NASHVILLE, Tenn. (AP) _ Farmers who want their cows to produce more milk should have them listen to Garth Brooks rather than Tchaikovsky or Guns N' Roses. But, whichever, it's better than no music at all.

''Moo-sic really does get cows in the milking mood,'' said researcher Bethany Lynn Welch, a 15-year-old sophomore at Mapleton High School in Ashland, Ohio, competing in the 43rd annual International Science and Engineering Fair.

At the Nashville Convention Center this week, hundreds of would-be scientists put on display projects for tackling such modern-day dreads as Alzheimer's disease and global warming. There also was the question of calculating the most efficient way to shuffle a deck of cards.

It's riffle, riffle, riffle, overhand shuffle, according to Aric John DiPiero, 17. Three successive riffles - cutting the deck in half, raising the corners slightly, causing the cards to fall alternately together - and an overhand shuffle will randomly shuffle the cards.

DiPiero, a senior at the Center for the Arts and Sciences in Saginaw, Mich., said he got his idea from two scientists who have proved mathematically that seven riffles would randomly shuffle a deck of cards.

''Seven riffle shuffles is required,'' he said. ''But three riffles and one overhand would bring the deck to total randomness on the average.''

Welch, whose research considered the effects of music - and silence - on milking cows, exposed a dairy herd to two-week stretches of varying background.

Her findings showed daily milk production per cow was: No music, 61 pounds; classical, 62; hard rock, 64; and country, 65.

Winners in 13 categories will be announced at the close of the fair Friday.

''They have a lot of ideas about global change, hunger, things like that,'' said Alfred S. McLaren, president of Science Service Inc. and publisher of Science News. ''They genuinely recognize the need.

''I was absolutely amazed at the amount of science coming out of this. Some of these projects would be comparable to doctoral or postdoctoral work.''

''Walking around here, you wish you had a pocketful of money to help these kids get where they're going,'' McLaren said, adding that a quarter to half of the contestants would have ''formidable obstacles'' getting into college.

The environment was the hottest topic - the environmental sciences category attracted 177 finalists - and mathematics was least popular with 26 finalists.

Michael Stephen Demo, 16, a senior at Satellite High School in Satellite Beach, Fla., examined the biochemical effects of sulfuric and nitric acids on lichens, which are sensitive to pollutants.

Christine Marie Voll, 18, a senior at North Vigo High School in Terre Haute, Ind., considered the effect of ink and pigment on photodegradable plastic.

''We have so much stockpiling of plastic and garbage,'' she said. ''I'd like to see that stopped or slowed with the use of photodegradable plastic.'' Photodegradable plastic, which breaks down in sunlight, could be used in weed control on farms and then turned under by plowing, Miss Voll said.

Kimberly Rochelle Looney, 18, a senior at Martin Luther King Jr. Magnet High School in Nashville, called her medicine and health category project ''Auditory Brainstem Response and Cortical Evoked Potential Screening.''

She measured whether a new chemical compound could act like acetylcholine, a neurotransmitter that helps humans process information.

''I used rats in my study to see if more information was processed in the brain,'' she said. ''It was.''

Ms. Looney says the results of her research conducted at Meharry Medical College in Nashville could be applied to Alzheimer's patients.