Browsing by Author "Hargreaves, Belinda Jane"

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    Fecal Kinetics and Digestibilities of Hays and Supplements Estimated by Marker Methods in the Horse
    Hargreaves, Belinda Jane (Virginia Tech, 1998-11-20)
    A marker model of fecal kinetics using chromic oxide (Cr) or ytterbium chloride (Yb) is being developed for grazing horses. The model consists of removal of feces at a constant rate from a single compartment, the prefecal mass. It was tested in experiments on stall-fed horses in the context of digestion balance trials. Following the preliminary work of Holland et al., (1998), three improvements in experimental design were tested. First, the rate constants were determined from both the administration and post-administration curve of the one-compartment model. Second, markers were administered three times a day to reduce diurnal variation in fecal marker concentration. Third, yttrium (Y) and Yb were tested were tested as internal markers, for the estimation of digestibility of hay and supplements, respectively. Eight horses were fed Diet 1 (orchardgrass/alfalfa mixed, OG) or Diet 2 (tall fescue/alfalfa mixed, TF) in Exp.1, and Diet 3 (OG plus fat-and-fiber supplement, OGFF) or Diet 4 (OG plus sugar-and-starch supplement, OGSS) in Exp.2. Balance-marker experiments were conducted for 17 and 20 d, with 7 and 10 d of dietary accommodation in Exp.1 and 2, respectively. Chromic oxide and Yb were administered orally and fecal samples were collected every 8 h for 8 d. Dry matter, Cr, Yb and Y were measured in feeds and feces. In balance experiments, estimates of DMD (DE) using Y, were determined precisely (SE 1 to 3 %) for hay and hay and supplement diets. Linear relationships, correlations and calibration curves were determined, validating Y as a marker. Mean daily fecal Cr data (Ct) at time t (days) including a delay (d) were fitted to a single exponential, with one rate constant (k), rising to an asymptote (Ca): Ct = Ca - Ca·e-k(t-d) Diets 1 and 2 had two sets of Ctdata, total collection (a) and fecal grab data (b), and each set was used in model development. Diets 3 and 4 had two sets of Ct data (both using fecal grab data), Cr marker dilution (3Cr and 4Cr) and Yb marker dilution (3Yb and 4Yb). For pooled data, delays of 3 to 6 h (Diets 1a, 1b, 2a and 2b) and delays of 5 to 7 h (Diets 3Cr, 4Cr, 3Yb and 4Yb) gave best fits (highest estimates of R²). The delays introduced to the Cr model for both 3Cr and 4Cr diets did not correspond to the preliminary study (Holland et al., 1998), where a 2 h delay gave the best fit in the model for horses fed hay and supplement. The present estimates may more realistically relate to mouth-to-cecum transport times, because the marker was administered three times a day instead of once, and the initial part of the tracer curve was more precisely defined. The results showed that fecal Cr kinetics could be calibrated precisely (SE 1 to 3 %) to predict fecal DM output of horses fed Diets 1b, 2b, 3a but not 4a. Similarly, fecal Yb kinetics could be calibrated to predict fecal DM output of horses fed Diet 3b but not 4b. The rate constants yielded turnover times (TT) that were longer with hay and supplement diets, than with hay alone, and which contrast with previous findings in the horse. However, the longer TT were similar to slower rates of marker excretion in sheep fed concentrates instead of all-roughage diets, suggesting that the lower fiber content retarded the rate of propulsion of digesta through the digestive tract. For two of the eight models of fecal kinetics, the rate constants of the post-administration curve were not well determined by the data, and rate constants from the administration curve were used. In future experiments, more frequent fecal sample collection during the post-administration period may improve rate constant determination. Improvements in diurnal variation of fecal marker concentration were obtained by dosing three times a day. But discrepancies between Cr and Yb concentration means of diurnal samples and combined samples showed incomplete mixing, the major source of tracer error. Therefore more frequent marker administration and fecal samples should be tested in future experiments to achieve more thorough mixing in the prefecal mass for modeling fecal kinetics, and in the small intestine for estimating digestibility.
