Sarah Flack, Sarah Flack Consulting
Amanda Gervais, University of Vermont Extension
In this video, offered by the eOrganic Dairy Team, grazing and organic certification expert Sarah Flack demonstrates how to calculate paddock size and stocking rates for pastures an organic dairy farm.
My name is Sarah Flack and I'm a grazing consultant. I also do organic farm inspections. Today, we are going to quickly run through how you can figure out how large your paddock needs to be to feed a herd of animals for a day. Then you can go on and do some stocking rate calculations to figure out how many total acres of pasture you need in order to provide the amount of dry matter from pasture to your animals that meets your farm goals.
Let's use an example here. We'll assume that it is a herd of dairy cows and there are 50 in the herd. This farmer's goal is to provide 30 pounds of dry matter per cow from the pasture per day. So this is a farm that's providing the majority of the dry matter from pasture. They're supplementing just a little bit of grain in the barn.
The first thing we need to do is determine what the total dry matter requirement is of the herd for a whole day. So I'll use my calculator--I'll take the 50 animals times the 30 pounds. I come up with 1500 pounds of dry matter per day. That's the requirement of that whole 50 cow herd from pasture.
Now that we've used the grazing stick, and have gone around the pasture and measured how much available grazeable dry matter is available in a whole acre, we came up with 1200 in our example. The next thing we are going to do is divide the 1200 into the 1500 and so we get 1.25. An acre-and-a-quarter is how much you need in order to provide the 1500 pounds of dry matter.
That means for every 24 hours, if you are using 24 hour paddocks that you are putting your animals in, each paddock would need to be an acre-and-a-quarter in size. So each paddock is providing the 1500 pounds of dry matter to the whole herd for that day. And you can go on later with those numbers once you know how long it's going to take each of your paddocks to grow back up to the full pre-grazing height -- in this case to about 8 or 9 inches of height. You can figure out how many total acres that you'll need to graze the whole herd now that you know how much it will take to feed them for 24 hours.
So now that we know that the herd needs an acre-and-a-quarter to feed them for 24 hours, let's figure out how many total acres are needed to feed that herd at different times of the year.
In the spring when the grass is growing very rapidly, it's going to take about 18 days for the pasture to grow back up to the correct pre-grazing height (in this case, the farmer's goal is to graze it when it is about 8-9 inches tall). So we take the 18 days and multiple it by 1.25 (an acre-and-a-quarter), and now we know that the farmer needs 22.5 acres in the spring to rotate throughout that's giving the cows a fresh paddock every day that is an acre-and-a-quarter in size.
Now later in the summer, when the speed that these plants out here in the pasture are growing at slows down, you'll need to add more acres in the rotation. So when you bring the cows back to the paddock, it's always at the correct pre-grazing height. This farmer's goal for the pre-grazing height is about 8 to 9 inches of grass and clover height when the cows come back into each paddock. Now, instead of taking 18 days for the plants to grow back, it's going to take more like 28 to 30 days in the middle part of the summer. On some farms, that will be significantly longer than that, so you need to use the numbers that are appropriate for your own area. Assuming the farmer is putting the livestock into this acre-and-a-quarter paddock every day and it is a 30 day regrowth period, we take the 1.25 and multiply it by 30. The farmer now needs 37.5 acres to rotate throughout to provide the same amount of dry matter intake to the cows.
You can see the farm has gone from needing 22.5 acres in the spring to 37.5 acres during the summer. There may be times in the summer where the regrowth periods are even slower than that and you would need even more acres. But this is a way to give you some ballpark numbers of how many acres you need at the different times of the year for this particular 50 cow herd.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.