the problem with kelly on horse racing is the inaccuracy of ones pricing,hence the use by some of half kelly or even a lesser %,kelly is an exact formula for calculating your optimal stake on a given event,dependent on the odds available and the true odds of such an event occurring,it works perfectly in card counting on blackjack,the only slight disparity is by the counter,in calculating the true odds,because of them not being able to exactly determine the amount of cards left in the shoe,but that is only a miniscule %,so the counter will finish in front over the long haul,but in horse racing,pricing up a race,one doesn't have all the variables to get the pricing precise enough to use kelly efficiently,the ones that can get it close enough,will use full kelly