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    Vitamin E Status of Thoroughbred Horses and the Antioxidant Status of Endurance Horses
    Hargreaves, Belinda Jane (Virginia Tech, 2002-03-05)
    Two times are critical for the horse ¾ the first few days of its life and the last few moments of a race. Vitamin E is critical in regard to immune competence in the first and antioxidant status in the latter. Studies conducted at the Middleburg Agricultural Research and Extension (MARE) Center include the development of horse feeds that replace sugar and starch with fat and fiber. The previous fat source of the pasture supplement under development was corn oil, which contains much vitamin E, was replaced with a cereal by product, which contains relatively little. Vitamin E has been studied in horses to a limited degree but not in grazing Thoroughbreds, thus the MARE Center gave me the opportunity to study vitamin E in Thoroughbred mares and foals. Middleburg is located in the Blue Ridge Mountains on Northern Virginia close to the site of one of the toughest endurance races in the world. This allowed me the opportunity to study vitamin E and antioxidant status in the horse during endurance racing. Initial studies of vitamin E supplementation to mares during the last trimester of gestation were disconcerting with no changes in serum concentrations of a-tocopherol (vitamin E). Studies conducted during the post-partum period revealed evidence of responses to vitamin E supplementation, as increased a-tocopherol concentrations were observed in mares' milk and in foal serum. Foals are born with virtually no circulatory antibodies and the supplementation of a synthetic form of vitamin E to mares demonstrated an increased passive transfer of immunoglobulins to foals. Natural vitamin E has shown a greater bioavailability than synthetic forms, in other species and was tested here at the MARE Center on mares. A greater passive transfer of immunoglobulins was observed with natural vitamin E supplementation compared with the synthetic forms, with immunoglobulin M concentrations in foal serum remaining higher for a longer period after birth compared to foals of non-supplemented mares. The transfer of a-tocopherol via the milk was also increased in concentration and duration in mares supplemented with natural vitamin E. Bioavailability of five oral forms of vitamin E (3 natural and 2 synthetic) were tested and one natural form was also administered intravenously so that clearance of vitamin E could be used to calculate the efficiency of absorption of the oral forms. Efficiency of absorption for oral treatments was not determined because of the slow turnover time of the intravenously administered vitamin E, which confounded all subsequent baseline serum a-tocopherol concentrations. Of the salvageable data, serum a-tocopherol concentrations were higher in grouped treatments at 9 and 12 h post dosing. Lipid fractions revealed possible insufficient absorption of the oral doses of vitamin E and possibly tissue saturation following intravenous doses of vitamin E. Serum concentrations of a-tocopherol were generally higher following natural forms of oral vitamin E administration. As vitamin E is the most important antioxidant in cells, it is often supplemented to endurance horses competing in 80 and 160 km races. Vitamin E protects lipid cell membranes from peroxidation by free radicals, which are increased during strenuous exercise resulting in oxidative stress. The antioxidant status of horses is severely tested during endurance racing and so a study was conducted to monitor changes in circulating antioxidants during three endurance races. Interesting novel findings in the horse were the maintenance of serum a-tocopherol and the depletion of erythrocyte glutathione and plasma ascorbate during two 80 km and one 160 km races. Associations were found between increased muscle cell enzyme leakage and decreased antioxidant status during endurance exercise and although associations do not prove a causation of oxidative stress, they do provide motivation to search for a cause and it is tempting to propose that oxidative stress damaged muscle cell membranes in endurance horses. Further, these findings propose a connection between muscle cell damage and a new form of exertional rhabdomyolysis (ER) that has been observed in endurance horses, where oxidative fibers are damaged compared to the typical glycolytic fiber damage associated with known forms or ER. An increased understanding of vitamin E utilization in the horse will improve the health and welfare of all horses, but especially newborn foals and the athletic endurance horses.