GROWING HORSES
By Dr. Randel Raub, Purina Mills, Inc.
Although much knowledge has been gained regarding the nutrient requirements of the young growing horse: there are still many unanswered questions and much controversy concerning optimum nutrition for growth. Confounding factors of genetics, environment, non-nutritional management, and their interactions with nutrition make the establishment of accurate and precise optimal nutrient requirements for growth complex and difficult.
We must define to what degree our nutritional management affects the horse during different stages of its growth. It would seem logical that nutrition would have the most profound effects during the most rapid periods of growth. Therefore, concerns have been directed toward the last third of gestation and the possible indirect effect broodmare nutrition has on fetal development. The early postnatal period is another period of rapid growth; thus, the effect of broodmare nutrition on lactation may affect the growth of the foal. Nutrition also will have a direct effect on growth and development from the initiation of creep feeding programs through maturity.
Maternal Influences on Growth
Concerns regarding the maternal effects on foal growth and development should include the prenatal period, particularly during the last two thirds of gestation. Evidence suggests neuroendocrine control of growth hormone secretion in the horse may occur before parturition, which is earlier than in other species and may emphasize the importance of broodmare nutrition during gestation and lactation. Copper supplementation in the mare is reflected with increased copper concentrations in the liver.
Linear increase in fetal body weight occurs after 200 days of gestation, and increases in length are greatest during mid-gestation. Growth in length of the cannon bones continues rapidly until eight to nine months of gestation and then slows; however the pastern bones reach almost adult length by the time of birth. In general the more distal the bone the sooner it reaches its mature size and shape. At birth the third phalanx (coffin bone) no longer has an active growth plate and postnatal growth proceeds perichondrally.
Foals produced by multiparous mares are heavier and tend to be taller through early lactation compared to foals produced from mulliparous mares. However, these differences are no longer evident by approximately 120 days of age.
Foal Growth
Growth patterns seem to be similar across breeds, geography and diet. Several studies indicate height and weight measurements of a horse's growth rate when compared to estimated mature body size. At 6 months of age a horse has obtained 83% of mature height and 46% of mature weight, at 12 months 90% and 67%, and at 18 months 95% and 80%. Much of the growth in length from the knee and hock to the ground has occurred by4 to 6 months of age. Growth data shows that Thoroughbreds from 1989 to 1990 grew slightly heavier and taller compared to Thoroughbreds from 1958 to 1976. This may be related to genetic selection for larger, faster growing horses and/or increased nutrient intake, improved nutrient requirements and improved health management. Data from 1979 showed that colts of the same age were heavier and taller than fillies and this difference persisted to maturity. However, data from Quarter horses showed no difference in height between colts and fillies after 48-60 months of age.
The effects of supplemental feeding programs for young, growing horses may be most profound during the transition period when the young horse begins receiving most of its nutrients from feed sources rather than mare's milk. This transition period is usually associated with weaning (5 to 5 months of age), a period of rapid growth that may compound the effects of feeding programs on growth. Creep fed foals (at approximately 1.5% of body weight) generally show a greater growth response than non-creep fed foals. However, differences in weight and height between creep and non-creep fed horses that may be evident at the weaning to yearling stage are not maintained provided the non-creep fed horses receive adequate nutrition for growth until maturity. Growth in the foal has been expressed in curvilinear fashion as it relates to age. Studies in other species have shown that daily growth is variable and may not follow a precise growth curve. Growth in children has been suggested to be a series of 0.5 to 1 centimeter spurts, each approximately lasting less than twenty-four hours and separated by periods of stasis. Daily measurements of growth in horses suggest similar fluctuations may exist. Thus, the current growth curve would represent a more stair step type of pattern with a negative rate of gain for 2-3 days post weaning. Where on the growth curve a horse should be at a given point in time is dependent on breed, the growth potential of the individual horse and the desired rate of the owner.
Growth and Developmental Orthopedic Disease (DOD)
There is perhaps no greater source for questions in equine growth physiology than developmental orthopedic disease (DOD). What degree, if any, of DOD can be considered Norman? If and DOD is abnormal, then what degree can be considered acceptable or recoverable, such that future athletic performance will not be compromised? Is there a perceived increase in the incidence due to a greater ability to detect DOD or due to changes in management practices that predispose horses to the disease, or both? Our ability to recognize and detect SOS has most likely increased, which leads back to the question of how much of what we detect is normal or inconsequential regarding the development and maturity of the tissue. Whether changes in management practices have affected the incidence of SOS is difficult to ascertain.
There appears to be some degree of heritability for DOD, which will vary with the specific joints and possibly breeds. Selection against SOS could be feasible, however, it would have to be based on progeny testing since stallions free of DOD have produced foals with a significant incidence of DOD lesions. It has been suggested that DOD lesions occur in the first three months of life, which is the most intense postnatal growth phase. In addition, the hind limbs of the horse tend to grow at a more accelerated rate than the front limbs and there is a greater frequency of DOD associated with the hind limbs. In addition to these variables timing may also play a role in the development of DOD. There may be a "window of opportunity" along some point of the growth curve or at some transition period that slows for a greater susceptibility to SOS. Heritability and early postnatal susceptibility suggest and emphases on nutrition and management during gestation and early postnatal growth.
