Wednesday, January 11, 2006

Rotational Stocking Calculations

In line with my interest in fencing costs, I've been looking for pasture layout plans and information on Rotational Stocking (also referred to as Rotational Grazing). I found a fabulous website run by Agronomy at Perdu University. It actually gives a formula using a grazing stick for determining how many animals a given pasture can support based on the forage per acre inch, grazing efficiency, animal weight, and intake. Once you get the hang of the following formula, you can check your skills using Perdu's Pasture & Forage Fun page ;)

(Total Forage/Acre) x (Acres) x (% Grazing Efficiency) _____________________________________________ = Days
(Animal Weight) x (Intake Rate in % of Body Wgt) x (# of Animals)

It's really quite simple, once you get used to the terminology.

Total forage per acre inch:
Each type of forage has a different dry-matter output, measured in pounds per acre inch. Basically, if you were to mow an inch off an acre of forage, the cutting would weigh within a certain range, depending on the type of forage. Bluegrass would have a higher weight range than Switchgrass. Now, this weight is also dependant upon the percentage of ground covered by the forage. Obviously, a nice dense field of Switchgrass would have a higher yield than a sparce field of Bluegrass. Cover percentage is mostly determined by sight and placed within three categories: under 75%, 75-90%, and above 90% cover. Use the Grazing Stick table from Perdu as reference. Once you find the proper range for the type and cover of your forage, multiply it by it's height (minus 4 inches to sustain). Switchgrass at 75-90% cover yields on average 250 lbs/acre inch. If it is 10" when you start grazing, and you must leave 4", that gives you 6" of feed. 250 Forage/Acre Inch x 6" = 1500 Total Forage/Acres. Got it?

Pretty self-explanatory...

Grazing Efficiency
From what I understand, the more paddocks within your pasture, the greater the grazing efficiency will be. Continuous Grazing has 40% efficiency, while seperating into 24+ Paddocks has 75% efficiency. High efficiency is good. Again, refer to the Grazing Stick chart.

Animal Weight
The average expected weight of your animals. I would treat all young as if they were adult size - they probably eat just as much since they are growing. If the weight difference between males and females is substantial, multiply the average weight of a gender by the percentage of that gender in your flock. If your 10 males weigh 140 lb on average, and your 40 females weigh only 110 lbs, then

(140 * 0.20) + (110 * 0.80) = 116 total average weight

Intake rate in percentage of total body weight
This is the amount of food your livestock will eat in relation to its own weight. Generally somewhere between 2-4%.

Number of Animals
This is the number of animals you wish to sustain on the given pasture/forage.

If you know how many pounds of forage you have available, combined with the efficiency of your paddock rotation, then you can figure out how long you can sustain a given number of animals based on their food intake. It really does make perfect sense ;)

Say we have 24 acres of Orchardgrass/Legume Mix with a decent but not prime cover, 12 inches tall, divided into 8 paddocks, and we're dealing with some lovely Shetland sheep of the smaller variety - mostly ewes and lambs with about 15% rams, averaging out to a generous estimate of 110 pounds each. Taking a look at the chart, or speaking with experienced farmers, we will find that they eat 3.5-4% of their body weight while grazing. I prefer caution, so let's take 4%. I think up here, the total number of days available for pasturing is 180. I could be too cautious, but that seems like a fair estimate.

(Total Forage/Acre) x (Acres) x (% Grazing Efficiency) _____________________________________________ = Days
(Animal Weight) x (Intake Rate in % of Body Wgt) x (# of Animals)

Pasture Height: 12" - 4" = 8"
Forage Type: Orchardgrass/Legume Mix
Percent Coverage: 75-90%
- Pounds of Forage/Acre Inch = 275
- - Total Pounds of Forage/Acre = 2200
Pasture Size: 24 acres
# Paddocks: 8
- Grazing Efficiency (based on # paddocks): 60%
Livestock Type: Sheep
Average Weight: 110 lbs
% Intake/Body Weight: 4%
Number of Days: 180
Number of Animals: ?

(2200 Total Forage/Acre) * (24 Acres) * (0.60 Grazing Efficiency) __________________________________________________ = # Animals
(110 lbs Animal Weight) * (0.04 Intake Rate/Body Wgt) * (180 Days)

2200 * 24 * 0.6
110 * 0.04 * 180

_____ =
40 sheep

That doesn't seem like a a great amount of sheep for 24x3 acre paddocks. Now, if you split the pasture into even more paddocks, thereby increasing your Grazing Efficiency, you should be able to fit more animals in the same area. A 24 acre pasture with 24 paddocks, each 1 acre, will have a grazing efficiency of 75%. If you redo the calculations using this figure, you get

2200 * 24 * 0.75
110 * 0.04 * 180

____ = 50 sheep

That seems like a better number to me, but still not that hot. Lets improve the yield of our forage to above 90% and see what that gives us

2600 * 24 * 0.75
110 * 0.04 * 180

____ = 59 sheep

Great! By increasing the number of paddocks and improving the forage cover, we've increased our flock by almost 50%. Wait - There's more!

The optimum grazing period for sheep is 3-5 days. Our sheep are currently taking 7.5 days to cut through 1 full acre of forage (180days/24 paddocks = 7.5). If we cut the paddocks in half, doubling their number to 48, our flock will have 3.75 days in each paddock, thereby hitting the "optimum grazing period".

So, now we have 48 paddocks, each 1/2 acre large. Thirty days after the flock has finished grazing the 1st pasture, they are onto the 10th paddock, but the forage in the first paddock has grown back to 12" and is prime for eating. We have 2 options - thresh it, or set another flock of 59 on it, and every 33.75 days thereafter. Since there are 48 paddocks, but a flock can only eat through 9, we'll need 5 more flocks of 59 to deal with the forage.

Total number of 110 lb sheep eating 4% of their body weight daily on 24 acres separated into 48 paddocks, using 12" Orchardgrass/Legume Mix at 90% coverage, with 5 cycles of 59 sheep = 295.

This of course depends highly upon the rate of regrowth of our forage. Slow-growing varieties might not be able to keep up every month, plus a drought could seriously affect our 90% cover, not to mention our sheep could be significantly larger due to the abundance of lovely fresh forage, and therefore start eating more... Or, perhaps you want to thresh the fields for winter sustenance - three cycles of 59 sheep, threshing in between each cycle. I haven't looked into winter feed costs yet, so I'm not too sure what the cost of feeding 295 sheep over the winter is.

I think I'd be more comfortable using the 75-90% cover calculations of 50 sheep/flock, for a total of 250 for 24 acres, or about 10 sheep/acre (which, incidently, is exactly what Maple Ridge Sheep Farm suggests for their Shetlands

Now you know how to calculate livestock/acre ;)

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