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Vita. / Digitized by Kansas Correctional Industries

An experiment was conducted to study the effect of prolonged nutritional stress on the reproductive performance of twelve Holstein dairy heifers. Energy intake was reduced to 60% NE(m). The animals lost 20% of their body weight during energy restriction (97 days) and gained 29% during the high energy feeding (44 days). Estrous cycles were irregular, delayed, or ceased completely. The conception rate was only 25% during energy reduction. Cycles were reestablished and conception rate improved (67%) during the high energy feeding. Heifers were designated as fertile or infertile according to their success or failure to conceive during the energy reduction period. Serum thyroxine (T₄) levels were slightly decreased (P > .05) while triiodothyronine (T₃) levels were significantly depressed (P < .01) during energy restriction in both fertile and infertile heifers. Serum thyroxine levels were slightly increased (P > .05) in the fertile heifers, but significantly increased (P < .01) in the infertile heifers during high energy feeding. Serum triiodothyronine levels were significantly increased during high energy feeding in both fertile (P < .05) and infertile (P < .01) heifers. Concentrations of blood cortisol were relatively higher in the fertile than in the infertile heifers during the ad libitum and energy restriction periods. This suggests that the fertile heifers possessed elevated adrenal activity that the infertile heifers did not have. Concentrations of blood progesterone of the infertile heifers were severely depressed (P < .01) during the submaintenance feeding while those of the fertile group were not affected (P > .05). Blood glucose levels were significantly reduced during energy reduction and significantly increased during high energy feeding in both groups of heifers (P < .01 for infertile heifers). Blood glucose levels of the infertile heifers at time of insemination (period of energy restriction) were as low as their average levels of the period. Glucose levels of the fertile heifers at time of breeding (period of energy restriction) were greater than their mean concentration during that period and were similar to the mean levels of glucose during the ad libitum period. Low progesterone levels are indicative of ovarian inactivity under such stressful conditions. Adrenal inactivity, (as reflected by low cortisol secretion) and low blood glucose levels at time of insemination of the infertile heifers are probably associated with infertility during submaintenance feeding in dairy heifers.

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Five experiments were undertaken to identify metabolites and hormones that could mediate the effect of dietary energy status on reproduction, particularly pulsatile secretion of luteinizing hormone (LH) postpartum dairy cows. / In the first two experiments, the concentration of progesterone (P4) in tailhead adipose tissue and plasma in 12 cows at different stages of pregnancy and lactation was determined as was P4 produced in vitro by explants of tailhead adipose tissue. Concentration of P4 in adipose tissue was correlated with that of plasma P4 near estrus and during the luteal phase of the estrous cycle, and P4 was released in vitro by fat mobilization. / In the third and fourth experiments, blood was collected continuously for 16 h from four ovariectomized cows offered maintenance or restricted energy diets after priming with P4 or estradiol (E2) using a crossover experimental design. The results indicated that P4 released during body fat mobilization is minor and is not related to LH secretion. Dietary energy restriction influenced plasma LH concentration and pulse amplitude but the effect was modulated by P4 and E2 priming. Dietary energy restriction decreased glucose concentration but did not influence plasma non-esterified fatty acids (NEFA), cortisol, P4 and insulin levels. Cortisol was negatively related to LH pulse frequency. Glucose and insulin were positively and negatively correlated with LH pulse amplitude, respectively. Cortisol, NEFA and glucose jointly had a negative correlation with LH concentration. / In the fifth experiment, blood samples were collected daily for 60 d and every 10 min for 8 h on 18, 36 and 54 d postpartum from 24 cows (12 ovariectomized) fed low (1.4 Mcal/kg DM) (L) or high (1.7 Mcal/kg DM) (H) energy in a 2 x 2 factorial treatment design. LH pulse frequency was reduced at 18 d postpartum in ovariectomized cows, but not in intact cows, fed L. First postpartum ovulation occurred later in intact cows fed L compared to those fed H. Energy balance and plasma glucose concentration were lower, but plasma NEFA, $ beta$-hydroxybutyrate (BHB) and E2 concentrations higher, in cows fed L compared to those fed H. E2 concentration in intact cows fed L was elevated for a prolonged period prior to first ovulation. Diet had no influence on plasma P4 and insulin concentrations. Plasma E2 and BHB concentrations were positively correlated with LH pulse frequency in intact cows across diets and ovariectomized cows fed L, respectively. NEFA were negatively correlated with LH pulse amplitude in ovariectomized cows fed L. Glucose, NEFA and P4 were negatively, but BHB, E2 and insulin positively correlated, individually or in association, with LH concentration. / Overall, the results suggest that the effect of dietary energy status on LH patterns and timing of onset of postpartum ovulation is modulated by priming with or presence of ovarian steroids. The relationships of metabolites and hormones with LH patterns appear to change with dietary energy level, ovarian status and mutual associations among the metabolites and hormones. These parameters, especially glucose and BHB, may be potential mediators of the effect of dietary energy status on LH patterns. (Abstract shortened by UMI.)

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M.S.

Please read the abstract in the section 00front of this document / Dissertation (MSc (Agric))--University of Pretoria, 2007. / Animal and Wildlife Sciences / unrestricted

Reproductive data from 30 research herds were on 31,132 breeding periods of 11,347 dairy cows. Cows were sired by 1,101 sires and had 66,184 services to 1,320 service sires. Several measures of reproductive pe.rformance were calculated. These included conception rate, number of services, service period length, days open, age at first breeding, calving interval, days between services, and return to estrus lag. First, second and third service period were each analyzed separately, while fourth and later service periods were pooled. Heritability was estimated using the sire component of variance and the estimate of the total variance derived from MIVQUEO and maximum likelihood analyses. The data set was restricted to daughters of sires used in multiple herds. Heritability estimates were less than .07 for all traits in the heifer service period except age at first breeding (.2 by maximum likelihood and .13 by MIVQUEO). Similarly, with the exception of conception rate, none of the measures of reproduction had heritabilities greater than .05 for all three remaining service period groups. Conception rate measured as a trait of the male (service sire) ranged from .08 to .135 for second and third service periods. Conception rate as female trait (sire) had heritabilities ranging from .09 to .249 for second and third service periods. Low heritability estimates obtained in this and other studies suggest that large progeny or service sire groups will be necessary to identify the small genetic differences between bulls. Many genetic and phenotypic correlations were forced positive due to a part-whole relationship or due to the fact they were simply different bounds for the same measure. A few correlations were in the range from .50 to .90, but many were not significantly different from zero due to large approximate standard errors. Repeatabilities based upon pairwise comparisons were in the range from 0 to .13. Repeatabilities for the reproductive performance of virgin heifers with first parity ranged from .01 to .06 and were generally smaller than later parities. Repeatabilities based upon repeated measures on the same cow ranged from 0 to .12. Predicted Differences for female (sire) and male (service sire) reproduction were calculated by Best Linear Unbiased Prediction. This analysis included 207 bulls which were in the data both as sire and service sire. Correlations between proofs for male and female reproduction ranged from -.13 to .13. These results suggest limited genetic relationships between male and female fertility. / Ph. D.

Graduation date: 1997