Does oral magnesium aspartate supplementation affect reaction speed in horses of different breeds?

Magnesium aspartate supplementation has previously been shown to reduce reaction speed (RS) in 6 Standardbreds. To confirm these findings and explore the potential role of magnesium or aspartate, 6 Thoroughbreds, 6 Arabians and 6 ponies were fed 3 different hay-based rations (A. control: 0.03g Mg/kg BW; B. Control plus magnesium aspartate: 0.05g Mg/kg BW and C. Control plus sodium aspartate: 0.03g Mg/kg BW) in a Latin square design with a 7-day washout period.
Initially horses underwent a blind crossover test to score sedation and measure RS, using a previously validated method, when administered a mild anxiolytic (0.04 mg/kg BW acepromazine [ACP]) or a placebo. Horses were then fed each dietary treatment for 7 days, undergoing RS testing and sedation scoring (SS) on Days 1 and 7. A linear mixed model for RS and a generalised mixed model fitted for SS were fitted, with Day, Breed and Treatment as fixed effects and Horse and Group as random effects using ASReml-R.
With ACP there was a reduction in RS (P = 0.004) and greater sedation (P = 0.001) versus placebo. There were individual differences in response to the diets but overall no significant dietary effect on RS or SS after either one or seven days of supplementation. This may indicate that either the magnesium content of the ration must be at least 0.06 g/kgBW or that the previous results were compounded by breed or that the Standardbreds were a homologous group. Magnesium supplementation should not be relied on to modify reaction speed in performance horses.