LOUISVILLE, Ky. (AP) _ The U.S. Postal Service is switching from mowing to mooing by bringing in cattle to trim grass at its main Louisville post office.

Four steers or cows, or maybe even beefalo, will begin grazing a 2 -acre grassy area around May 1.

An internal memo says officials can't scientifically predict ''how effective the cows can be toward reducing the number of times we have to cut the grass.''

But that didn't stop bureaucrats from being oh-so-precise in their forecast: a savings of 210 employee hours or $4,130.01 a year. That assumes, of course, that neither rain nor sleet nor traffic noise will stay the trusty bovines from the swift depletion of their appointed grounds.

The idea of using cattle as clippers surfaced when maintenance officials were looking for ways to mow the area more cheaply, said Harvey Slentz, director of operation support for the Louisville division, which covers most of Kentucky and portions of Indiana.

If the experiment works, it might be expanded to other divisions, officials said.

Slentz and Gene Muncy, manager of maintenance engineering, researched the idea and got approval from zoning, police, health and environmental authorities.

They also rounded up a rancher to lend the cattle, which Muncy said will likely be steers or beefaloes, a cross between beef cattle and buffalo.

In a memo, they said officials ruled out using goats (''prohibited by zoning regulations'') and planting grain (''harvesting equipment is dangerous'').

The area is securely fenced, cutting down the risk of an animal getting loose. ''If you see the gate open, call Maintenance Control,'' the memo advises. ''If you see a cow trying to climb the fence, call us too.''

About the only expenses for the post office will be $2,000 to run a water line and build a shed.

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