In last month’s livestock article, we talked about how to estimate the amount of available forage in a pasture. Once we have that estimate, we still need to know where to place our temporary fence. This will depend on several factors: the dimensions of the pasture, the number of animals to be grazed, and the desired forage utilization rate.
That may sound like a lot of numbers to keep track of, but in practice each one helps determine all the others. Taking it step by step helps to keep things manageable.
Continuing with the example from the article on estimating available forage, we know that our pasture has 2,129 pounds of dry matter per acre.
To know how long that will last a herd of cows, we need to know how many cows are in the herd, and how much they weigh – information that, as managers, we’re likely to have. In this example, we have 68 cows, weighing 1,200 pounds each.
A mature dry cow needs to consume 2% of her body weight in average-quality forage dry matter each day. (We can look that figure up in a book, or online.) For a 1,200 pound cow, that works out to a 24 pound daily dry matter requirement.
To find out how much the herd as a whole needs to eat each day, we multiply that 24 pounds a day by the 68 cattle in the herd. That gives us a figure of 1,632 pounds of dry matter a day.
We already estimated that each acre of our pasture has 2,129 pounds of dry matter – more than our herd’s daily requirement. We might be tempted to just divide 1,632 by 2,129 and conclude that we need to set aside 0.77 acres a day to graze this herd.
But wait! If we do that, we’re assuming that the cattle are going to eat the forage right down to ground level, the way we clipped it when estimating the amount of available forage. That would be a 100% utilization rate – not the healthiest approach to pasture management.
A more moderate and sustainable approach would be to use the “take half, leave half” rule of thumb. Since this pasture is past its peak growth of the season, and the cattle are mature and dry, we can up our desired utilization rate to 70%. That is, we want our herd to eat 70% of the available forage in this pasture, and leave the rest to regrow and protect the soil.
Seventy percent of 2,129 pounds is 1,490 pounds. So, to achieve a 70% utilization rate, we need our herd of cows to eat 1,490 pounds off of every acre we graze them on. We divide their intake requirement of 1,632 pounds by 1,490 pounds per acre to find that our herd needs to graze right at 1 acre per day.
Now that we know how much of our pasture to fence off temporarily for each day’s grazing, we can work out where to put our temporary fence. We know that our pasture is 183 feet wide. We also know that there are 43,560 square feet in an acre. Dividing 43,560 by 183 tells us that we should run our fence across the pasture 238 feet into the pasture to separate out one acre for a day’s grazing:
Since there are seven acres in this pasture, we’ll be able to graze our herd here for a week.