How many square feet in an acre? Most of you can answer 43,560. Now, how many square yards in an acre? This is not usually common knowledge, but it is 4840. Why is that important? Read on to find out.
The most critical aspect of adaptive grazing management is to target short grazing bouts at high animal density, and then allow the forage a long rest period. When plants are allowed to rest after being grazed they can use their carbohydrate stores and photosynthesis in residual green leaves to regrow quickly assuming the grazing animals are not there to nip off the new growth before it can really make a start. There are a myriad of techniques employed to achieve this goal, but with any of them an estimate of the size of the allocation is necessary.
When I was first trained to layout temporary paddocks, I learned that you need to know: 1) the animal demand (total animals x target level of forage consumption), 2) the amount of available forage (lbs per acre above a target grazing height), and 3) target forage utilization efficiency (how much of the grazable forage is left behind). We often use a percent of body weight to estimate the demand, with most cattle groups needing between 2 and 2.5% of body weight intake. So, for example, 20 head of 1000 lb cattle at a 2.5% of body weight intake will need 500 lbs. of dry matter every day. If we want good individual animal performance a 75% utilization rate might be our goal, meaning you need to offer the animals 667 lbs of available forage. If the pasture is estimated to have 2000 lbs/acre available forage, then we need to allocate 1/3 of an acre.
Finally, the 1/3 acre paddock is stepped off and set up with temporary electric fence. Of course this estimate of the amount of pasture to allocate is a starting point, and it needs to be adjusted as the animals are moved. If they are consistently given less than they need (less residual than you planned), animal intake and performance will be reduced, and the plants will be grazed too short. If you give more than they needed, the animal performance will be good, but you will leave more residual than you planned. With each move, the grazier needs to critically evaluate the previous allocation and make adjustments up or down.
So, back to the title of this article. As I said previously, I was trained to layout paddocks in square feet. We know that an acre is 43,560 square feet, so 1/3 of an acre is 14,375 square feet. I was also trained to make a 3 foot step to make the math easy. Once one dimension is stepped off, then the other dimension can be calculated. So, if the paddock is to be 75 steps wide (225 feet), then the other dimension needs to be 64 feet or 21 steps. This seems complicated, but really is simple math, just a series of several calculations. As I got more experience with this I started carrying a small calculator so I could keep up with all that in the field as it was complicated enough to where I had difficulty doing it all in my head.
One day while laying out pasture I had forgotten my calculator and ran into the problem of multiplying everything by 3 (to go from my 3 foot pace to feet). I realized suddenly that all I had to do was remember how many square yards there are in an acre, and then I could skip the conversion from feet to yards. So, I now know there are 4840 square yards in an acre. I could step off the 75 steps in one direction and then calculate that I need to step off the 21 steps in the other dimension to get my 1/3 of an acre (1613 square yards). With a little practice I found I could do that one step in my head pretty easily.
One additional complication to this for me is that as I age that 3 foot pace is more and more difficult to achieve without really stretching. Again, in a moment of insight I realized it didn’t matter how long my pace was as long as it was consistent and I knew how many of my square paces were in an acre.
So, now my more comfortable pace is 32 inches, and there are 5400 of my square paces in an acre. This has pretty much allowed me to work without the calculator and to make the daily adjustments I need to make to keep my utilization efficiency consistent despite changes in pasture mass and paddock shape.
If you estimate the area to give grazing groups and use square feet per acre as a reference number, learn how many of your square paces is in an acre. If you don’t use any math to allocate pastures, try it using the square pace approach to make it a quicker and easier concept to apply. I find that the doing this math in my head while moving fence is great mental exercise and I can keep both hands free. Most importantly, estimating the size of your allocations allows for rapid and effective adaption to changing conditions when cattle are moved frequently.
Matt Poore, Professor of Animal Science at North Carolina State University, a Farmer in Halifax County, Virginia, Director of the Amazing Grazing Program, and Chairman of the Alliance for Grassland Renewal.
The Alliance for Grassland Renewal is a national organization focused on enhancing the appropriate adoption of novel endophyte tall fescue technology through education, incentives, self-regulation and promotion. For more resources or to learn more about the Alliance for Grassland Renewal, go to www.grasslandrenewal.org
The Alliance for Grassland Renewal 