Horse owners should take precautions when grazing pastures after the first killing frost. Frost damaged pastures can have higher concentrations of nonstructural carbohydrates, leading to an increase in the potential for founder and colic, especially for horses diagnosed with Equine Metabolic Syndrome, laminitis, obesity or Cushings.

To reduce the chances of adverse health effects, we recommended horse owners wait one week before turning horses back onto a pasture after the first killing frost.

Horses should be removed from a pasture when a majority of the forage is grazed down to 4″. The entire pasture should then be mowed to 4″ (since horses do not graze uniformly), drug to disperse manure piles and the horses should be rotated to a different pasture or housed in a drylot. This time of year (fall), horses will likely need to be kept in a drylot due to limited forage re-growth.

Ideally, owners will slowly transition horses back to hay diet (from a pasture diet) in preparation for winter feeding. We do not recommend over-wintering horses on pasture due to plant damage from digging, pawing, and hoof traffic.

Finally, ingestion of dried or wilted (but not fresh) maple leaves is associated with the toxicosis. Toxicosis normally occurs in the autumn when normal leaf fall occurs. Red blood cell damage has been reproduced in horses ingesting 1.5 to 3 pounds of dried leaves per 1,000 pounds of bodyweight.

Horses are the only species for which maple leaf toxicity has been reported. Horses are often depressed, lethargic, and anorexic with dark red/brown urine after the first day of ingestion. They may progress to going down with labored breathing and increased heart rate before death.

Horses should be fenced out of areas where wilted maple leaves are plentiful. Although dried leaves may remain toxic for 4 weeks, they are not generally believed to retain toxicity the following spring.

Written by Krishona Martinson, PhD, University of Minnesota. This and other horse nutrition articles can be found at http://www.extension.umn.edu/agriculture/horse/nutrition/.