Champion mare Bayakoa, winner of the Breeders' Cup Distaff the past two years, suffered a shocking defeat yesterday when she finished last in Santa Anita's $157,200 Santa Maria Handicap, which was won by Little Brianne.

Bayakoa went off as the 1-5 favorite of the crowd of 29,087 in Arcadia, Calif. But, coming off a three-month layoff and saddled with the highest weight of her career, 128 pounds, she faded down the stretch to finish 11 lengths behind the winner in the field of four.

Ridden by Laffit Pincay Jr., Bayakoa led early, but faded as Little Brianne, with Julio Garcia aboard, rallied from fourth to win by two lengths over Luna Elegante. Somethingmerry finished third, another 2 1/2 lengths back and 6 1/2 lengths ahead of Bayakoa.

Little Brianne ran the 1 1/16 miles in 1:41 3/5, two-fifths off the stakes record. She returned $14.40 as the third betting choice.

Aqueduct:

In New York, Arthur Klein's Bravely Bold led from start to finish and won the $108,400 Coaltown Breeders' Cup Stakes.

The winner was ridden by Diane Nelson to a one-length victory over Kechi. Man It's Cold, coupled in the wagering with Bank Of Sharacco, finished third.

Bravely Bold, running the six furlongs in 1:10 3/5, returned $18 and earned $65,940.

Oaklawn Park:

Overpeer, ridden by Jon Court, ran down Silent Reflex in the final sixteenth of a mile to win the $38,200 King Cotton Stakes in Hot Springs, Ark.

Overpeer ($3.20), part of a Robert Holthus-trained entry, matched the stakes record of 1:03 3/5 for 5 1/2 furlongs. Silent Reflex was second, a half-length back. Mr. Prosperous was third.