In what is shaping to be the biggest change in first-run syndication business in two decades, Oprah Winfrey will announce on her show Friday that she will end her daily syndicated talker Sept. 9, 2011, after 25 years on the air. 

This is anything but a retirement. Winfrey will shift her efforts to OWN: The Oprah Winfrey Network, which she jointly owns with Discovery Communications. The cable net expects to launch at the end of next year in at least 74 million homes. 

“If you think the last quarter-century has been something, then ‘don’t touch that dial’ as together we plan to make history in the next 20 months and beyond,” wrote Tim Bennett, president of Winfrey’s Harpo Prods., in a letter to company employees Thursday. 

Although the announcement has been anticipated for the past several weeks, it still is a blow to local TV stations around the country that have counted on Winfrey’s show for high ratings, usually as a lead-in to local news programs. Ratings for “The Oprah Winfrey Show” have drifted downward in the past few years and posted an all-time low in June, but the CBS/Paramount-syndicated talker remains the king of afternoon television in most markets. 

The show has averaged a 5.4 household rating this season, which translates to about 7.3 million viewers daily. It also scores a strong 3.0 rating in the key 18-49 demographic.  Although that doesn’t make “Oprah” the highest-rated syndicated show — a title held by “Wheel of Fortune,” with about 10 million daily viewers — it’s far above anything else in daytime, though it recently finished behind courtroom show “Judge Judy.” 

The announcement comes at the end of a big week for “Oprah,” which nabbed its best ratings in two years Monday when Winfrey interviewed Sarah Palin.

With local stations across the country already reeling from drastically reduced ad revenue, how incumbents deal with replacing the Winfrey show — and how rival stations counter with their own programs — will be crucial to their long-term health once the recession ends.

In addition, Winfrey has been the most successful incubator of new syndie talent, including Dr. Phil, Rachael Ray and Dr. Oz, so it’s unclear who among syndie distributors will step up with new strategies for getting shows launched in her absence.

“There’s no doubt that the landscape could shift dramatically, especially in early fringe, because of Oprah’s exit,” said one syndie vet. 

The very style of talk shows likely will shift as the remaining first-run players, and any newcomers, take up the slack in Winfrey’s wake. 

CBS/Paramount inherited the distribution rights to “Oprah” when it acquired King World Prods. in 1999 for $2.5 billion. The end of the show will be a blow to the syndicator but not as powerful a loss as if it had actually owned the show. Winfrey’s Harpo is the owner and producer of the program.

“We have the greatest respect for Oprah and wish her nothing but the best in her future endeavors,” the show’s distributor, CBS TV Distribution, said in a statement. “We know that anything she turns her hand to will be a great success. We look forward to working with her for the next several years and hopefully afterwards as well.”

By Alex Ben Block