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Research into the growth and development of Thoroughbreds managed on commercial breeding farms is reviewed in this paper. As horses are not meat producing animals, less research has been performed quantifying growth, muscle and bone development than with other production animals. However, variations in growth and development are linked with both value and wastage in young Thoroughbred horses, and can have a long-term impact on racing performance. In Thoroughbred horses, the breeder aims for optimum, or consistent, growth rather than maximum growth. Factors affecting equine growth including environment, nutrition, season and management are discussed in sections covering conception to weaning, weanling to yearling and yearling to 2 years old when horses are nearly fully grown and racing begins. The importance of reference curves for different ages, sex and locations is highlighted. While average daily gain (ADG) declines as the foal gets older, there are seasonal and management influences to consider. Month of birth has an influence on both birthweight and the expected ADG at different ages, which are believed to be related to ambient temperatures and pasture availability for the mare and growing horse. Weaning leads to a growth setback and ADG declines through winter, but increases in spring for the yearling as temperatures and pasture quality and quantity increase. Management factors such as yearling sale preparation are associated with an increase in growth rate, with gains of over 1 kg/day common during the yearling sales preparation process of 6–12 weeks. ‘Home-bred’ horses that do not go to a yearling sale grow at a slower rate but will reach the same endpoint in mature size. Growth of Thoroughbreds is influenced by country, and regions within a country, related to genetics, climate, management practices, market preferences and sale timing. More research is needed into factors affecting growth and development of Thoroughbred horses on commercial farms and the influence differing rates of growth and development have on wastage (such as developmental orthopaedic disease) and commercial outcomes such as yearling sale results and, ultimately, racing performance.

THE DIGESTIBLE ENERGY REQUIREMENTS OF LACTATING PONY MARES J. D. Pagan, H. F. Hintz and T. R. Rounsaville Cornell University, Ithaca, NY 14853 Summary The energy requirements of lactating pony mares were studied. Twenty-two mares were divided into three groups and fed one of three diets consisting of different ratios of corn, alfalfa meal and soybean meal. All three diets were fed at a level of intake that supplied the National Research Council (NRC) recom- mended amount of crude protein for pony mares during the first 3 mo of lactation. Diets 1, 2 and 3 supplied 93.0, 74.8 and 57.2 kcal of digestible energy (DE). kg body weight -1 9 d -1 , respectively. The DE content of each diet was determined in a 3 “ 3 Latin square di- gestion trial using mature pony geldings fed at a rate of 1.5 % body weight/d. Mares and foals remained on trial for an 11-wk period. The mares and foals were weighed each week, and foal heart girths and heights at withers were measured. Least-squares regression of foal average daily gain with mare weight and energy intake of the mare as independent variables showed no significant effect of energy intake of the mare on foal growth. A regression equation that describes the influence of energy intake of the mare on mare weight change is Y = .0031-.0033

Energy balance studies using indirect calorimetry were conducted with four mature equids ranging in weight from 125 to 856 kg. Each animal was fed three different levels of intake of the same diet. The amounts of digestible and metabolizable energy required for zero energy balance were determined by calculating regression equations for energy balance against energy intake. It was concluded that the maintenance requirements of equids vary linearly with body weight. No advantage was found for the use of weight for the comparison of equids within the range studied. The digestible energy (DE) requirement for equids confined to metabolism stalls was DE (kcal/d) = 975 + 21.28 X W where W equals body weight in kg. If a factor for activity similar to that suggested by the National Research Council was included, the requirement could be calculated by the equation: DE (kcal/d) = 1,375 + 30.0 X W.

Energy expenditure was measured in four geldings (433 to 520 kg) during submaximal exercise on a racetrack using a mobile open-circuit indirect respiration calorimeter. A total of 304 5-min measurements of O2 consumed and CO2 produced were taken. Measurements were made with and without riders. The amount of energy expended by the horses was exponentially related to speed and was proportional to the body weight of the riderless horse or the combined weight of the horse plus rider and tack. Total energy expended by the four horses walking, trotting, cantering was best described by the equation: Y = e3.02 + .0065X where Y = energy expended (cal X kg-1 X min-1) and X = speed (m/min). Digestible energy (DE) required above maintenance was calculated as (Formula: see text).