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Diagnosis of Nutritional Problems in Horses
Sarah L. Ralston, VMD, Ph.D., dACVN,
Department of Animal Science, Cook College, Rutgers University
Fact Sheet #894 – Reviewed 2004
Nutrition is frequently implicated as a cause of disease or poor
performance of horses. Sudden changes in feed or feeding schedules,
toxins present in feeds or forage plants, and excesses or deficiencies
of nutrients can all result in clinical problems. Diagnosis of the
nutritional cause is necessary to effect a cure. Finding the source of
the problem may be as simple as a getting a thorough history (i.e., the
horse got into the grain bin). However, in many cases a more thorough
investigation may be necessary.
When a problem arises that is suspected to be linked with the
nutritional management of the horse, a complete history, evaluation of
the horse(s) involved, feed samples, and tissue samples may be necessary
to pinpoint the cause. Each case will be different, but the following
discussion details a general sequence of inquiry, including which tests
would be appropriate in certain situations.
A complete history of the horse(s) should be
recorded. This will direct subsequent actions. Important information
1. How long have the horse(s) been on the feeding/management
If no changes occurred—either in feeds used or management—for
over one month or more, and signs had a slow onset or are vague,
consider long term problems such as energy, protein, trace mineral,
or fat soluble vitamin deficits/excesses.
If no obvious changes were made but signs were of sudden onset,
consider the possibilities of spoiled or contaminated feed, or
exposure to toxins.
If any changes were made before the onset of clinical signs, they
should be highly suspect. Problematic changes can be as obvious as a
sudden change in feed type/amount or as apparently innocuous as
receiving a new batch of feed, hay, or bedding.
2. Is the horse turned out in a paddock or pasture?
If yes, look for toxic plants. Always check for signs that the
plants were actually consumed by the horses since many toxic species
(i.e., buttercups) are unpalatable and normally do not pose a threat
unless the pasture is severely overgrazed and/or the horses are
underfed. Also check the water source and trees/shrubs around or in
the pasture (leaves and bark of some species are potentially toxic),
and for evidence that clippings from lawn mowing or trimming were
put in the area.
3. How many horses are affected?
If only one horse in a herd or stable is affected and the other
animals are on the same feeding regimen, the possibility of a
feed-induced problem is reduced. However, it may be an idiosyncratic
reaction, or the animal may have ingested something the others
avoided. If more than one horse is affected, try to find the common
denominator (i.e., feed, age, type of supplements, type of activity,
etc.). Also establish how long the horses have been on the regimens.
Verify the reported rations by actually examining the feed room and
weighing out the amounts reported to be fed. Ask the people feeding
the horses if the animals are actually consuming the amounts to be
fed. For example, one horse may have consumed all of a new feed
while the rest refused it.
4. Complete feed history: how much of what and when?
Hay: Check for mold and assess overall quality. Is it
green and soft, or yellow and fibrous? Does it contain noxious
weeds or other contaminants? If alfalfa, check for blister
beetles, though they are often hard to find. If a chronic
nutritional imbalance is suspected and the same hay has been fed
for over a month, nutrient analysis of the hay may be useful.
Concentrates: Check for any evidence of mold, abnormal
odor, or contaminants. A representative sample should be taken
(see below) for nutrient analysis, especially if the feed was
custom mixed. If a commercial feed is being used and erroneous
formulation is suspected, get the lot number and manufacturer
information from the bag. If feed samples are sent to the
manufacturer for analysis, it is important to save some of the
feed for independent analysis in case a dispute ensues.
Supplements: Record all supplements’ label information and
the amount and frequency at which they were fed. Potentially
toxic levels of nutrients such as vitamins A and D, selenium,
and iron can result from feeding multiple supplements. It is
important to calculate the total intake (from all supplements
and concentrates) if signs of toxicity are present (see Table
Water source(s): Inadequate water will result in an
increased incidence of impaction colic. Water may also contain
high amounts of various trace minerals that would interfere with
the absorption/utilization of nutrients. If galvanized steel
tanks and copper pipes are in contact with each other, high
levels of zinc can leach into the water.
Salt: Regardless of other supplements, salt (NaCl)
should be available free choice at all times.
5. Other signs of disease?
Check vaccination and internal parasite (deworming) schedule. If
weight loss is one of the complaints, the affected animal’s teeth
should be carefully checked. Liver and kidney problems should also
be on the list of things to rule out.
Samples to Take
1. Feed samples:
When taking feed samples, especially for trace mineral analysis,
it is important to avoid contaminating the sample. Wear gloves when
handling forages and use the scoop normally used to deliver
concentrates to place the samples in a clean sealable plastic bag.
When sampling pastures, take forage samples from a least ten
sites, preferably from areas that are obviously being grazed by the
animals. Clip the grass and legumes, do not pull them up by the
roots. Do not use rusty implements to cut the samples!
Table 1: Common Nutritional
Problems: Differentials to Consider
Inadequate feed, poor dentition, parasite
infestation, heavy metal toxicity, malabsorption,
iron, copper deficit, iron deficit (rare)
Developmental Orthopedic Disease
Excess energy intake, inadequate or imbalanced
intake of Ca, P, Cu, Zn, I, Se; sudden increase in
plane of nutrition
Excess carbohydrate intake, electrolyte deficit,
vitamin E deficit (?)
Sudden change in feed, grain overload, toxic plants
Inadequate water intake, excessively fibrous feed,
sudden change in feed
Impaired GI motility
Alfalfa hay or other alkalinizing feed, Ingestion of
Grain or carbohydrate overload, toxin exposure
Metabolic disease, obesity
For hay samples, ideally a hay corer should be used to
obtain core samples from at least 10 bales. If grab samples
are obtained, take them from the center of at least 15
2. Clinical samples:
samples are useful mainly to rule out
non-nutritional disease problems and to test for toxic
heavy metals. Calcium and sodium concentrations in the
blood are tightly controlled by hormonal mechanisms and
do not reflect dietary intake. Plasma concentrations of
micro-minerals such as copper, zinc, and selenium may
reflect problems with intake but are so variable that
only severe imbalances will be detected. Iron status is
best determined by measuring serum ferritin, not blood
iron. Anemia is NOT diagnostic for an iron deficiency
because there are many other, more likely causes of
anemia in horses. The only vitamin for which assays are
routinely available is vitamin A, and this is not likely
to be deficient.
Urine: Creatinine clearance ratios or
fractional excretion can be determined for phosphorus,
potassium, and sodium which are reflective of actual
intake. Urine calcium content varies radically over a
24-hour period and is not a reliable measure. Blood and
urine for these analyses must be collected at the same
Postmortem: GI contents, liver, and kidney
biopsy: Most beneficial for chronic trace mineral
imbalances (especially copper and zinc) or toxin