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Inorganic P is often added to growing horse diets because organic P, or phytate-P, is believed to have lower digestibility. If horses can efficiently digest organic P, then the need for inorganic P may be reduced. Much of the P in grain-based concentrates fed to growing horses is in the form of phytate-P. Little is known about the ability of growing horses to degrade phytate-P or whether horse age affects mineral digestion in horses. The objective of this study was to examine the effect of age on P, Ca, and Mg digestibility as well as phytate-P degradation. Four yearling geldings and 4 mature geldings were fed a diet of alfalfa cubes, timothy cubes, and a pelleted concentrate. The diet contained 0.28% total P and 17.4% of that P was in the phytate form. There was a 14-d diet adaptation period followed by a 4-d fecal collection period. Apparent total tract P digestibility was higher for yearlings than mature geldings ( = 0.036; 7.7 and -6.6% for yearlings and mature geldings, respectively). Phytate-P disappearance was 94.8% and did not differ between ages ( = 0.190). Apparent Ca digestibility was lower in mature geldings ( = 0.043), but apparent Mg digestibility did not differ between ages ( = 0.414). Phytate is broken down in the gastrointestinal tract, but the low P digestibilities suggest that either degradation occurs after the site of P absorption or liberated P is recycled back into the gastrointestinal tract. Yearlings can utilize organic P as well as mature horses; therefore, diets without inorganic P are acceptable for growing horses.

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.

Eight weanling Standardbred horses were used to evaluate the effects on growth and plasma glucose of two differently processed diets with identical ingredients fed at a rate of 3% of body weight, as fed, daily. The daily ration was divided into two equal feedings at 7:00 am and 7:00 pm. The hay and grain diet (diet HG) consisted of 50% alfalfa hay cubes and 50% of a commercial growing horse texturized grain mix fed as is, without any further processing, with each part fed in separate containers. The completely pelleted diet (diet P) consisted of the same proportions of the hay cubes and grain mix that was ground, pelleted, and fed in a single container. Two groups of four horses were fed either diet P or diet HG for two 21-day periods such that each horse consumed each diet for 21 days. On the last day of each feeding period, 7-mL blood samples were drawn into heparinized evacuated blood tubes for determination of plasma glucose concentration. Blood samples were drawn 30 minutes before the afternoon feeding, immediately before, and every 30 minutes thereafter for 5 hours. Analysis of variance found greater (P = .027) average daily gain for horses fed diet P versus diet HG. A nonsignificant difference was seen in feed efficiency (P = .057) for horses fed diet P versus diet HG. Plasma glucose changes were compared across treatments using area under the curve analysis. No difference (P > .05) was found in the postprandial plasma glucose changes between diet treatments.

Summary This study demonstrated that the feeding of treatment diets with calculated dietary cation-anion balances (DCAB) of +370.43 (H) and -25.69 (L) did not have significant effects on blood pH, pCO 2 , and HCO 3 -. Serum Ca 2+ , P, Na + , and Cl - as well as plasma PTH did not differ (P > .05) between the two treatment groups. Serum K + was higher (P 2+ , P, Na + , K + , and Cl - excretion in the young growing horse. Urine Ca 2+ and Cl- levels were higher (P + , and K + in the urine were higher (P

Summary Rates of growth were measured in two-year-old (Experiment 1) and nine-month-old (Experiment 2) horses fed high (12 or 14 per cent) or low (6 or 8 per cent) crude protein diets with one of two levels of exercise (0 or 12 km trotting per day at 12 km/h). In the non-exercised horses feed intakes and growth rates were greater on the high than on the low protein diets. Exercise increased feed intakes and growth rates of horses on the low but not the high protein diets, so that in the exercised groups there were no significant differences in feed intakes or growth rates between the horses on the two dietary protein levels. Exercise increased the apparent digestibility of dry matter, energy and crude protein. There were no differences in skeletal characteristics caused by either dietary protein level or exercise. These results suggest that the growing horse, if exercised regularly, may be given a diet of lower protein concentration than currently recommended without significant reduction in growth rate.

Summary This report reviews the recommendations of the National Research Council (NRC) published in 1978 regarding nutrient requirements of horses and discusses its conclusions in the light of more recent evidence published elsewhere. It appears that most of the results of studies made available since the publication of the NRC requirements agree with the NRC values. However, there are some differences and more research is needed. The energy requirements for racehorses appear to be greatly underestimated by the NRC. The values for most vitamins and trace minerals need further study. It is suggested that the role of nutrition in the development of the growing horse, particularly of the skeleton, should be examined more closely. Resume Cet article passe en revue les recommandations du National Research Council (NRC) publiees en 1978 au sujet des besoins alimentaires des chevaux; il en discute les conclusions a la lumiere des plus recentes connaissances publiees ailleurs. Il semble que pour la plupart, les etudes posterieures a la publication du NRC soient en accord avec les valeurs du NRC. Quelques differences existent cependant et de nouvelles recherches sont donc necessaires. Les valeurs pour la plupart des vitamines, et pour les oligo elements, demandent de nouvelles recherches. Il apparait que le role de l'alimentation dans le developpement du cheval en croissance, notamment en ce qui concerne le squelette, devrait etre etudie plus particulierement. Zusammenfassung Dieser Artikel bespricht die Empfehlungen des “National Research Council” (NRC) von 1978, die sich auf die Ernahrungsbedurfnisse von Pferden beziehen. Des weitern werden die Schlussfolgerungen im Lichte neuerer Erkenntnisse diskutiert. Es scheint, dass die meisten Resultate, die seit der Veroffentlichung der NRC-Empfehlungen zuganglich geworden sind, mit den NRC-Werten ubereinstimmen. Immerhin bestehen einige Differenzen, die vermehrte Forschung rufen. Die energetischen Bedurfnisse fur Rennpferde scheinen vom NRC ganz erheblich unterschatzt zu werden. Die Werte fur die Mehrzahl der Vitamine und Spurenelemente mussen eingehender untersucht werden. Es wird vorgeschlagen, dass die Rolle der Ernahrung fur die Entwicklung des wachsenden Pferdes — vor allem der Einfluss aufs Skelett — naher unter die Lupe genommen wird.