The annual balloting for horse-of-the-year honors began this week, with Forego facing the first serious challenge to the national championship he has held since 1974.

Seattle Slew will attract considerable support in the Eclipse Award voting by the racing secretaries of the thoroughbred Racing Associations, members of tha National Turf Writers Association and columnists and editors of the Daily Racing Form. Each organization has one group vote for each divisional champion and the horse of the year. There is a possibility the decision for the top award will be split this year for the first time since 1972, when Secretarist nosed out La Prevoyante.

The case for Seattle Slew rests on his being the only 3-year-old in the history of Triple Crown series to get through the Kentucky Derby. Preakness and Belmont Stakes while still undefeated. What he did was unique. He also has a higher winning percentage than Forego in 1977.

Slew was not beaten until early in July when his owners, believing their colt could fly over the Rockies and land, running, foolishly sent him to California for the Swaps Stakes at Hollywood Park. That was his last appearance of the session. The confrontation with Forego in the fall never materialized.

Overall, Seattle Slew won six of seven races to go with his three-for-three-mark as a 2-year-old. Horse that have achieved considerably less have, on many occasions, been accorded the national title.

Forego's record was not as impressive, statistically, as Seattle Slew's.

The 7-year-old gelding probably slipped a bit in performance from 1976.

He won only four of seven races, finished last in the slop at Saratoga in the Whitney Handicap and, for the fourth straight year, failed to sweep the Handicap Triple Crown series.

He lost both the Suburban and the Brooklyn after having captured the Metropolitan. He would have been better of to have skipped the Nassau County, after the Metropolitan, than to have gained such a costly victory at the weights.

Still, Forego is the horse of the year again. What he attempted to do this season was much more difficult than that which Seattle Slew accomplished.

The 3-year-old participated in a series of events in which he and his rivals carried equal weight. Slew's competition, was suspect, inasmuch as he never ventured outside his generation. How impressive is it to say that he could handle Rum Dusty Run, when Dusty didn't dust off the opposition once all year?

The best line between Forego and Seattle Slew is provided, perhaps, by Silver Series, a good 3-year-old that developed nicely to defeat Affiliate, Cormorant and Run Dusty Run in three summer stakes.

Silver Series tailed off slightly in August but came back to run one of his better races in the Woolward Handicap. He finished second, a respectable distance behind Forego - with Forego carrying 133 pounds to 114 on his younger rival.

Could Seattle Slew, on his best day, have spotted Silver Series that sort of weight over 1 1/8 miles and defeated him so handily? If your answer is yes, even allowing for the age scale, you have to be kidding.

The weight differential that Forego overcame in the Woodward was that made his races competitive all season long. He assigned 133 pounds for the Metropolitan, 136 for the Nassau County, 138 for the Suburban, 137 for the Brooklyn, 136 for the Whitney and 133 for the Woodward.

This is a strata reserved for only the best of thoroughbreds. Seattle Slew may earn the opportunity to show, some day, he is this kind of runner. But, beginning this winter, he shapes up as a 180-pound highweight, not 133 or, heaven forbid, 136, 137 or 138. He is not in Forego's league, not until he proves it. They are pounds apart.

Seattle Slew is, of course, the champion 3-year-old of '77. Our Mims merits the 3-year-old filly title. Affirmed and Lakeville Miss earned 2-year-old laurels. Cascapedia was the best older filly or mare, Affiliate the premier sprinter. The turf title hangs on the results of Saturday's $200,000 classic at Aqueduct.

Now if only someone would come up with a category for Cum Laude Laurie, everything would be lovely. Until then, the Ruffian, Beldame and Spinster victor gets my vote as the "Most Deprived" performer of the year.