Fetal Development and Foal Growth
Procreation in any species borders on the miraculous. How else can one describe a phenomenon where two microscopic entities intersect, join forces, and grow into a living, breathing creature that, in the case of the horse, will weigh 1,000 pounds or more at maturity. Science, of course, does not deal in miracles. It involves itself in no-nonsense facts and observations that have unraveled some of life's greatest mysteries. The matter of equine reproduction is a case in point. Science can take us step by step through the process from the time sperm meets egg until approximately 11 months later when the foal is born.
The placental membranes not only are a literal life support system for the growing fetus, but also provide clues after birth as to why problems occurred (if they did during pregnancy).
Photo: Barbara D. Livingston
Unfortunately, the "miracle" does not always occur. There are pitfalls all along the route and any one of them can terminate a pregnancy before it reaches term.
The estrous cycle of the mare has been explored in these pages in some detail in recent issues (see The Horse of December 1998), so we will begin our discussion of fetal development and foal growth at the climactic step in any given cycle--fertilization.
Fertilization occurs in the oviducts, also known as fallopian tubes. The sperm is deposited directly into the uterus by the stallion, or by a human using artificial insemination. Once that occurs, the race is on as individual spermatozoa propel themselves toward a meeting with an ovulated egg, or ovum.
Each oviduct, which is a highly coiled tube, connects the tip of a uterine horn with an ovary. The portion of the oviduct nearest the ovary is called the infundibulum. It is shaped something like a catcher's mitt with finger-like projections from its end called fimbriae. Just like a catcher receiving the ball from the pitcher in baseball, it is the job of the infundibulum to "catch" the egg when it is released from the ovary and to send it on its way down the oviduct to the uterus.
As the egg moves downward, the sperm moves upward. Both receive assistance as they travel. The oviducts are heavily lined with hairlike projections called cilia. They beat rhythmically, thus helping propel both sperm and egg to a meeting place.
That meeting place normally is an area just below the infundibulum called the ampulla. Once fertilization has occurred, the egg continues on its way to the uterus, passing through the isthmus, that portion of the oviduct where it joins the uterus.
The uterus will now become home for the embryo until it reaches maturity and parturition occurs. The embryo's new home is designed to house, nourish, and protect the growing fetus. To this end, the equine uterus possesses characteristics that are unique among domestic animals. Two of those key characteristics are vital to reproduction.
First, the mare's uterus possesses a remarkable ability to defend itself against massive bacterial contamination. It has the ability to clear endometrial inflammation efficiently.
Second, placental development and function begin relatively late in gestation in the horse. This means that uterine gland secretions must support the embryo for a large part of the first trimester.
Failure of either of these two key characteristics can result in pregnancy termination.
Science has done much to educate horse owners concerning the life and times of sperm and ova. Applying this knowledge helps insure that there is a meeting of sperm and egg. It is known, for example, that the lifespan of sperm within the mare's reproductive tract is, on average, 48 hours. There are exceptions to every rule, with some sperm remaining viable for only a few hours while others survive five or six days. By comparison, the equine ovum might only survive for six to 12 hours. As a result, the horse owner knows that insemination or breeding one to three days before ovulation results in a higher pregnancy rate than does, for example, insemination four days or more before ovulation.
Using ultrasound equipment and palpation, the practitioner can predict with a high degree of accuracy when ovulation will occur, and he/she can time artificial insemination or natural cover accordingly.
If the egg is not fertilized during its brief lifespan, it degenerates. When the egg is fertilized, there is a completely different reaction on the part of the mare's reproductive system as the egg continues its journey to the uterus. This journey, by the way, will last for six to seven days.
By the time the embryo passes into the uterus, it is already in the process of cell division. At the time of its entrance into the uterus, it has usually developed to the morula or blastocyst stage.
Once the embryo reaches the uterus, it must signal its presence to prevent regression of the corpus luteum. If the corpus luteum should regress, there would not be production of a sufficient amount of progesterone to maintain the pregnancy.
