ARS Equine Colostrum Refractometer
Patrick McCue, DVM, Ph.D., Diplomate American College of Theriogenologists
Colorado State University

Colostrum - 'Liquid Gold' for the newborn foal

Colostrum is the thick, sticky yellow fluid produced by the mammary gland of the mare during the last few weeks of pregnancy. It is comprised of carbohydrates, fats, proteins and electrolytes.  Colostrum is rich in antibodies or immunoglobulins that are critically important for immune protection of the newborn foal.  The process by which foals acquire antibodies from the colostrum of their dam is called passive transfer of immunity.

The epitheliochorial-type placenta of the horse prevents in utero transfer of antibodies from the mare to the fetus. As a consequence, foals are born without any antibody protection. The newborn foal is thus entirely dependent on antibodies from colostrum for protection against infection during the early neonatal period.

The mammary gland selectively concentrates antibodies (primarily IgG class and some IgM class antibodies) from the blood of the mare during the last two to three weeks of pregnancy. The mammary gland is capable of directly producing some IgA class antibodies that are found in colostrum. The average IgG concentration in equine colostrum is approximately 7,000 mg/dl (70 g/L) and ranges from 3000 to 12,000 mg/dl (30 to 120 g/L).

Under normal circumstances, a foal will stand and nurse within 1-2 hours after it is born.  Specialized cells in the small intestinal mucosa called enterocytes absorb the colostral antibodies by a process termed pinocytosis. The enterocytes engulf droplets of colostrum from the intestinal lumen, transfer the droplets in small vacuoles across the cell and then discharge the contents into lymphatic vessels. The lymphatics subsequently transport the antibodies to the blood stream.

The capacity of the small intestine to absorb and transfer antibodies is greatest during the first 6-8 hours after birth.  It declines substantially after 12 hours and within 24 hours after birth the specialized intestinal cells are replaced by cells incapable of transferring antibodies.  Consequently, ingestion of colostrum by the foal after 24 hours of age will not result in an increase in blood antibody levels.

Foals need to ingest approximately two to three liters of good quality colostrum within the first few hours of life to acquire sufficient antibodies to protect against infectious diseases.

Mares produce colostrum only once in the course of a pregnancy.  Colostrum will be rapidly depleted as the foal nurses throughout the first day of life.  Within 24 hours after foaling, the mammary gland no longer contains colostrum and the milk produced thereafter does not contain high concentrations of antibodies.

Failure of Passive Transfer

The term 'failure of passive transfer' or FPT refers to inadequate uptake of antibodies from colostrum by a neonate.  Failure of passive transfer puts the foal at significant risk of infectious diseases early in life. It has been estimated that failure of passive transfer occurs in 10-20 % of foals.

The most common cause of FPT is premature lactation. Mares that drip or run milk for hours prior to giving birth are losing colostrum that is vital to the survival of the foal. Other causes of inadequate transfer of antibodies include poor quality colostrum, failure of colostrum production (i.e. due to fescue toxicity), inability or lack of desire by the foal to nurse, prematurity, dysmaturity, foal rejection by the mare and failure to absorb antibodies that are ingested.  Young maiden mares and aged broodmares may produce colostrum of lower quality than middle-aged mares.

Determination of passive transfer success is determined in foals 24 hours of age or older.  Foals with blood IgG levels of = 400 mg/dl are considered to have failure of passive transfer. Foals with levels between 400 and 800 mg/dl are considered to have partial failure of passive transfer, while foals with IgG levels > 800 mg/dl have adequate passive transfer of antibodies.  Note: assessment of foal blood IgG levels at 12 hours of age may identify early potential cases of FPT and allow for supplementation with oral frozen-thawed colostrum, if necessary. Please consult with your veterinarian regarding IgG testing in neonatal foals.

Evaluation of colostrum

Evaluation of mare colostrum immediately after foaling and prior to nursing can be helpful in predicting if failure of passive transfer is likely to occur.  This is especially true in situations in which colostrum has been lost due to premature leakage from the mammary gland, or if the colostrum quality is poor.

Quantitative measurement of IgG levels in colostrum can be obtained by a radial immunodiffusion (RID) assay.  Unfortunately, the RID technique is not a test that can be performed easily on a farm and takes approximately 24 hours to obtain results.

