For most species, the extent of foetal growth determines the length of the pregnancy. Small species such as the dog have a shorter pregnancy (58-67 days) than large animals such as an elephant (645 days). Pregnancy lasts for about 11 months in the horse but as with all biological events the exact timing varies. For most mares, foaling will occur within a 10 day period (335 to 345 days) but it can vary between 320 and 370 days; foals born at less than 326 days are considered premature. Although there can be large differences between horses in terms of the length of their pregnancy, when bred regularly, individuals tend to be fairly consistent year on year. Various other factors can be involved in determining the duration of a pregnancy. In humans, male foetuses normally remain in utero for several days longer than females and multiple pregnancies last for a shorter period. As in humans, sex can also influence the length of a mare’s pregnancy; mares with colts will be pregnant for 2 to 7 days longer than those with fillies. However, absolute foal growth does not always determine the length of pregnancy in the horse. Species, such as the horse, that naturally have a breeding season usually give birth during a specific time of year when food is available. Mares foaling in late spring and during the summer will tend to have a shorter pregnancy than those, such as the Thoroughbred, that foal in January/February. Use of artificial lighting during the last third of pregnancy can shorten the pregnancy by up to 10 days. It is of interest to note that both Zebras and Donkeys tend toward a 12 month pregnancy.
A pioneering study was conducted at Cambridge in 1938 to determine any maternal influence on foetal development and hence birth weight. Mating large Shire females with small Shetland males (using artificial insemination!) produced offspring larger than those from pure Shetland crosses, but smaller than those from Shire crosses, while crossing Shetland females with Shire males produced offspring no heavier than those of pure Shetland crosses. It was plain that there was a very strong maternal effect. A Shetland foal will weigh around 20kg and that of a Shire about 70kg. For a startling contrast, a new borne Elephant calf will weigh ~120kg! Generally we assume that foals borne of light horses will weigh the equivalent of 10% of mature weight whereas foals from draft horses will average around 7% and those of Shetlands about 13%. Large mares will have bigger foals than lighter mares and this is highly relevant in the Thoroughbred breeding industry. This is because the heavier foal grows faster, has a higher final mature weight and will be likely to perform better on the track. Mare age and parity affect foal birth weight. Thoroughbreds produce the heaviest foals between 7 and 11 years of age in contrast, maiden mares and multiparous mares produce lighter foals. Thoroughbred foals weigh about 50kg when new born and those that weigh less than 45kg are less viable and require additional care. Thoroughbred colts are generally heavier than fillies at birth and tend to grow at a faster rate. Since heavier Thoroughbred foals are desirable, attempts have been made to manipulate birth weight by overfeeding mares through providing excess supplementary feeding in late pregnancy. None of these strategies work. However, disease in the form of strangles during pregnancy has been shown to markedly reduce foal birth weight.
It should be clear from the foregoing that ultimately, the pregnant mare will have stored a lot of energy and nutrients in the foetus and it is crucial to the wellbeing of the newborn foal that this is done properly. It is also worth remembering that the foetal membranes and associated tissues will also create a demand for nutrients and energy that must be supplied throughout the pregnancy. A well-developed placenta is needed to ensure that the foetus is “well fed” during the pregnancy and that it develops normally. For example, a Thoroughbred foal weighing about 55kg will be accompanied by a placenta weighing around 6.5kg. Furthermore, formation of colostrum in late pregnancy and subsequent milk production will depend on adequate nutrition in late pregnancy.
Initially the developing foetus is rather slow growing. During the first 6 months of the pregnancy of a typical Thoroughbred mare weighing ~550kg the foetus grows very slowly attaining a weight of only ~4kg by the end of this period. Although a heart beat may be detected as early as day 21, the requirement for energy and nutrients by the developing foetus is trivial at this stage of the pregnancy and thus during the first 180days a mare should be fed for maintenance if doing no work, for lactation if feeding a foal (common amongst Thoroughbreds) or for work if used in riding activities. This presupposes that the mare is in good bodily condition. A desirable score would be 5 on a scale 1 to 10 or 2.5 to 3 on a scale 1 to 5. The goal when feeding the pregnant mare is to achieve and maintain the appropriate condition score. The estimated date of foaling will affect the nature of the feeding programme because feed availability changes with the seasons. Assuming that the mare is a non-lactating Thoroughbred due to foal in January she will be at grass for the first 6 months of the pregnancy and will obtain sufficient energy and nutrients from the grass alone. However, some owners prefer to feed a small volume of a commercial product at that time to guard against any mineral/vitamin shortfalls and thus provide some “insurance” against deficiency. A mare foaling in June will have her first 6 months of pregnancy through October to March and thus grass is unlikely to be a great supplier of nutrients and energy during this period although any deficiencies can easily be made up with good quality hay or haylage and a supplement if deemed necessary.
Overweight mares can be managed to lose weight under veterinary supervision only after the first 90 days of pregnancy have passed. Underfeeding during the first 90 days can lead to a higher risk of early embryonic death and foetal abortion when compared to those mares maintained at constant bodyweight. After the first 90 days it is safe to put an obese mare on to a controlled weight reduction programme. Feed intake and exercise should be adjusted to achieve a weight loss of no more than 1% of the mare’s bodyweight by the end of each week (1kg/100kg bodyweight) until she reaches and maintains the desired condition score described above. Starvation or the use of poor quality forages should not be allowed but it would be a good idea to feed a low energy, broad spectrum product to guarantee adequate protein/mineral/vitamin intakes during any period of weight reduction.
During the last 90 days of pregnancy foetal growth accelerates. At 224 days (~7.5 months) a Thoroughbred foal will weigh ~9kg and 60 days later it will have more than doubled in weight to ~19kg. After another 60 days to term it will have more than doubled in weight again to ~45kg, a gain of 433g per day! In the last 90 day period the weight gains are impressive and demand a huge increase in both energy and nutrient supply to the mare. Clearly the early foaling Thoroughbred mare will depend totally on conserved forage and concentrate food since there will be minimal grass growth during the winter period. The grass in the field may be green but its nutritional value will be zero although it might provide some occupational therapy. The goal as stated earlier, is to maintain the mare in the right condition score so that concentrate usage will have to be judged against forage quality. Normally ration composition should not exceed 30% concentrate with 70% forage at term since really poor quality forages should never be fed to pregnant mares. Concentrate feeding can be roughly incremented by 10% per month starting with 10% concentrate and 90% forage at the start of month 9 of pregnancy. If very high quality haylages are available then concentrate usage may be restricted to one of the low volume balancer products designed specifically to provide micronutrients rather than energy. The mare foaling at grass during mid summer is likely to require no concentrate feed at all other than a low volume product designed to provide just vitamins and minerals.
Pregnancy in the horse is characterised by there being an extended period of time during which there is very little foetal growth followed by a period of very rapid growth over the last 90 days (up to 1kg/day for a Clydesdale foal in utero). This exceptional growth must be carefully supported nutritionally. Strategies to achieve this depend on mare condition and when the mare is expected to foal. The latter determines the type of feed available which can vary from an all-grass diet to one that is a mixture of conserved forage and concentrate.