The mare's hormonal system has been preparing for pregnancy since the beginning of the estrous cycle. The concentrations of blood progesterone in a normal pregnancy where the corpus luteum has continued to function, for example, begin to increase one to two days after ovulation and reach maximum proportions when the fertilized egg reaches the uterus. Progesterone is the key hormone in preparing the uterus for accepting and maintaining a pregnancy.
During its early days in the uterus, the fertilized egg, or embryo, is a restless little entity. It moves throughout the uterine lumen from the tip of the horn where it entered, to the body of the uterus, and into the other horn. Peak mobility of the embryo occurs between Days 12 and 14. On about Day 16 of gestation, the embryo ceases moving and fixation occurs.
Once the embryo stops moving about (migrating), it establishes itself at the base of one of the two uterine horns. Researchers tell us that the mean day of fixation for ponies is Day 15, and for horses, it is between Days 15 and 17.
During the process of cell division, a yolk sac has formed to provide early nutrition for the embryo. Between Days 22 and 24, the allantois, an embryonic membrane, is formed. On Days 21 through 40, the allantoic sac becomes larger and the previously large yolk sac becomes enveloped in the developing umbilical cord.
A heartbeat can be detected about Day 21.
After fixation, the shape of the migrating yolk sac changes from spherical to an irregular, almost triangular shape under the influence of increasing uterine tone. The embryo increases in size from three to four millimeters in diameter per day between Days 17 and 24.
By 40 days, the chorioallantoic membrane that lines the placenta is nearly complete, and the growth of the embryo will increase dramatically. Ultimately, the fetus will be enveloped by the amnion, the membrane that lines the chorion. The chorion is the outermost placental membrane. The amnion contains the fetus and the amniotic fluid around it.
By approximately Day 40, the yolk sac has disappeared and the umbilical cord has been formed.
To many scientists, this is the end of the embryonic stage and the developing foal is now termed a fetus.
With the use of ultrasound, the sex of the fetus can be determined at an early stage. Described as the optimal time for this technique is the period between Days 59 and 68 (see sidebar on page 37).
By 45 days of gestation, the attachment of the fetus-containing sac to the endometrium of the uterus is reaching an advanced stage. Little projections or villi of fetal origin interlock with the endometrial crypts. This interlocking action forms the microplacentome, which is the tissue that functions as an exchange unit for pregnancy. The microplacentomes continue their support and development of the pregnancy for four months.
In the space between the placentomes, uterine glands continue to secrete uterine milk. As the fetus matures, the placental membranes increase dramatically in vascularity. This facilitates the placenta's ability to function as an exchange surface for such things as respiration, elimination of waste products, and nutrition between dam and fetus.
The placental membranes not only area literal life support system for the growing fetus, but also provide clues after birth as to why problems occurred (if they did during pregnancy). Basically, the placenta is a mirror of uterine health.
Comparatively speaking, during the first six months of fetal existence, there isn't a great deal of growth. In fact, says Dave Beckman, DVM, of Anchorage, Ky., the fetus at six months is about the size of the average cat. The majority of fetal growth occurs in the final three months of gestation.
During this period of rapid growth, there also is an increased demand for nutrients on the part of the mare's body. During the early months, the relatively slow fetal growth puts little nutrient demand on the mare. However, in those final months when the fetus is growing rapidly, the mare's nutritional demands will increase greatly. The fetus will grow from the size of a cat at six months gestation to weigh about 125 pounds--depending on breed--at birth. More about nutrition and optimum physical condition for a pregnant mare a little later.
The prime question in the mind of the breeder after ovulation, followed by artificial insemination or natural breeding, is the question of whether the mare is pregnant. There are many ways to determine this very early in the pregnancy, and they range from the basic to the sophisticated.
The most basic, non-invasive method involves teasing with a stallion. This is not a sure-fire approach, but if the mare has been teased regularly and records kept, it will provide a strong indication.
The mare which becomes pregnant after ovulation will show a definite change in behavior as opposed to one who is open and beginning a new estrous cycle. Normally, the pregnant mare will resist all advances by a stallion as her reproductive system devotes itself to maintaining a pregnancy.
A very common approach to detecting pregnancy is via transrectal palpation. Characteristic changes associated with pregnancy can be recognizable as early as Day 15. However, they are more consistent at Day 16 or Day 17 post-ovulation.
