Recently, I’ve been trying to use some of the mathematical approaches to grazing management. This had always seemed a little too complex for me, but I decided to give it a go. I wanted to see how many pastures, how many days, and how much rest period I needed. To find this information out, you will need to have some numbers in mind already.

###### Days of Grazing and Days of Rest (4/8)

These two numbers you will choose for yourself. They will depend on what forage you are grazing. In my example, we will be grazing bahiagrass pastures. And we have research showing that the ideal rest period for bahia is 28 days. This is the point where we get the biggest bang for our buck in terms of forage yield and nutrition. For my example, I want to graze for 4 days. To determine how many pastures I need, I divide my days of rest by days of grazing and add 1 to account for the grazing in the current paddock.

###### Number of Paddocks (Pastures)

###### Number of Animals (Animal Units)

Next we need to think about how many head of cattle we are grazing. In a cow-calf operation, this number might change with the year. But the next number is where we can take that factor into account. For my example, we are running a herd of 50 head.

###### Average Weight of Animals

This number is simply the average weight of the cattle in your herd. But there are two important factors in this simple number. First, most people underestimate the weight of their cattle. Unless you’re an order buyer and looking at cattle and their weights almost every day, you might not be calibrated to guessing cattle weights. Surveys show that producers most often underestimate their cattle’s weight by ~150lbs. Second, this is where we can add the weight provided by the cows’ calf. If you grow 500 pound calves, you can add half that to your cow average. In our example we have an average cow weight of 1100lbs and we’re growing out 5 weight calves, so our number would be 1350lbs (1100lbs+(500/2)). (‘5 weight calves’ is cattlemen’s slang for calves that weigh 500lbs)

###### Forage Eaten per Animal

For this estimate, we usually use a percentage. Most large animals consume between 2.2 and 3% of their body weight in * dry* forage. I like to work things on the conservative side, so in this example we’ll use 4% to account for any trampling or other waste that might happen in the pasture.

###### Forage Yield per Acre

We need to know just how much forage is out there. This number can be difficult to estimate and can vary widely. In fact, this number will change monthly. Factors such as fertilization, season, forage variety and stand coverage will determine how much forage you have. For our example, we will be using a fertilized bahiagrass pasture in May so our estimate will be 1420lbs dry matter/acre. Again, this number is an annual average, so it provides us a good starting point that might need to be adjusted monthly.

Herbage production for Argentine (blue lines) and Pensacola (gray lines) bahiagrass under medium (100 lb N/A; solid lines) (Mislevy et al. 2003) and low (53 lb N/A; dashed lines) nitrogen fertilization. (Vendramini et al. 2013) UF/IFAS

###### Forage Utilization

This number is a percentage reflecting how much we want to remove from our pastures when we graze them. A good range is between 40 and 70%. There’s a grazing saying that goes, “take half, leave half,” meaning you only graze half of the forage you have available; that way, you have enough left over for adequate regrowth. So for our example, we will use 50% utilization.

###### Acres per Paddock (Pasture)

Now the moment of truth. You have your numbers; now let’s find out how many paddocks (pastures) and total acres you’ll need for this herd. Here is the overall equation:

So in our example, we need 8 paddocks (pastures) of 15.21 acres to properly graze this herd, for a total of ~122 acres. Again, many of these number we used are very forage dependent and therefore variable. I used to not like this equation because I felt like I was working backwards. I know how much land I have, I know my pastures, and those are difficult factors to change. But if you just plug some numbers in, you can play around with the formula and make it best fit your operation.