# How to estimate forage volume.

If you turn a group of animals into a paddock (pasture subdivision) how can you determine how long the forage will last? If you are moving a temporary fence, how much pasture should be blocked out to provide X days of grazing for a certain size herd?

Either of these questions can be roughly calculated using two rules of thumb:

* An acre inch of green forage contains about 150 pounds of dry matter.

* One animal unit will consume about 30 pounds of dry matter per day.

An acre inch is considered to be one inch of height in a well-established, mixed stand of green forage spread out over 43,560 square feet. Thus, if the average forage height is eight inches in a one-acre paddock, and you want to graze off half this height, there would be four acre inches of forage or about 600 pounds of dry matter available. Dry matter is about 20 percent of the weight of fresh-cut green forage. Thus, if the forage is dry as the result of a droughty period, or has been allowed to mature too long and is mostly stems and dry leaves, the dry matter percentage will be higher (but the forage will not be as high in quality).

One animal unit is considered to be a 1,000 pound cow. Other animals can be converted with a bull being 1.25 AUs, a mature horse 1.25 AUs, mature sheep.2 AUs and mature goats. 17 AUs. Weaned young at the foraging stage are considered to be 60 percent of their parent while those still exclusively nursing are not considered in the AU calculation.

Using this rule of thumb, dry matter consumption per day would be: cow - 30 pounds, bull - 37.5 pounds, horse - 37.5 pounds, sheep - 6 pounds, goat - 5.1 pounds, grazing calf - 18 pounds, grazing lamb - 3.6 pounds and grazing kid - 3.06 pounds. Any supplemental feeding, e.g., hay, would reduce the dry matter required to be obtained from grazing.

These rules of thumb would need to be tailored to your individual situation by documenting actual results.

To apply these rules of thumb to the initial question, assume your herd has 20 ewes, two rams, and 36 weaned lambs. Total AUs would be about 8.72 (20 plus two plus 36 times.6 divided by five, since sheep are .2 AUs). 8.75 times 30 pounds of dry matter would be a requirement of 261.6 pounds. Your four acre inches of grazing would last about 2.29 days. Another method to arrive at the 261.6 would be to add up the sheep's requirements (20 plus two plus 36 times .6) or 43.6 times six pounds of dry matter per day per sheep AU.

For the same size sheep herd, to provide three days grazing in one paddock, three times the daily requirement of 261.6 would give a total requirement of 784.8 pounds of dry matter. With 600 pounds of dry matter available per acre, you would need to fence about 1.3 acres of pasture into a paddock.

Forage management is an art, rather than a science, due to the number of interacting variables involved. A period of warm, rainy weather may mean you have to concentrate on keeping seed heads clipped off to maintain grass growth vigor. A droughty period may mean sacrificing one or more paddocks arid supplemental feeding to try to allow the rest of the paddocks to obtain some regrowth.

Rules of thumb, such as the foregoing, offer a starting point. After that, it's up to you.