The most sophisticated approach, and one becoming more common by the day, is the use of ultrasound.
What the practitioner is looking at when seeking to determine pregnancy is changes in both the uterus and the cervix. In a normal, pregnant mare, one would expect to find excellent uterine tone and a closed, narrow, and elongated cervix. One researcher characterized such a cervix as being like a No. 2 pencil.
Back to fetal development. The size of the fetus at birth is often determined more by the mare's uterine capacity than by genetics, although genetics do kick in once the foal has been born.
In one bit of research at Colorado State University, a Shetland pony mare was inseminated with semen from a draft horse stallion. The pony mare delivered a small foal during a normal birth, but the foal soon outgrew its mother once it was on the ground and nursing.
Two papers presented at the International Symposium on Equine Reproduction held in South Africa in July also indicated that the mare exerts considerable influence on the size of the growing fetus.
In one paper, a researcher from Poland--Marian Tischer, who studied embryo transfer foals--found that "irrespective of genetic makeup, the ultimate height of the horse is decided by nourishment during gestation and less so by the milk capacity of the mare."
The second study was carried out by researchers in England who studied the influence of maternal size on fetal and post-natal development in the horse. They reported that, "Maternal size significantly affects fetal growth, presumably by means of limiting the area of uterine endometrium available for attachment of the diffuse epitheliochorial placenta."
Once the foal is born, genetics and nourishment are highly influential in growth and development. A foal which has the genetics for large size can do considerable catching up in the first few months of life, particularly if it receives proper nourishment and has been properly nourished while in the uterus.
Still another report presented at the South Africa symposium provided some new insights into determining the age of a fetus when no record has been made of insemination or breeding dates. Presenting the report was Catherine Renaudin, DVM, of the University of California, Davis.
Involved in Renaudin's study were 10 healthy, pregnant mares. Beginning at 100 days of gestation, Renaudin ultrasonically scanned the fetuses of the mares every two weeks--from 100 days gestation to parturition. She and her colleagues measured eye length and width, the aorta, gonads, femur, and kidneys.
The researchers found that aortic diameter, biparietal (bone in the skull) diameter, and femur length grew linearly with age, while the other parameters were not linear.
Using the information they obtained from the measurements, they established growth charts. The researchers have suggested that this data might be used to estimate fetal age when breeding or ovulation dates are unknown. A key to successful use of the data involves normal development of the fetus.
The horses used in the study, Renaudin said, were primarily Quarter Horses and Thoroughbreds. This means, she said, that the charts should be accurate for fetal development in those breeds, but might not be accurate if applied, for example, to miniatures or warmbloods. She is now at work to establish a fetal growth chart for Arabians and Morgans.
Definite changes in fetal development were observable in each two-week scan, Renaudin said.
The question of whether gender influences gestation length often has been debated. A study at Michigan State University involving more than 500 births over a period of time at a Michigan Standardbred farm produced some significant data relative to this discussion. It was found that the gestation length for colts was 344.4 days compared to 342.2 days for fillies. The average gestation length was 343.3 days.
The study also found that the sire could influence gestation length. Certain sires were consistently associated with gestations of less than 340 days, while other sires were consistently associated with gestations of 350 or more days, irrespective of foal gender.
It also was found that the time of breeding could have an effect. Gestation length decreased 2.5 days per month as mares were bred later in the season.
Unfortunately, as mentioned earlier, there are many pitfalls along the way for the developing fetus, plus a host of potential problems for the growing foal once it is delivered.
The magnitude of potential problems for both the fetus and the growing foal was outlined in a report presented at the American Association of Equine Practitioners meeting in Denver in 1996. Authoring the report were Paul S. Morely, DVM, PhD, The Ohio State University, and Hugh G. G. Townsend, DVM, MSc, University of Saskatchewan in Saskatoon. In an introductory paragraph in the report, they had this to say concerning the study:
"The ultimate goal of horse breeders is to produce viable foals that are physically capable of performing their intended activity. Authors often state that certain diseases affect foal production, are frequently seen, or are considered important, but objective measures of this frequency or importance are not available for most diseases. This information would be useful when developing management protocols intended to prevent the occurrence of disease, and for directing long-term research goals of the industry. The purpose of this study was to evaluate the reproductive performance of Thoroughbred mares, examine the risk of morbidity (illness) and mortality in foals during the first year of life, and assess foals' physical acceptability at one year of age."
