this calculation than the other way around. Let’s use 50% utilization rate for our example calculation.
Animal nutrition requirements, expressed as daily dry matter (DM) in pounds, are easily found online. While there absolutely are variations among animal personalities, the general numbers will work just fine for our purposes. It is more important to recognize the difference that life stage has on cattle requirements. For a dry cow I use 2% of her body weight, for a nursing cow I use 3%. So my typical 1,000-pound cow needs somewhere between 20 to 30 pounds of DM per day. I’ll use 25 pounds for the purposes of this conversation, in order to run a calculation for my entire herd over the course of both nursing and dry cow seasons.
Speaking of seasons, the last thing we need to consider in order to calculate carrying capacity is the growing season of our region. You obviously can’t graze something that isn’t there, and so we have to put a time limit on our cow’s grazing nutritional requirements. If you are in a climate that allows year-round grazing then by all means use 365, but for the rest of us, our average growing season can be generally be found with a quick internet search. Keep in mind that this number is forage-specific, so if you are finishing animals on warm-season annuals, then your number will be much smaller than mine grazing cool-season perennials. Southern Ohio’s growing season is usually mid-April through mid-October, or roughly 180 days.
Cow Day Math – The Formula
So, we’ve finally arrived to my favorite part — formulas! We just have to determine which question we are seeking an answer to. It’s the same formula, just run differently to answer either:
1. How many cows can I put on my X acre
farm?
2. How many acres do I need to support my X cows?
If your acreage is set (option 1) then you are looking for your land’s carrying capacity, which looks like this:
Acreage Carrying Capacity = Forage Production x Utilization Rate x Available Acreage Carrying Capacity ÷ Daily Intake x Growing Season
Using my example numbers from above, this formula would work out like this:
Carrying Capacity = 6000 x .50 x 70 ÷ 25 x 180 = 46.6 cows
So in our theoretical scenario I can sustainably run 46 cows on my 70 acres of pasture. Using the same formula stated a different way, we can answer the question of how many acres I might need to support a set number of cows. Why ask this question? Maybe we have a potential new market for X number of animals, or maybe our financial calculations tell us that we need to move X number of cattle in order to be profitable. How many acres would I need to rent, plant or fence in order to take advantage of a potential opportunity? Using a set number of cows (option 2), the formula changes to look like this:
Acreage Required = Number Of Cows x Daily Intake x Growing Season ÷ Forage Production x Utilization Rate
Already knowing what the number should end up to be, here is what my example numbers look like:
Acreage Required = 46 x 25 x 180 ÷ 6000 x .50 = 69 acres
Buckle up, it is time for me to get on my soap-box for a minute. Taking a second look at both of these formulas you can see the multiplier effect that increased utilization rate has on the formulas, you can exponentially increase your carrying capacity. Changing only the utilization rate from 50% to 70% (the low end of the MiG range), I would increase my carrying capacity to 65 cows… a 150% increase in inventory that represents a change in net income (in my direct marketing world) of $45,600! That is the value in managing your pastures with excellence, and fully leveraging the benefits of rotational and management-intensive grazing techniques.
What About Multi-Species Operations?
Up to this point we’ve kept the conversation to just cows for simplicity, but everyone knows that sustainable and regenerative systems are anything but simple. If you are running a herd of mama cows or stocker steers, there’s absolutely no reason for you not to consider adding in a small ruminant as well. You’ll increase your forage utilization, differentiate yourself in the market, and increase your bottom line. Who doesn’t want all of that?
To take the same information already presented, but alter it for multi-species purposes, we have to remove the “cow” and replace her with an “animal unit” or AU. Don’t feel bad for her getting replaced; she is still the basis of the AU measure, where a mature cow (and her unweaned calf) represents 1 AU. In actuality, we are concerned more with her DM requirement, since that is what will change as we begin substituting different classes of livestock. So when we equate 1AU with 25 pounds of DM per day, then the first formula doesn’t change except to yield a carrying capacity as AUs instead of cows.
Still with me? Awesome! To translate our results into various combinations of animal species, the final thing we need is an equivalency chart. There are many different versions of this chart running around, but most differences are subtle and unimportant. The one I’ll provide here is adapted from the NRCS National Range and Pasture Handbook:
Class of Animal | Animal Unit Equivalent |
Cattle, Cow with Calf | 1.00 |
Cattle, Bull | 1.40 |
Cattle, Weaned Calf | 0.60 |
Cattle, Yearling | 0.70 |
Cattle, 2 Year Old | 0.90 |
Bison, Cow | 1.00 |
Bison, Bull | 1.50 |
Sheep, Ewe with Lamb | 0.20 |
Sheep, Ram | 0.25 |
Sheep, Weaned Lamb | 0.12 |
Goat, Mature | 0.15 |
Goat, Weaned Kid | 0.10 |
Jackrabbit | 0.02 |
Prairie Dog | 0.004 |
I went ahead and included the jackrabbit and prairie dog data, since pastured rabbits are the next big thing in food and you just never know what is next after that. In case anyone is wondering, according to my math I can sustainably graze 2,330 rabbits on my 70 acres!
Silly example or not, that is how you take the final step in adapting our formulas for multi-species use. Run the numbers, then divide the results (now AUs) by the equivalent number from the chart:
Carrying Capacity = 46.6 ÷ 0.02 = 2,330 rabbits
More than likely, you will want to add enterprises to your existing herd instead of replace all your cows with rabbits. In this case, just subtract the desired number of cattle AUs from the total carrying capacity, then divide the remaining capacity by whatever enterprise you are considering adding.
I hope this article has helped shed some light on the concept of carrying capacity, factors that influence it and how to adapt the formulas for multiple species. Additionally, I can’t let you go without reiterating the value of rotational and management-intensive grazing methods, whether you are grazing cows or rabbits. Now, when the inevitable question gets asked about how many cows you run per acre, your answer can be just like mine: It depends.
About Paul Dorrance
Paul Dorrance owns and operates a pasture-based livestock operation in Ohio, marketing 100 percent grass-fed beef and lamb, as well as pastured non-GMO pork, poultry and eggs directly to consumers. Previously an active duty Air Force officer, Paul still serves as a pilot in the Air Force Reserves.
Learn more about Paul by listening to his Tractor Time podcast interview here!