There has been little work addressing possible connections to vitamin status and DOD. The pathology of the various forms of DOD does not suggest a vitamin deficiency or excess issue. Vitamin K deficiency has been investigated recently as having a possible role in DOD but the normal diet of the horse appears to have adequate amounts of vitamin K. Deficiencies of calcium, phosphorus ratios (normal 2:1, Ca:P) and excesses of specific minerals that render other minerals deficient due to binding activities also have been implicated as contributors to DOD.
Although DOD has been shown to be independent of growth rate, feeding high amounts of energy and accelerated rates of gain related to overfeeding seem to have a consistent link o DOD. Several controlled studies feeding horses at 130% NRC for growth have resulted in increased incidences of DOD. In addition, an epidemiological study of 270 Thoroughbred foals showed that the 10% diagnosed with clinical DOD had greater than average rates of gain. Fetlock incidences of DOD occurred before 180 days of age and were associated with foals born in January, February and March. Thoroughbred foals born at this time are generally larger at maturity than foals born in April, June and July. However, this may be more related to selection of specific horses to be bred at a given time of year than to an actual time of year affect. Hock, shoulder and stifle DOD incidences occurred at 300 to 350 days of age and those horses with such incidences were heavier than average at 25, 120 and 300 days of age. In contrast, some studies have shown no effect on the incidence of DOD due to accelerated growth rate. Also, rapid growth did not effect bone mineral content or bone mineral deposition and any observable affect of DOD was no longer evident by 240 days of age.
Besides the amount of energy fed and subsequent growth rates, the source of energy may be a factor in DOD. Protein when fed in excess of tissue needs can serve as a source of energy, although not a very efficient source. When fed at amounts approaching 130% NRC protein did not result in increased incidence of DOD. Dietary energy in the form of soluble carbohydrates, which usually compose much of the energy in concentrates fed to growing horses, has been suggested to be a contributing factor in DOD. The effects of soluble carbohydrates on DOD may be mediated by hormonal interactions. A possible substitute for carbohydrates as an energy source for growth could be fat. Fat has been shown to be well utilized for growth in horses. In addition, fat may not elicit the same hormonal response suspected of contributing to DOD as carbohydrates. A greater number of DOD lesions have been shown to occur in horses fed a diet at a digestible energy level of 129% NRC compared to horses fed at 100% NRC for digestible energy. However, the source of the additional energy in the 129% NRC diet was primarily added fat. To add further confusion, studies have shown that horses fed with a conventional carbohydrate based diet for rapid gain resulted in no more clinical or radiographic evidence of bone abnormalities than horses fed with a 10 per fat diet.
Exercise, or the lack of, may have effects on DOD directly or in combination with nutrition. Based on gross examination of joint surfaces, weanlings subjected to forced exercise and fed a conventional diet for rapid growth tended to have fewer degenerative lesions when compared to non-exercised weanlings fed for moderate growth. Furthermore, horses stalled for excessive amounts of time seem to have the greatest incidence of DOD when compared to horses in pasture of forced exercised.
Although the amount and form of nutrients growing horses receive is important, the nutrient balance also is very important. Imbalances between energy and protein, or between energy and protein and other nutrients may result in deficiencies of some nutrients relative to the rate of growth. Energy, protein, minerals and vitamins, their interaction with one another, and their effects on bone growth and development may be particularly sensitive to nutrient balance as it relates to DOD. The information regarding nutrition and DOD would indicate that many other interactive factors should be considered when assessing the nutritional management of growing horses. Genetics, exercise and health management could influence the sensitivity of individual animals to DOD in regard to specific nutrient interactions and deficiencies and various amounts and forms of energy.
Currently, formulating rations with quality ingredients, in the amounts and balances based on sound research, provides the best approximation to meeting the total nutrient requirements of the growing horse. However, optimal nutrient requirements that meet the genetic potential of an individual for growth while ensuring the development of a structurally sound skeletal system are not yet known.
Take Home
·
Management of the growing horse starts at conception.
Management of the growing horse starts at conception.·Pay attention to individual growth rates more than average growth curves.
Pay attention to individual growth rates more than average growth curves.·The most dynamic growth occurs during the first 3-4 months postnatal.
The most dynamic growth occurs during the first 3-4 months postnatal.·The greatest fluctuation in weight is seen during weaning.
The greatest fluctuation in weight is seen during weaning.·Nutrition is not a cure but can be a contributor to DOD.
Nutrition is not a cure but can be a contributor to DOD.·The best nutritional management to help reduce the potential for DOD is to feed balanced, high quality diets at an amount resulting in no more than a moderate, consistent rate of growth.
The best nutritional management to help reduce the potential for DOD is to feed balanced, high quality diets at an amount resulting in no more than a moderate, consistent rate of growth.Purina feeds are the best balanced, highest quality feeds for growthand will only get better.
