WASHINGTON (AP) _ Fees for grazing livestock on public land administered by the Forest Service will be reduced slightly beginning March 1, the Agriculture Department agency said today.

F. Dale Robertson, chief of the Forest Service, said monthly fees for grazing stock on national forests in 15 Western states will be lowered five cents to $1.81 per ''head month.''

The decrease will apply to forests in Arizona, California, Colorado, Idaho, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Utah, Washington and Wyoming.

A ''head month'' was defined as a month's use and occupancy of public range by one weaned or adult cow, bull, steer, heifer, horse, burro or mule; or five sheep or goats.

The fee reduction is expected to apply to more than 9,000 livestock owners who hold grazing permits for national forest land. More than 1.9 million animals, mostly cattle, are grazed on the ranges each year. Fees are expected to total about $10.2 million.

Grazing fees are determined by a formula that takes into consideration the ''fair market value'' of forage involved, along with changes in private gazing rates, beef cattle prices and the costs of livestock production.

Officials said the current reduction is based primarily on increases in cattle production costs, which in 1989 exceeded gains in both beef cattle prices and the private grazing rates.

No fees were announced for 1990 grazing on national grasslands in nine states. A Forest Service spokeswoman, Diane Hitchings, said those will be announced in a week or so.