Managed Intensive Rotational Grazing. That’s a mouthful. MIRG for short.
MIRG is a system of pasturing animals to maximize pasture growth. Proponents call it shepherding grass.
Rotational grazing is nothing new. Dividing up a pasture into paddocks to prevent overgrazing goes back to the earliest agrarian societies. What is fairly recent is intensive rotational grazing. Instead of pasturing animals in a large paddock for a month or two, then moving them to the next paddock, MIRG is pasturing animals in very small paddocks for just a few days at a time, or in some cases hours.
The heart of the idea stems from giving pasture the optimum amount of time to re-grow before being grazed again. Short grazing periods stimulate growth, long grazing periods destroy pasture.
Colorado State University
There are many benefits to the animals as well:
- Animals are exposed to more varieties of forage because they cannot simply return to their favorite alfalfa patch every day
- Animals are forced to move and exercise more
- Fewer parasites build up in the soil and affect the animals
- Manure is more evenly spread
Finally, perhaps the biggest benefit of all is soil health. MIRG and techniques like it build up far larger amounts of topsoil than any other known method. Rather than continually depleting the soil and subsidizing with chemical fertilizers, MIRG allows me to build healthier pastures that are self-sustaining.
Contrary to what it may seem, grazing is necessary to healthy pastures. If pastures are not grazed they turn into deserts. This was brought to my attention by the compelling TED talk by Allan Savory, a pioneer in the field of holistic land management.
Since acquiring our sheep last Summer I have had this nagging desire to figure out MIRG and apply it to our farm.
Tonight, I finally figured it out.
My first question was, can MIRG be used with a small scale of animals? We have 3 ewes and 1 ram currently, expecting lambs this Spring. The answer appears to be yes; I have found no literature saying there is a minimum amount of animals required. As long as the space requirements are met, the benefits of rotational grazing applies to 4 sheep just as much as 40,000.
My second question was, how do I size the paddocks for the number of sheep I have? The numbers are all over the map on this, and “stocking density” isn’t a terribly popular subject online. Thanks to a University of Minnesota Extension presentation, I was able to get some hard numbers and convert them for our farm.
The calculations use “Animal Units” or AU’s which are a livestock standard based on a mature 1,000 pound cow. The MIRG approach assumes 33 AUs per acre per day. So that’s 33 cows, or about 33,000 pounds. Keep in mind, this is dependent on the pasture productivity. It could be as low as 25 AUs or high as 80 AUs. But for the purposes of getting started, I am going with the 33 AU number.
My last question was, how big do I need to make the paddocks and how many of them do I need? If MIRG requires 33 cows per acre, that equates to 1,320 square feet per cow. Since our mature sheep are approximately 200 lbs, they will need 1/5th the space, or 264 sq ft.
The grazing periods should be as frequent as you can handle, no more than 5 days per paddock. Once per day seems to be a desirable number in most of the literature. But I have a day job, so I’m settling for twice per week. That means I will increase the spacing by 3.5, which is ~900 sq ft per sheep. Multiply by 4 sheep and we get to 3,600 square feet per paddock.
Square paddocks are recommended as opposed to rectangular or wedge-shaped which do not tend to get evenly grazed. So that means I’ll be making 60’x60′ paddocks (out of electric fencing) in addition to lanes for the sheep to follow to get to each paddock.
Finally, I calculated out how many times I would have to move the sheep to allow the paddocks 30 days of rest between grazing. It’s 10.
To recap, modified MIRG for 4 sheep:
- 33 Cows (AUs) / Acre = 165 Sheep / Acre (Using 0.2 AU)
- 1,320 sq ft / cow = 264 sq ft / sheep
- 3-4 day rotation = ~900 sq ft per animal
- 4 sheep = 3,600 sq ft total
- 3,600′ = 60’x60′ paddocks
- Requires 10 paddocks to achieve 29-day rest period
This is subject to change depending on the number of lambs born this Spring, and the productivity of our pasture. The rule of thumb was to graze the animals when the grass reaches 9-inches and to move them when its down to 2″ or 3″.