WASHINGTON (AP) _ Mixing herbicides with oil instead of water could increase farmers' earnings and benefit the environment by reducing the amount of herbicide needed, Agriculture Department scientists say.

''Herbicides are commonly mixed with water and some type of oil to boost their weed-killing ability,'' said Chester G. McWhorter, a plant physiologist. ''We've found that it is possible to totally eliminate the water and still have effective weed control.''

He and James Hanks, an agricultural engineer, found in four years of field tests at Stoneville, Miss., that barnyard grass and johnson grass could be controlled with less herbicide when it was mixed with paraffinic oil instead of water.

The herbicide and oil was applied at a combined total volume of as little as one quart of liquid per acre, compared with the usual herbicide-and-water mixture of 20 gallons per acre.

The lower volume requires fewer trips with spray equipment to treat an entire field for weeds.

Paraffinic oil is similar to mineral oil. McWhorter said the mixture of herbicide and paraffinic oil provided better weed control than herbicide mixed with soybean oil, cottonseed oil, No. 2 diesel fuel, kerosene or jet A fuel.

Agricultural engineers at the ARS research center in Stoneville have developed an ultra-low-volume herbicide sprayer system that would allow farmers to apply herbicides at volumes of less than 1 gallon per acre, the researchers said.

''The ULV sprayer applies smaller, more concentrated droplets that are more toxic to the weeds,'' McWhorter said. ''Also, oil spreads much better than water on leaf surfaces.

---

WASHINGTON (AP) - The 1992-93 apple marketing season will be one of the most competitive U.S. exporters have experienced in the last several years, say trade experts.

''U.S. exporters have had two excellent years, both in terms of export volumes and prices received, due in part to the extremely limited European apple crop in the 1991-92 season,'' says a Foreign Agricultural Service report this month on horticultural products.

''The reverse holds true this year,'' it said. ''Bumper European apple crops along with a strong U.S. dollar, and a European recession are severely hampering U.S. apple exports to Europe.''

It said Asian and Central and South American markets are making up for some of the shortfall, but U.S. apple exports are trailing last season's.

''Average to above-average Southern Hemisphere crops (excluding a large decline in Argentina) soon to be marketed, could further aggravate the world apple market situation,'' the report said.

Apple production in 25 major producing and trading countries is forecast to rise 31 percent, to 24 million metric tons, in the 1992-93 marketing year, the report said.

World exports are forecast to increase 5 percent, to 3.7 million tons, it said.