Summary Data were compiled regarding the success of commercial embryo recovery rate, transfer pregnancy rate, and embryonic loss in recipients, with respect to influence of donor parity and age, between 1985 and 1988. There were significant differences between maiden and subfertile mares for embryo recovery rate (40/66 versus 81/282), transfer pregnancy rate (34/49 versus 38/78), and embryonic loss in recipients (4/34 versus 13/38). There were also significant differences between recently foaling and subfertile mares (146/276 versus 81/282) and for younger mares (two to eight and nine to 17‐year‐old) compared with older mares (18 to 28‐year‐old) for embryo recovery rates (80/132, 94/183 and 93/309, respectively). Transfer pregnancy rates differed signifiantly between mares aged two to eight‐years‐old compared with those aged nine to 17 and 18 to 28‐years‐old (59/84, 50/97 and 50/90 respectively). No differences were observed among any of the age groups regarding embryonic loss in recipients. There was an effect of year on embryo transfer pregnancy rate in 1985 compared with subsequent years that could have been attributed to any of a wide variety of external factors.

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Mathematical nutrition models have been developed for beef and dairy cattle to estimate dietary energy intake needed to change BCS. Similar technology has not been used to improve nutrition and feeding strategies for horses. An accurate equine nutrition model may enhance feeding management and reduce the costs of unnecessary overfeeding and promote an optimal level of fatness to achieve reproductive efficiency. The objectives of this study were to develop and evaluate a mathematical nutrition model capable of accurately predicting dietary energy changes to alter BW, rump fat (RF) thickness, and overall body fat (BF), which is needed to maximize profitability and productivity of mares. Model structure was similar to a previously developed model for cattle, and literature data for Quarter Horse mares were used to parameterize the horse model in predicting DE requirement associated with BCS changes. Evaluation of the horse model was performed using an independent dataset comprising 20 nonlactating Quarter Horse mares. Pretrial BCS was used to assign mares to 1 of 4 treatment groups and fed to alter BCS by 1 unit as follows: from 4 to 5 (Group 1), 5 to 4 (Group 2), 6 to 7 (Group 3), and 7 to 6 (Group 4). The BCS, RF thickness, and BW were measured for each mare before the commencement of the feeding trial and once per week thereafter for the duration of a 30-d feeding trial. Initial and target BCS, percent BF, and BW data were collected from each mare and inputted into the model. Mares were individually fed according to the DE suggestions proposed by the model to achieve the targeted BCS change within 30 d. The coefficient of determination of observed and model-predicted values (model precision) was 0.907 (P < 0.001) for BCS, 0.607 (P < 0.001) for percent BF, and 0.94 (P < 0.001) for BW. The BCS was highly correlated to percent BF (r = 0.808; P = 0.01). We concluded the reparameterized model was reliable to predict changes in BW and BCS, but more work is needed to improve the predictions of initial and final body composition.

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Chair of Advisory Committee: Dr. Clay A. Cavinder Studies performed on stallions have characterized detrimental effects on semen quality resulting from thermal stress by testicular insulation, but few have investigated the effects of exercise-induced increases in core body temperature on stallion semen parameters. To our knowledge, this is the first study that correlates subcutaneous scrotal temperature and stallion spermatozoa quality using a subdermal scrotal thermal sensory device. Stallions were assigned to a non-exercised (non-ex; control; n=4) or exercised (ex; n=4) group. A motorized equine exerciser was used to work stallions 30 min/d for 4 d/wk during a 12-wk period from July through October. Temperatures (subcutaneous scrotal, subcutaneous neck, rectal, and ambient) were recorded before exercise, immediately after exercise, and 60 and 120 min post-exercise. Humidity data were obtained later to determine THI.No deleterious effects were observed from implantation of thermal sensorydevices. An interaction of treatment and time (P < 0.0001) was evident for rectal and neck temperatures. The relationship between scrotal and rectal temperatures was highest (r s =0.761), and other correlations existed between scrotal, neck, and ambient iv temperatures, as well. Mean rectal temperature in the ex group increased 1.9°C (P < 0.0001), while there was a slight increase in scrotal temperature of 0.8°C (P > 0.05) from 0 min to 22 min. Although an increase in core body temperature was successfully induced by exercise protocol, scrotal temperatures were not significantly affected, and no treatment effects were found in any of the semen parameters measured (P > 0.05).Therefore, no significant changes in fresh or cooled semen parameters resulted from exercise or increases in core body temperature resulting from exercise protocol used in this study.Results of this study indicate that thermal sensory devices are a safe and effective way to measure subcutaneous scrotal and neck temperatures. Although an increase in core body temperature was successfully induced by exercise protocol, scrotal temperatures were not significantly affected, indicating efficient thermoregulation of the testes by the scrotum. Since the testes experienced no significant thermal insult during the exercise protocol, no significant changes in fresh or cooled semen parameters were evident as a result of exercise or elevated core temperature.v DEDICATION This thesis is dedicated to the two people I admire most, my parents. They forged the career path that I have chosen in life by providing me the opportunity to grow up with horses. Although this path has taken me far from home and family, I know that no matter the distances I travel, the unwavering love and support of my parents will always sustain me. Mom and Dad, I will forever be grateful for the sacrifices you have made to see me achieve my goals. Thank you; I love you; and I miss you every day.To my beloved horse and friend of only 5 short years: you will always be the horse to which I will compare ...