Daily Phosphorus Losses in Growing Horses Studied

Daily Phosphorus Losses in Growing Horses Studied

A recent study from Swedish researchers found that current feeding recommendations might be overestimating fecal inevitable phosphorus losses in growing horses at rest.

Photo: Anne M. Eberhardt/The Horse

Some owners might supplement their horses’ diets with phosphorus (P) in order to meet the National Research Council’s (NRC) guidelines for recommended amounts of the mineral, but a recent study from Swedish researchers found that the NRC’s latest guidelines (most recently updated in 2007) might be overestimating fecal inevitable P losses.

”From an environmental perspective this is important because the global (phosphorus) reserve is running out and phosphorus excreted from animals contribute to eutrophication (high concentration of nutrients causing oxygen depletion in water),” reported Anna Jansson, professor at both the Swedish University of Agricultural Sciences and Holar University College in Iceland.

Jansson and a research team set out to evaluate P balance (intake vs. fecal output) and fecal P losses in growing horses, as well as to determine the proportion of the soluble, inorganic P in feces, which is responsible for eutrophication.

During the study the team evaluated 14 young Standardbred trotters in training fed free-choice fescue/timothy haylage. Using a crossover design, the team added one of two supplements to pelleted alfalfa for six days:

  • A high-P diet: 150 g/day of a supplement containing 6.44% monocalcium phosphate, 6.76% calcium carbonate, 5.93% magnesium phosphate and magnesium oxide, 0.5% vitamin E, and 0.0015% sodium (which amounted to 130% of 2007 NRC recommendation for P); or
  • A low-P diet: 50 g/day of a supplement containing 1% vitamin E and 0.002% sodium selenite (which amounted to 70% of 2007 NRC recommendations for P).

Horses had a 12-day washout between consuming the different supplements. Researchers weighed feed amounts offered and feed refusals daily in order to accurately determine dry matter intake (DMI). They collected feces and urine during the last three days of each six-day period for analysis.

Key findings from the study included:

  • Study horses' P requirement was between 17-32 grams per day, since P retention was observed with the high-P but not the low-P diet;
  • P excretion was higher in the high-P diet than the low-P diet; and
  • The high-P diet was associated with a higher proportion of soluble, inorganic P in feces (which likely contributes to eutrophication).

The team also concluded that the study results indicate young horses have fecal inevitable P losses of approximately 10 milligrams per kilogram of body weight per day, which is lower than current NRC P loss estimates of 18 milligrams per kilogram of body weight per day.

”According to this study, the P recommendations suggested (for) growing horses (at rest) in the old version of NRC (published in 1989) is okay, but the results do not support the increase that was suggested in the 2007 issue,” concluded Jansson.

The study, “Phosphorus balance and fecal losses in growing Standardbred horses in training fed forage-only diets,” will appear in an upcoming issue of the Journal of Animal Science

About the Author

Casie Bazay, NBCAAM

Casie Bazay holds a bachelor of science degree in education from Oklahoma State University. She taught middle school for ten years, but now is a nationally certified equine acupressure practitioner and freelance writer. She has owned Quarter Horses nearly her entire life and has participated in a variety of horse events including Western and English pleasure, trail riding, and speed events. She was a competitive barrel racer for many years and hopes to pursue the sport again soon.

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