A Little Grazing Math

photo-1444323696443-1b5cedba84bcSimple equations to ease your way

Forage dry matter (DM) availability can quickly be estimated on a per acre basis using a USDA NRCS “Grazing Stick”.

  • Contact information for where you can get one for your state
  • A simple tutorial on using a grazing stick
  • If a stick isn’t easy to come by, a quick Google search will point you to the many other estimation methods.

Temporary grazing paddocks can then be constructed using electrified polywire and tread-in posts that contain the appropriate amount of forage dry matter needed to support the stock density desired.

Example

If you want to move cattle forward daily to fresh grazing paddocks and you have 100 head of 800 lb. steers, first calculate the forage DM available per acre in the area you will be grazing.  Let’s assume your NRCS Grazing Stick formula shows you have an estimated 3000 lbs. of forage DM available per acre.

Each 800 lb. steer will need to consume up to 3.5% of his body weight every 24 hour period to achieve desired gains (3.5% is a standard in the industry.  Depending on your goals and whether you’re offering supplemental feed, this number may vary).

800 lbs.*.035 consumption = 28 lb forage DM daily per head

100 head*28 lb per head = 2,800 lb forage to feed the herd for a day

Let’s say you do not want the steers to consume more than 50% of available DM (take half, leave half is a good rule of thumb for re-building soils and increasing productivity over time).

3000 lb forage dry matter per acre * .5 (% available for consumption) = 1500 lb forage DM per acre available for consumption

So now we have a simple equation to work through in order to set your paddock size, stocking density, and frequency of movement.

Let’s say you plan to move the cattle once in 24 hours. 

Forage DM needed by the herd every 24 hours = (# of acres per paddock) *(# of paddocks)*(forage DM per acre available for consumption)

2800 lbs. = (# of acres) * 1 *1500

2800/1500 = # of acres

So they’ll need about 1.9 acres (we’ll round up to 2)

We now have the size of the paddock needed (2 acres) and the frequency of rotation (1 day).  To figure out the stocking density in lbs./acre, we simply divide the weight of the herd by the acreage.

800 lbs. each*100 steers = 80,000 lbs.

80,000 lbs./2 acres = 40,000 lbs. per acre.

If you want a higher stocking density, then you need to decrease the acreage in each paddock and increase the frequency of moves.

Let’s increase the stock density to 200,000 lbs. /acre.

Stocking density= (weight of the herd)/(# of acres)

200,000 lbs./acre = 80,000 lbs. / (# of acres)

200,000* (# of acres) = 80,000

To get a stocking density of 200,000 lbs./acre, each paddock would need to be .4 acres.

To figure out the number of paddocks you’ll need, simply plug .4 into the previous equation.

2800 lbs.=1500 lbs. per acre * (# of acres) * (number of paddocks)

2800 lbs. =1500 lbs. per acre *.4* (number of paddocks)

We end up with 4.66 which we can round up to 5. 

So the herd will need to consume 50% of the forage from each of five paddocks over the 24 hours in order to meet their nutritional needs.  Each paddock will be a bit less than half an acre (.4 or 2/5 of an acre) and you’ll need to move them four times to hit all five paddocks. 

With these equations, you can determine any of the variables by knowing the others.  So if you know your forage DM, forage intake per animal and how much residual you’ll leave, you can determine an appropriate stocking rate.  If you know your herd size and average weight, you can find out how many acres you need (for the whole season, just multiply the forage DM intake per day by the number of days).  If you know how big you want to make your paddocks, you can determine the stocking density and number of moves you’ll need to make.