Twenty mature geldings, averaging 535 kg, were used to determine the influence of dietary selenium (Se) on the blood levels of Se and Se-dependent glutathione peroxidase (SeGSH-Px) activity in the horse. Horses were randomly assigned within breed to four treatments consisting of five horses each and fed a basal diet containing .06 ppm of naturally occurring Se. Diets were supplemented with .05, .10 and .20 ppm Se, as sodium selenite. Blood was drawn for 2 wk before, and for 12 wk following, the inclusion of supplement Se in the diets. Whole blood and plasma Se concentrations and plasma SeGSH-Px activities were determined from all blood samples. Selenium concentrations in plasma and whole blood increased linearly from wk 1 to wk 5 and 6, respectively, in Se-supplemented horses. After these times, no significant changes in Se concentration were observed in Se-supplemented or in unsupplemented horses throughout the remainder of the 12-wk trial. Plasma Se reached plateaus of .10 to .11, .12 to .14, and .13 to .14 micrograms/ml in horses supplemented with .05, .10 and .20 ppm Se, respectively. Whole blood Se reached plateaus of .16 to .18, .19 to .21, and .17 to .18 micrograms/ml in horses supplemented with .05, .10 and .20 ppm Se, respectively. Plasma SeGSH-Px activity was not significantly affected by dietary treatment. Therefore, this enzyme was not a good indicator of dietary Se in these mature horses.
Effects of high dietary levels of energy, protein and Ca and deficient levels of Ca on skeletal growth and development in the equine were studied in two experiments. The weanlings were fed grain-based diets. In Exp. 1, Group 1 (Ctl) received all nutrients at NRC recommended levels for growth; Group 2 (HE) received 150% of their digestible energy (DE) requirement; Group 3 (LC) received 150% of DE and 35% of Ca requirements. In Exp. 2, Group 1 (Ctl) again received all nutrients at NRC recommended levels for growth; Group 2 (HEP) received 150% of DE and 275% of CP requirements; Group 3 (HEPC) received 150% of DE, 275% of CP and 500% of Ca requirements. In addition, all groups had access to pasture. The HE treatment resulted in greater (P less than .10) increases in BW and total cortical width. However, LC weanlings had lower (P less than .10) gains in third metacarpal length and radiographic bone density. Cumulative increases in BW and wither height were greater (P less than .10) for the HEP and HEPC weanlings than for the Ctl weanlings. Increases in third metatarsal length also were greater (P less than .10) for HEP weanlings than for Ctl weanlings. Higher planes of nutrition increased body weight gains and growth rate of several long bones. However, skeletal development (e.g., cortical area) may be compromised by a high rate of growth.
Nineteen weanling horses (average age = 147 d) were divided into exercised (EX; n = 10) and nonexercised (NEX; n = 9) groups, with age, sex and breed represented as equally as possible. The EX group was exercised on an automatic walker at a medium trot for up to 20 min, 5 d each week. Both groups were fed to meet 100% of their protein and 110% of their energy requirements (NRC, 1978). The EX group's diet was supplemented, on exercise days, with corn starch to meet the additional energy requirements for exercise. The experiment was conducted over a 111-d period. Body weight was measured at 10-d intervals, and height at the withers and metacarpal circumference was measured at 20-d intervals. Radiographs of the distal radius, metacarpal joint and the proximal and distal ends of the third metacarpal were taken at 147, 218 and 255 d of age to determine bone density and to observe any possible bone abnormalities. There were no differences between groups in weight or wither height gain; however, gain in third metacarpal circumference was greater (P less than .01) in the EX group than in the NEX group from 167 to 215 d of age. Bone density in the EX group increased by a greater amount (P less than .06) than in the NEX group by the end of the trial. There were no lameness problems or bone abnormalities observed in either group. Exercise training of horses during the weanling to yearling age period was shown to improve the stress-bearing characteristics (radiographic bone density and metacarpal circumference) of the third metacarpal without affecting the quantity of body growth.
Thirty foals of mixed breeding, from two consecutive years, were used in two 120-d experiments to evaluate the effects of supplemental feeding (creep feed) on growth in nursing foals. At 10 d postpartum, foals were randomly assigned either to a creep-fed group (CF) or an unsupplemented group (NCF). Initial measurements of body weight (BW), height at the withers (WH), third metatarsal length (MtIII) and third metacarpal length (McIII) were made at 10 d of age and at 30-d intervals thereafter. Medial and lateral cortical peak values for radiographic bone density, cortical width and cortical area at the midpoint of the third metacarpal were used to assess bone quality. Creep feed containing National Research Council (NRC, 1978) recommended levels for all nutrients was offered at 1.5% of BW per day. When compared with unsupplemented foals, supplemented foals had greater gains in BW (P less than .05), in WH (P less than .10) and in MtIII (P less than .05). Mean gains during the trial in BW, WH, MtIII and McIII for the CF foals were 133.3 kg, 22.8 cm, 2.1 cm and 1.9 cm and for the NCF foals were 117.6 kg, 21.2 cm, 1.5 cm and 1.9 cm, respectively. The mean value for the lateral cortical peak was slightly lower (P less than .10) for the CF foals. No differences (P greater than .10) were observed for the medial cortical peak, cortical width or cortical area of the third metacarpal. The results of this study indicate that a creep feeding program that supplies NRC-recommended nutrient levels can increase the rate of skeletal growth with little decrease in quality of bone.
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