News release issued June 25, 1998, effective immediately.
For media inquiries, contact Caroline Booth Lara, Communications Specialist, (580) 224-6379.
  email: cblara@noble.org

An Example: Calculating Forage

Let's take, for example, a 200-acre farm, mostly bermudagrass with some volunteer annuals, running a 50-cow herd. It has four pastures, all about the same size. Three are used for grazing and are continuously grazed, with the last pasture used for hay.

With normal rainfall, the grazing pastures produce excess bermudagrass in the spring which declines steadily through summer, and are "limped" through the fall until frost. Hay is fed until the next spring. The hay field produces 200 to 250 bales most years.

It is mid-June and the grass in the grazing pastures is about 4 inches tall and declining. The hayfield grass is about 8 inches tall and thin with a residual of dead winter annuals mixed in. Past history indicates that 75 percent of the bermudagrass is produced by midsummer. The best chance for rain after June is in September, over two months away. Minimum grazing height is 3 inches.

We prefer not to graze below 3 inches and the grass height in the grazing pastures is 4 inches. This means 1 inch of grass is grazeable. It is estimated there are 200 pounds (dry matter) of grass per acre inch. On 150 acres, this represents 30,000 pounds of available forage (150 acres times 200 lbs./acre inch). Harvest efficiency under continuous grazing is about 50 percent. Thus, only 15,000 pounds of forage will be consumed by the cows. Cows require on average about 30 pounds of forage (dry matter) per day. Fifty cows eating 30 pounds of dry matter per day equals 1,500 pounds of total forage consumed daily. The available 15,000 pounds of forage to be consumed by 50 cows will last about 10 days (15,000 lbs. available in pasture / 1,500 lb. daily consumption by herd). Even considering regrowth potential, the pasture will be at the minimal grazing height within two weeks.

Drought management strategies to be implemented include selling large calves, dry-open cows, heavy bred cows and late calvers. Let's say this leaves on our example cattle operation 30 head of uniform, productive cows, some with calves at side, all cows having the best chance of being bred back within a short time. Pastures will then be subdivided (halved), including the hay field. Gates are closed and the cattle are rotated between pastures.

The hay field has 5 inches of usable forage (8 inches tall minus the minimal graze height of 3 inches), and covers 50 acres. The 200 pounds of forage per acre inch figure still applies. Five inches multiplied by 200 lbs./acre inch multiplied by 50 acres equals 50,000 total pounds of forage available. Assume 50 percent efficiency and the total amount of forage to be consumed is 25,000 pounds. The 30-cow herd has a demand now of 900 pounds per day (30 head times 30 lbs./head/day). The 25,000 pounds will last almost 28 days, or four weeks, which puts us into mid July, assuming no regrowth. That relates to two weeks per half of the hay field since it is in the rotation. The grazing pastures are rested until then, at which time the rotation begins through all eight pastures, based on a 30-day cycle. If the grazing pastures regrow to a height of 6 inches, there will be 90,000 pounds of forage available by mid July. Increase the harvest efficiency to 70 percent under the rotational grazing regime, and the consumable forage equates to 63,000 pounds. With the daily requirement for the cow herd being 900 pounds per day, the 63,000 pounds will last 70 days. Add the 70 days to the 28 days that the cattle grazed on the hay field initially and you have 98 days of grazing ahead. This puts us into September – through the dry season and into the time of year we expect our next seasonal rains.

The bermudagrass will continue to regrow during the rest periods. An opportunity could present itself in the form of a surplus of grass, if heavy rains occur unexpectedly. Should this occur, some of the small pastures could be removed from the grazing rotation and hayed during the latter half of the growing season.

This illustrates the methodology in estimating the forage inventory, the planning for its use, and the management of livestock and pastures during drought conditions.

Although this example uses only bermudagrass, the same principles apply to other forage types. Estimated forage per acre inch will vary considerably between forages and sites. Forage production estimates based on clipped samples increase the accuracy. For more information on estimating standing forage production, contact county Cooperative Extension Service specialists or The Noble Foundation.

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(The Noble Foundation is a privately funded, nonprofit organization headquartered in Ardmore, Okla. The Foundation conducts agricultural and plant biology research; provides grants to numerous other charitable and educational organizations; and assists farmers and ranchers through educational and consultative programs.)