NEWMARKET trainer Luca Cumani has dropped the first bombshell of the close season by announcing he will not be retaining a jockey next season. The news comes as a blow to Ray Cochrane, who was first jockey to the trainer this year. Cumani, who has one of the keenest business brains in racing, sounded a warning that the days of jockeys receiving massive retainers could be a thing of the past. ''There are lots of freelance jockeys available now,'' argued Cumani. ''In the past, you retained a jockey in order to ensure you had a jockey of a certain standing when he was needed. While there are lots of freelancers, that problem no longer exists. ''The economics of racing at present mean that owners are looking at cutting expenditure whenever possible.'' Cochrane will remain linked to Cumani's famous Bedford House stables and is expected to share the bulk of the riding with champion apprentice Jason Weaver. There also remains the distinct possibility that Cumani could well welcome back Frankie Dettori, with whom he broke contact earlier this year. Dettori, expected in Hong Kong this week, will be first jockey to John Gosden, Sheik Mohammed's principal trainer, in 1994. But he is also expected to ride for many of the Sheikh's other trainers, perhaps even Cumani.