Many riders seek calming methods for excitable horses in competition. Oral and intravenous products are available to improve a horse’s performance in the show ring or in races by “taking the edge off.” Magnesium is one such product, potentially serving as an antagonist of N-methyl-D-aspartate receptors in the central nervous system or as a calcium channel blocker in the peripheral vasculature to lower blood pressure. Because magnesium sulfate (MgSO4) can elicit sedative effects, the Fédération Equestre Internationale (FEI), in keeping with its official and stringent no drug policy, considers this a prohibited substance for participants in FEI events. That said, it is difficult to test for and control because of the presence of endogenous magnesium distributed throughout all tissues.
Study on Magnesium Use in Horses
In a collaborative study, practitioners at The Ohio State University and Colorado State University’s veterinary medical clinics examined the pharmacokinetics of magnesium and its effects on several clinical variables.
In the study, the research team administered six healthy, adult mares aged 4-18 intravenous magnesium sulfate 50% solution over five minutes. The dosage was 30 grams of magnesium for a 500-kilogram (approximately 1,100-pound) horse, which approximates the dose riders give their horses at competitions. The researchers evaluated a variety of physiologic variables:
Plasma electrolytes:
- Magnesium increased fivefold within five minutes but dropped to 1.8-fold increase by 60 minutes.
- Calcium concentrations reached their lowest value at 45 minutes and returned to baseline by 330 minutes.
- No abnormalities were found in cerebrospinal fluid, electrolytes, or cytology.
Urinary fractional excretion:
- Magnesium in urine increased fourfold by 30 minutes, peaked by 180 minutes, and remained twofold over baseline at six hours.
- Urinary calcium remained unchanged overall, but fractional excretion of calcium increased threefold by 30 minutes and returned to baseline by three hours.
- Urine sodium increased at five to 10 minutes.
- Urine potassium decreased from 10-120 minutes.
Mean arterial blood pressure dropped within five minutes and did not reach baseline levels for six hours.
Horses’ head height dropped within 15 minutes and remained significant for four hours. However, the horses did not appear to be tranquilized.
Competitors might give their horses supplements like magnesium to overcome persistent temperamental problems or issues training has not resolved. A horse with a dropped head could just be relaxed, so head height is not reliable for regulatory purposes in determining if a horse has received magnesium.
Biomarkers for Regulatory Control of Magnesium in Horses
The authors concluded that three biomarkers might be useful for regulatory control of exogenous magnesium: a) plasma magnesium; b) the calcium:magnesium ratio in plasma; and c) the urinary fractional excretion of magnesium. These values not only changed significantly from baseline normal values but also likely remained altered during the period for drug testing after competition. A calcium:magnesium ratio that drops to one-third of baseline and remains low for 12 hours might be indicative of exogenous magnesium administration.
In equestrian sports with drug rules, screening for magnesium supplementation can help level the playing field for fairness in competition and safeguard equine welfare.
Reference
Schumacher SA, Toribio RE, Scansen B, et al. Pharmacokinetics of magnesium and its effects on clinical variables following experimentally induced hypermagnesemia. Veterinary Pharmacology and Therapeutics May 2020; DOI: 10.1111/jvp.12883
Related Reading
- Disease Du Jour: Regulatory Veterinary Medicine for Horses
- Approach to Horses with Poor Performance
- Nutritional Management of the Competitive Equine Athlete
Stay in the know! Sign up for EquiManagement’s FREE weekly newsletters to get the latest equine research, disease alerts, and vet practice updates delivered straight to your inbox.