As the beginning phase of the study, information was obtained from the North American Jockey Club regarding all Thoroughbred mares with registered matings during the 1988 breeding season that were owned by residents of western Canada, and for all foals born to these mares during the 1989 foaling season. This information included the owners' names and addresses, breeding dates of mares, results of mating, and foaling dates.
Early in 1990, surveys were sent to owners and information was collected regarding their assessment of foaling difficulty, illness, and mortality during the first 14 days of life, and from Day 15 to one year of age.
Jockey Club records indicated that 2,811 Thoroughbred mares, owned by 1,120 residents of western Canada, were bred during the 1988 breeding season. The response to the survey form sent out in 1990 was from 46% of the owners, representing 43% of the mares bred--a total of 1,201.
The overall results were quite grim.
Among horses whose owners responded to the survey, 82% were reported to be pregnant following breeding. Of that number, 10% of the mares resorbed fetuses or miscarried, and fetuses were not found.
An additional 6% of mares aborted and the fetuses or premature foals were found by owners.
Approximately 1% of mares gave birth to live foals at less than 320 days gestation. Three percent gave birth to full-term foals which were born dead, and 80% of the 82% which had become pregnant gave birth to full-term, live foals.
Twenty-six mares reportedly carried twins (this computes to 2.6% of the mares which conceived). Of this number, 18 aborted. Three of the mares gave birth to at least one dead full-term foal, and four full-term sets of twins were born alive.
Approximately 1.4% of the foals delivered alive reportedly were premature. When compared with full-term foals, it was found that the premature youngsters had a significantly greater risk of health problems and mortality during the first 14 days of life and also a significantly greater risk of mortality between 15 days of age and one year of age.
Overall, the responding owners reported that 25% of foals had health problems during the first two weeks of life, and that 5% of that number died during the period. Another 27% of the foals which survived two weeks were reportedly affected by some health problem between 15 days and one year of age, and 6% of that number died during the time frame.
The most commonly reported health problems during the first year of life were upper respiratory tract infections, diarrhea, and musculoskeletal problems such as angular limb deformity, contracted tendons, joint or tendon laxity, and epiphysitis.
It also was found that health problems during the first year of life took a decided toll on the youngster's well-being at one year of age. The study showed that foals which suffered health problems during the first 14 days of life were five times more likely to receive an unfavorable physical assessment at one year of age. Foals suffering from health problems between 15 days and one year of age were seven times more likely to have a negative physical assessment when compared with those which did not suffer from health problems.
Angular limb deformity was the health problem most commonly reported in foals receiving unacceptable assessments.
Overall, the study showed that 42% of the foals involved had suffered some form of health problem in the first year of life, and that 11% of the afflicted foals died. Both the number of illnesses and the number of deaths were in excess of what other studies had indicated.
In their concluding comments, Morely and Townsend had this to say about their findings:
"Estimates obtained from this survey of morbidity and mortality during the first year of life were greater than previously reported. Previous estimates obtained from large surveys performed in Texas and England suggested that morbidity from any cause was less than 30%, and mortality was less than 5%.
"The case fatality rates of horses with upper respiratory tract infections and diarrhea were much lower than case fatality rates for infectious diseases, which occurred less frequently. The case fatality rate for horses with musculoskeletal problems was low during the first two weeks of life, but was much higher between 15 days and one year of age. This was likely attributable to a greater rate of elective euthanasia among foals with poor conformation.
"Angular limb deformity was the health problem most commonly reported in foals receiving unacceptable physical assessments, and assessments of long-term athletic potential were apparently not affected by the occurrence of infectious disease problems.
"It should be noted that this survey documented owners' assessments of health, which may have been different from those of veterinarians. Similarly, owners' assessments of whether foals were acceptable for sale might have been different from those of potential buyers."
