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Colostrum is the first milk that a mare produces after foaling. It is particularly rich in antibodies, which provide immunity for newborn foals.
Foals are born without a fully developed immune system and rely on a passive transfer of antibodies from their dam to help build their immune defenses. Consuming enough high-quality colostrum in the first 6 – 12 hours of life is critical for the foal’s health and survival.
Colostrum contains antibody proteins called immunoglobulins and other factors, which help combat diseases that could otherwise be deadly for foals.
Foals that do not get enough high-quality colostrum are at greater risk of infections, such as diarrhea, joint sepsis, and umbilical infections caused by pathogens. If your foal cannot be nursed for any reason, donor colostrum should be fed to support immunity. Another option is to infuse immunoglobi-rich plasma directly into the circulation to deliver antibodies.
You can determine whether your foal has consumed adequate colostrum with an IgG test from your veterinarian to measure the immunoglobulin concentration in your foal’s blood.
Importance of Colostrum for Foals
Foals are born with naive immune defenses. They do not receive any immunity from their dam while in the uterus. [1]
When a foal is born, it relies on colostrum from the mother to obtain protection from common diseases and start to develop an immune system. [2]
Colostrum is rich in several components including hormones, growth factors and antibodies. Many of these components support immunity and gut health, such as insulin-like growth factor, lysozymes, lactoperoxidase and lactoferrin. [5]
This first milk also contains high levels of vitamins and minerals, especially vitamin A.
Colostrum is also a source of oligosaccharides and essential fatty acids, which have been shown to promote gut development and improve thermoregulation (the ability to maintain a stable body temperature). [3][4]
Antibodies
The antibody levels in colostrum are 100 times higher than in regular milk. These antibodies provide newborn foals with early protection from harmful bacteria and contagious diseases.
Immunoglobulins are the main antibodies in milk. They are large bioactive molecules composed of short and long polypeptide chains. The three most abundant types are:
- Immunoglobulin G (IgG)
- Immunoglobulin A (IgA)
- Immunoglobulin M (IgM)
Newborn foals have specialized cells in their digestive tract to accommodate these large molecules, allowing them to absorb the immunoglobulins in colostrum. However, the gut is only “open” to absorb immunoglobins from colostrum during the first 24 hours post-partum. [1]
Once absorbed, the immunoglobulins travel to the circulatory system and bind to foreign invaders, helping to destroy them. [2] Foals will encounter many new and potentially damaging pathogens as they mature and this immune transfer is vital to their survival.
The ingestion of colostrum confers passive transfer of protective antibodies, which will last for 4-6 months. As the foal matures, so does its own immune system.
This is why routine vaccination is recommended for foals starting around the time of weaning – around 6 months of age – when passive immunity begins to wane. At this stage, the aim is to stimulate an active immune response to pathogens.
Failure of Passive Transfer (FPT)
If a foal does not receive adequate immunoglobulins from the dam’s colostrum, this results in a failure of transfer of passive immunity.
These foals have low IgG levels in their blood and are more susceptible to harmful and potentially life-threatening diseases, such as: [2][7][8]
Composition of Colostrum
Compared to milk, colostrum is thick, sticky, and light yellow in colour. The consistency of a mare’s colostrum changes within 24 hours of giving birth as the mare transitions to making milk.
Over this initial 24-hour period, the protein content decreases and lactose content significantly increases. By three weeks post-partum, the mare’s milk composition is fairly stable and remains similar for the rest of the lactation. [6]
Table 1: Composition of equine colostrum and mature milk. [9]
| Component | Colostrum | Milk |
|---|---|---|
| Fat | 1.7% | 1.2% |
| Protein | 18% | 1.7% |
| Lactose | 1.5% | 6.6% |
| Immunoglobulins* | 27% | 0 |
*Using brix refractometer
This table reflects the average composition of colostrum and milk. Colostrum composition could be affected by characteristics of the mare including: [9][10]
- Breed
- Parity
- Diet
- Season
Ensuring Adequate Passive Transfer
There are a few factors that influence adequate passive transfer of antibodies in the newborn foal. The two main parties involved in passive transfer are the mare and the foal – the mare has to produce colostrum for the foal and the foal has to be able to ingest it. If something goes wrong with either party, then adequate passive transfer will not take place.
Factors related to the dam include the quantity and quality of the colostrum produced. If the mare does not produce enough high quality colostrum, then passive transfer to the foal will be inadequate.
Factors related to the foal include the foal’s ability, or lack thereof, to consume enough colostrum during the necessary window. Since the gut is only “open” to absorb the antibodies from colostrum during the first 24 hours post-partum and most absorption occurs within the first 12 hours, foals that fail to meet their benchmarks in the post-foaling period are at risk of FPT.
Mare Factors: Quantity Produced
A healthy mare should produce an adequate volume of colostrum for her newborn foal. However, individual variation does occur and some mares are less productive than others during lactation.
Additionally, the mare’s body condition, age, and parity (i.e. the number of times she has foaled previously) can all affect colostrum and milk yield. Mares that are in poor body condition at the time of foaling, or those that do not receive adequate nutrition during lactation, generally produce less milk. [11]
Additionally, young, maiden mares produce less milk compared to older or multiparous mares (those that have had more than one foal previously). [11]
Further, there are several medical conditions that can negatively affect mammary development, lactation, and colostrum availability if they occur late in gestation. Common examples include fescue toxicosis and placentitis.