Qualitative assessment of colostrum can be performed using a colostrometer or by a sugar (Brix) refractometer.  The colostrometer measures the density or specific gravity of colostrum. Colostrum with high IgG levels has a greater density and therefore a higher specific gravity. Determination of the specific gravity of equine colostrum is dependent on accurate measurement of an exact volume (15 mls) of colostrum.  Small errors in volume measurement lead to inaccuracy of specific gravity determination.

Refractometry measures the concentration of dissolved solids in a solution.  In the case of a Brix refractometer, a small amount of colostrum is placed on the prism and the light plate is closed. The colostrum is then spread evenly across the prism. The refractometer is held in the direction of a light source and the deviation or refraction of light is evaluated on a scale as a percentage score.  Colostrum with a low amount of dissolved solids (i.e. low IgG level) will have a lower amount of light scatter and a lower percentage score.  Colostrum with high amounts of dissolved solids (i.e. high IgG levels) will cause more light scatter and a higher percentage score. Refractometer evaluation of equine colostrum has been shown to be highly repeatable (R= 0.98) and highly correlated with IgG levels (R=0.85) as measured by the radial immunodiffusion assay.1,2

ARS Equine Colostrum Refractometer

The ARS Equine Colostrum Refractometer is a Brix refractometer calibrated to evaluate mare colostrum. The test is rapid (less than one minute), repeatable, and uses a small volume of colostrum (1-3 drops).  The steps involved are:

     1. A small amount of colostrum (i.e. 1 ml) is collected
         immediately post foaling before the foal nurses
         (Figure 1).
Figure 1

     2. One to two drops of colostrum are applied to the prism surface
         of the refractometer (Figure 2)

Figure 2

     3. The daylight plate is closed, effectively spreading the colostrum
         across the surface of the prism.

     4. View through the eyepiece while holding the refractometer in
         the direction of a light source.

     5. Note the level or location of the color (blue) on the scale. The
         scale contains graduations (%) and a quality assessment
         score for accurate interpretation (Figure 3).

Figure 3

Interpretation and Colostrum Management

The relationship between refractometry percentage, IgG content as determined by RID assay and overall colostrum quality assessment is presented in the table below (modified from Knottenbelt et al
Successful passive transfer of antibodies from the mare to her foal depends on many factors, including quality and quantity of colostrum, timing of the ingestion of colostrum, health status of the foal, and absorption of antibodies by the small intestine. Assessment of colostrum quality evaluates only one component of the passive transfer process.

However, if it can be determined that the quality of the colostrum produced by a mare is inadequate, the foal can be supplemented with additional frozen -thawed colostrum of good quality or with an oral colostrum substitute containing equine antibiodies.

Refractometer evaluation of colostrum is also valuable in determining the quality of colostrum to be harvested and stored in a colostrum bank. Owners and farm managers are strongly encouraged to maintain a supply of equine colostrum frozen at - 20 C.  A total of 250 to 500 mls of colostrum may be safely harvested from postpartum mares without adversely affecting passive transfer to their own foals. The best colostrum donors are healthy mares that have had one or more foals previously and that have been vaccinated 4-6 weeks prior to foaling.  Ideally, the colostrum should have a refractometer score of 25 % or higher and be tested for the presence of anti-RBC antibodies, to prevent the possibility of neonatal isoerythrolysis or jaundice foal syndrome.  Frozen colostrum can be safely stored for up to 1-2 years. Please consult with your veterinarian regarding testing, banking and usage of frozen colostrum.


     1. Cash RSG.  Colostral quality determined by refractometry.
         Equine Veterinary Education 1999;11:36-38.

     2. Chavatte P, Clement F, Cash R, Grongnet J-F. Field
         determination of colostrum quality by using a novel, practical
         method. Proceedings of the American Association of Equine
         Practitioners Annual Conference 1998;44:206-209.

     3. Knottenbelt DC, Holdstock N, Madigan JE. Equine
         Neonatology Medicine and Surgery. Saunders, Edinburg,
         pp 393-394.

About the Author...

Patrick M. McCue, DVM, Ph.D.,
Equine Reproduction Specialist
Associate Professor, College of Veterinary Medicine and Biomedical Sciences,
Colorado State University

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