Granted, the above information came from one study in one part of North America, but it does, without doubt, demonstrate that merely getting a mare in foal is only the beginning of a long, sometimes torturous process, before a young horse reaches maturity and is ready to perform.
Perhaps one of the most frustrating things for all mare owners occurs when the foal has been carried to term, then is lost to dystocia (foaling difficulties). Because of the explosive nature of a mare's delivery, there is very little time in which to do anything about solving dystocia.
The time between rupture of the chorioallantoic membrane and delivery of the foal normally is about 20 minutes. Not much time to get a veterinarian on the premises if a problem exists.
Parturition, or birthing, in the horse has been divided into three phases, but to the casual observer, it is one relatively brief, continuous process. During the first stage, the fetus plays an active role, along with uterine contractions, in assuming the correct positioning for delivery through the birth canal. The second stage comes with the rupture of the chorioallantois and culminates in the delivery of the fetus. The third involves continued uterine contractions that result in expulsion of the placenta.
Any problems along the way in stages one and two can result in dystocia.
Unfortunately, many fetuses never get to the stage where normal parturition is an option. Many are aborted along the way. Abortion is the premature termination of a pregnancy resulting in the resorption or expulsion of a non-viable embryo or fetus.
Embryonic death before the time of pregnancy recognition--at Days 14 to 16--normally is a non-traumatic event for the mare, and she usually will return to estrus. An abortion later in the pregnancy can have a serious negative effect on her ability to be rebred.
Causes of fetal abortion generally are categorized as being either infectious or non-infectious. Listed as the most commonly diagnosed infectious cause of abortion in horses is equine herpesvirus 1 (EHV1) or rhinonpneumonitis. Abortion from this cause generally occurs between seven months of gestation and term, and it accounts for between 10-15% of all abortions. A preventive measure against "rhino" is immunization during pregnancy.
Bacterial and fungal causes of abortion primarily are caused by infective agents that enter the uterus through the cervix. The resulting placentitis can, in turn, bring about fetal infection and death.
Bacterial organisms that most commonly are cultured from aborted fetuses include Streptococcus spp, Escherichia coli, Pseudomonas spp, Klebsiella spp, and Staphylococcus spp. The most frequently recovered fungus is Aspergillus spp.
Abortions also can be the result of other causes, such as a genetic weakness, chromosomal factors, inadequate nutrition, vitamin or mineral deficiencies, ingestion of harmful plants (such as fescue during certain stages of the pregnancy), hormonal factors, environmental factors, physical factors, and certain medications.
After the Foal is Born
Once the youngster is on the ground and nursing, there still are enemies waiting in the wings to prevent normal growth and development. When one considers that foals attain approximately 90% of their mature height and 75% of their mature weight in the first 12 months of life, it becomes instantly clear that good health and proper nutrition are musts during this growing period. One of the most insidious enemies to normal growth and development is neonatal septicemia, also known as joint or navel ill. The newborn foal can be infected by micro-organisms entering its body through the umbilical stump.
However, septicemia does not restrict itself to the newborn. It also can be a cause of abortion or the delivery of a "septic" foal. The severity and course of the illness depend upon the specific micro-organism causing the problem. When the fetus is invaded, the result can be death, or the birth of a foal that is weak and ill.
The infection from the causative agent can spread to the bloodstream of a newborn, then to areas such as the intestinal tract, lungs, joints, bones, liver, kidneys, and central nervous system.
Although there are classic symptoms for septicemia, they might be subtle and the disease can progress rapidly. The basic signs include fever, depression, a weakened sucking reflex, diarrhea, and excessive resting or sleeping. The foal also might be reluctant to move, and its joints could be hot, swollen, and painful. If infection is through the navel stump, that area could be hot and swollen as well as have periodic discharges.
Foals suffering from septicemia might appear to recover, then suffer a relapse in eight to 12 months. In chronic cases, there often can be enlargement of the joints, coupled with arthritis and potential damage to growth plates.
Antibiotics can help. Professional help is a must. When a foal shows signs of septicemia, a veterinarian should be called without delay.