Fescue toxicosis is the most common cause of agalactia (lack of milk production) in mares in North America. Agalactia can occur in mares grazing on endophyte-infected fescue pasture during late-pregnancy. Affected mares exhibit little to no mammary development and fail to produce colostrum and milk. [12]
Another common issue is premature lactation due to placentitis (i.e. inflammation and/or infection of the placenta). Not only is this condition harmful to the developing fetus, it can also result in premature udder development and lactation during late-pregnancy. Mares with placentitis often begin leaking milk well in advance of foaling. In some cases, all the colostrum is wasted before the foal is born. [13]
These medical impacts on lactation underscore the importance of monitoring mares towards the end of gestation for udder development and evidence of premature lactation. It is normal for mares to “bag up” or develop an udder in the last 2 weeks of foaling, and milky secretions can be expressed when palpating the teat in the final days leading up to foaling.
Both the failure to develop an udder and premature udder development can be signs that there is an issue with the pregnancy.
Mare Factors: Quality
Colostrum quality is determined by immunoglobulin content. This can be measured in the barn by assessing levels of IgG, which is the most abundant immunoglobulin.
IgG concentration is measured using a Brix refractometer – a small instrument that provides immediate results. A brix reading of 23% or above indicates good quality colostrum. [14]
Transfer of passive immunity to the foal can be assessed by testing the foal’s blood for serum IgG levels. This blood test is performed by your veterinarian.
Foals with IgG concentrations greater than 800 mg per 100 mL of blood have received adequate immunity. IgG levels below 800 mg per 100 mL indicate a failure of passive immunity. [2]
Vaccinations
A vaccinated dam will transfer immunoglobulins induced from immune response to the vaccine to her foal through colostrum. The diseases horses are vaccinated against are typically high morbidity (common) and/or high mortality (fatal), so it’s important that foals are also protected against these pathogens.
To maximize the transfer of vaccine-related antibodies in colostrum, it is recommended to vaccinate the mare one month prior to her due date. This allows adequate time for the mare to develop antibodies against the targeted pathogen and produce colostrum containing these specific antibodies in high concentrations. [2]
These are called “pre-foaling vaccines” and include all the normal core vaccines recommended for horses in North America:
- Rabies
- Tetanus
- West Nile Virus (WNV)
- Eastern Encephalitis Virus (EEE)
- Western Encephalitis Virus (WEE)
Several risk-based vaccines can be given to the mare to provide immunity to the foal, depending on the situation. Examples include: [15][16][17]
- Rhodococcus equi: Vaccination of broodmares reduces pneumonia in foals
- Clostridium difficile: Vaccination of the mare produces antibodies in colostrum that could protect against enterocolitis
- Equine Herpesvirus 1 or rhinopneumonitis: Vaccination of the mare increases antibodies in the foal that may protect against respiratory disease
- Botulism
- Streptococcus equi equi or “strangles“
- Equine influenza
- Rotavirus
Consult with your veterinarian about which vaccines to give your gestating mare.
Foal Factors: Quantity Consumed
Foals need to consume enough colostrum to ensure they are getting adequate levels of immunoglobulins.
A foal should receive 2-3 L of colostrum within the first 6 to 8 hours of life over an average of 4 feedings. [4]
As they get older, healthy foals can consume as much as 27% of their body weight in milk daily, equivalent to 13 L for a 50 kg foal.
Sick foals may only tolerate 5-10% of their body weight in milk daily (equivalent to 2.5 – 5 L per day). This amount can be slowly increased as the foal recovers. [18]
Assessing Colostrum Intake
The quantity and quality of the dam’s colostrum, the timeliness of consumption and the quantity that the foal consumes all affect the immunity and development of the newborn foal.
Even healthy foals should undergo a wellness exam by a veterinarian once they’re about 24 hours old. At this visit, the vet can draw blood to test the IgG concentration and confirm that enough colostrum was consumed. Testing earlier than about 20 hours may not accurately reflect the foal’s IgG levels. [2]
Of course, if you are aware of any mare or foal factors that may impact passive transfer of immunity, it is best to reach out to your veterinarian right away. Prompt intervention can be the difference between a healthy foal and a foal that does not survive.
The most common causes of inadequate colostrum intake include: [19]
- Premature birth
- Delayed suckling
- Death of the mare
- Premature leaking of colostrum prior to foaling
- Small intestinal malabsorption
If failure of transfer of passive immunity is identified, your veterinarian can recommend safe and effective ways to increase IgG concentration. Both oral and intravenous routes can be used to provide supplemental IgG. [20]
Post-Foaling: 1-2-3-4 Benchmarks
Most horse-owners that have experience foaling out mares are familiar with the normal newborn foal benchmarks – or the 1-2-3-4 Rule.
The 1-2-3-4 Rule describes the time points at which a foal should stand, suckle, and pass meconium (the first manure). After birth, a foal should stand by 1 hour, successfully and independently nurse by 2 hours, and pass meconium by around 4 hours. Number 3 is reserved for the mare, who should pass her placenta within 3 hours.
For many healthy foals, they meet all of their benchmarks quickly, even within hour one. If one or more of these benchmarks is not met within the expected timeframe, you should contact your veterinarian for prompt assessment.
The foal’s gut is “open” to absorb the immunoglobulins from colostrum for only about 24 hours, though most of this absorption occurs within the first 12 hours after birth. After 12 hours, absorption rates decrease by approximately 80%. [2]
Given this, a foal that is slow to stand and nurse is at risk for failure of passive transfer. This is why monitoring the late-gestation mare is so important – owners or caret