A common malady in newborn foals is diarrhea. It can be mild, or it can be so devastating that it dehydrates the foal and can cause its death. The most common form of diarrhea, one that is familiar to everyone who raises foals, is the "foal heat diarrhea." In days gone by, it was theorized that the condition came about because the mare went into estrus. Later, research showed that the diarrhea more likely comes about because of normal changes in the foal's digestive tract when the foal is from six to 14 days of age. In other words, the diarrhea would occur even if the mare did not return to estrus during that time period.
Foal heat diarrhea usually is short-lived, running its course in two to three days. However, if the diarrhea persists beyond that point and the foal appears to become weak, even though it is nursing regularly, it is time to call in the veterinarian.
One of the causes of foal diarrhea is an infestation of internal parasites. Foals should be placed on a routine deworming program beginning at about eight weeks of age.
Maintaining a clean environment also can help stave off a diarrhea attack. Foaling stalls should be disinfected between foalings, for example.
It is estimated that 90% of all foal diarrhea outbreaks are caused by rotavirus, says Frederick Harper, PhD, of the University of Tennessee.
"It is imperative," he says, "that newborn foals get colostrum from their dams to supply antibodies against rotavirus. Foals with rotavirus will usually be 70 days of age, but can range from two days to six months of age. They do not nurse or eat creep feed and become depressed and develop diarrhea 12 to 24 hours after being infected. They may or may not have a fever. Diarrhea normally lasts two or three days, but might persist for a week. Foals under two weeks old usually have a watery diarrhea causing dehydration, which requires intensive care, including intravenous fluids."
Next to diarrhea, one of the most common problems in the newborn is pneumonia. Harper estimates that 9% of all foals get pneumonia, and of that number, 15% die.
Once again, both cause and cure can be complicated. Foals are normally between two and eight months of age when afflicted.
"Ill animals cough in the morning and after exercise," Harper says. "Fever can come and go, ranging from 100ï¿½ to 106ï¿½ F. The respiratory rate is also elevated. Foals may be depressed and not nurse or not eat creep feed. They may lose weight."
Harper lists four of the factors that can pre-dispose foals to pneumonia:
1. Stress, such as overcrowding, weaning, and handling.
2. Dust irritates their lungs and reduces respiratory defenses. Dust might be from bedding, hay, or a dirt paddock. Bedding, when not changed daily, has a build-up of micro-organisms, dust, and noxious gases, such as ammonia.
3. Poor nutrition and parasites weaken a foal's natural defenses.
4. Heat and high humidity predispose foals to pneumonia, as do fluctuations in temperature.
Harper goes on to outline a four-way approach to reduce the risk of foal pneumonia:
1. Foal outside if possible and house foals in a clean, well-ventilated stall.
2. Have foals on a scheduled deworming and vaccination program. Show horses and visiting mares with foals should be isolated in separate facilities.
3. Booster vaccinations should be given to mares before foaling to protect foals against respiratory diseases.
4. Make sure foals have an adequate intake of colostrum.
Still another problem that faces the newborn is developmental orthopedic disease (DOD). DOD is a broad umbrella for a plethora of orthopedic conditions that can afflict the foal and have a negative effect on proper growth and development. Included are osteochondrosis, osteochondritis dissecans, subchondral bone cyst, physitis, acquired angular limb deformities, acquired flexural deformities, and cervical ventral malformation. Each of these is a subject for discussion in itself, demonstrating that there is no simple explanation for DOD. Factors involved can include genetics, nutrition, endocrine problems, and exercise problems in foals.
Nutrition often is implicated, but this, too, defies a simple explanation. Many researchers believe that overfeeding a foal can lead to DOD. Conversely, a deficiency of necessary nutrients also can bring about developmental problems in the young horse.
It is a complicated problem, and one where professional help is a must if it is suspected.
Feeds And Feeding
The list of potential problems that can prevent normal growth and development of the newborn seems endless. In some cases, the mare owner is powerless to do anything about a potential abortion of the fetus or a developmental problem in the newborn. He or she can't change genetics or chromosome function, for example.
However, there are areas where the owner can insure proper fetal development through immunizing the mare against infectious diseases and making certain that she receives proper nutrition, particularly during the final trimester of gestation and during the early days of a foal's life when its energy demands are high and are supplied almost entirely by milk from the mare.
The importance of a plenteous supply of mare's milk for the newborn foal was demonstrated in a study undertaken at Beaufort Cottage Stables in Newmarket, Suffolk, England. A report on the research was presented at the Denver AAEP session by Jennifer C. Ousey, PhD.
"Foals frequently may be fed nutrients other than mares' milk for varying time periods," Ousey said in her introduction. "For example, orphaned foals may be raised on milk replacer until weaning, and sick foals, which are often intolerant of enteral (taken into the intestine) feeds, may be fed specialized nutrients administered intravenously for some days.
"To achieve growth, the foals must have a positive energy balance. Although gross energy intakes and fluid intakes have been reported, there are few data on how energy intake is utilized, particularly when foals are fed diets other than mare's milk.
"It is important to understand the energy balance in such circumstances because milk replacer products are often fed at higher concentrations than mare's milk. Moreover, sick foals often have gastrointestinal problems and it is not known whether they can utilize dietary energy effectively.
"Therefore, the aim of this study was to measure energy balance in healthy foals fed either mare's milk, milk replacer, or total parenteral nutrition (via intravenous feeding). The foals were studied during the first week postpartum, which is when most artificial feeding regimes are implemented."
A group of pony foals was divided into three groups. Group A, six in number, remained with their dams throughout the study period, feeding normally. Group B, four in number, were removed from their dams immediately following delivery and were housed in pairs. They were fed commercial milk replacer at two-hour or three-hour intervals. Group C, also four in number, were fed through a catheter inserted into a vein. They received a commercial nutrient formula containing amino acids, lipids, and glucose. The Group C foals were muzzled to prevent nursing, but remained with their dams.
The results left little doubt as to the value of a dam's milk for her foal. All six of the Group A foals remained healthy throughout the studies.
Three of the four Group B foals were constipated initially.
The first Group C foal studied received intravenous feeding for five days. When its muzzle was removed at the end of the five days and it was allowed to nurse, it took the youngster 36 hours to learn how to suck. The foal also developed ulcers.
Because of these difficulties, the other three Group C foals were fed intravenously for three days instead of five and were given preventive anti-ulcer therapy. However, two foals experienced mild colic and "shivered strongly" when they began nursing.
The researchers also found that gross energy intake for the normally nursing foals was high from as early as the second day after birth, while gross energy intake for foals on milk replacer initially was lower because of the foals' inability to digest the replacer immediately. As a result, the daily weight gain of foals nursing on their dams was greater than that for foals on milk replacer. Both of them outdistanced the foals being fed intravenously.
In her conclusion, Ousey had this to offer:
"The data obtained indicate that the preferred diet in energy (and nutrient) terms, is mare's milk. It is highly digestible, such that excess energy is available for growth from as early as Day 2 postpartum. An energy balance similar to that of foals fed mare's milk may be achieved by feeding milk replacer. However, because of its initial lack of digestibility, coupled with its high energy density, it is advisable to dilute milk replacer to the equivalent energy density of mare's milk, or greater for sick
foals that may have impaired gastrointestinal function. The clinical problems observed in Group C suggest that foals, as in other species, suffer significant GI damage associated with enteral feed deprivation."
Mare's milk, we can conclude from the study, is very important for normal growth of the newborn. However, not all mares are created equal in the milk production department and some mares, although on an optimum diet, might not produce enough milk for a rapidly growing foal. When this is the case, a creep feeding program might be instituted to make certain that normal growth and development continues.
It should go without saying that if nutrition is important to the newborn, it also is highly important for the unborn. Foals born to mares which do not receive proper nourishment during gestation could be born weak and be susceptible to more health problems than the foal born of a well-fed mare.
We stated at the outset that procreation is a "miracle" to the layman. After looking at the perils that await the foal once it enters the world, one would also be constrained to declare it at least a near-miracle when that youngster reaches maturity as a healthy, robust, performing animal.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: Rehabbing the Injured Horse