Graduation date: 1995

Two trials were conducted over consecutive years to examine the effects of ewe body condition and post-mating nutrition on ovulation rates and embryo survival. Trial 1 used 146 Polypay ewes ranging in age from 5 to 8 years in a 3 x 2 factorial array of pre-mating (high-H, low flushed-LF and low unflushed-L) and post-mating nutrition (high and low) treatments. Trial 2 was a 2 x 2 x 2 factorial array of 60 Polypay and 60 Coopworth x Polypay (C x P) three year old ewes, two pre-mating (H and LF) and two post-mating (high and low) treatments. To estimate timing and extent of embryo loss, two methods of embryo detection were employed during Trial 2. Real-time ultrasound was performed on all ewes at 21, 28, 34 and 45 days post-mating. Blood samples were also collected on these days for analysis of Pregnancy-specific Protein B (PSPB) levels. There was no effect of pre-mating treatment or ewe age on ovulation or conception rates for Trial 1. Pre-mating treatment was significant in Trial 2, with H ewes having higher ovulation rates than LF ewes. Pre-mating treatment, post-mating treatment, ewe age (for Trial 1) and genotype (for Trial 2) had no effect on mean litter size in either trial. Analysis of litter size among ewes with twin and triple ovulations showed pre-mating treatment to be significant among triple ovulators in Trial 1, where L ewes had much lower litter size (1.86) than LF (2.37) or H (2.60) ewes. In Trial 2, post-mating treatment was significant among the ewes with triple ovulations, with ewes on low nutrition having lower litter size (2.01) than ewes on high nutrition (2.59). Two-way interactions were significant among twin and triple ovulators in Trial 2. Breed x pre-mating interaction among twin ovulating ewes showed C x P LF ewes to have lower litter size (1.43) than the H ewes (1.87), while the opposite was true for Polypay ewes. Pre- x post-mating treatment interaction among triple ovulators showed LF ewes on low post-mating nutrition had lower mean litter size. This effect is largely due to lower litter size in the C x P triple ovulators on low post-mating nutrition. Data available from the ultrasound diagnosis showed little indication of treatment effects on amount or time of embryo loss. Ewes bearing single or multiple embryos differed in PSPB level at day 45, but not at earlier times. However, there were no differences in PSPB levels in ewes with twin or triple embryos at any of the trial days. Assay for PSPB failed to facilitate detection of the amount or time of embryo loss, as determined by ultrasound. / Graduation date: 1991

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 171-195). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 171-195). Also available on the Internet.

No description available.

The Elsenburg Merino flock has been divergently selected for the ability of ewes to rear multiple offspring since 1986. Updated genetic trends for reproduction are reported for the Elsenburg Merino resource flock. The objective was to determine whether genetic trends estimated previously for the Elsenburg Merino Resource flock changed significantly with the introduction of genetic material from the industry to the high (H) line. All analyses included the full pedigree file, consisting of 6547 individuals. Heritability estimates were 0.08 ± 0.02 for number of lambs weaned and 0.11 ± 0.02 for corrected weight of lamb weaned. The ewe permanent environment variance was estimated at 0.09 ± 0.02 and 0.11 ± 0.02 for number of lambs weaned and for corrected weight of lamb weaned, respectively. Genetic trends for the H and low (L) lines were divergent (P < 0.05) for all reproduction traits during the period prior to the observed breakpoints. Progress for number of lambs weaned in the H line stabilised after 1999 while a decline in response for weight of lamb weaned in the H line occurred after 2003. The change points may result from reduced selection intensity during the formation of reciprocal crossbred lines, or the introduction of unrelated industry sires in the H line. The pedigree was analysed and inbreeding trends computed for the H and L lines with the aim to test the significance of inbreeding within the lines. The software packages used for the statistical analyses were ENDOG v4.8 and POPREP web analysis software. The average inbreeding coefficients (F) were 1.47% and 0.73% for the divergently selected H and L lines. The rate of inbreeding (ΔF) per generation was 0.5% for the H line and 0.6% in the L line. The overall rates of inbreeding per generation were different in the H and L lines but within acceptable levels. The L line, however, showed an unwanted recent increase in inbreeding that will need to be considered in future. Since 2003, part of the Elsenburg Merino breeding flock was subjected to structured reciprocal within-breed crossing. Lamb survival traits and ewe reproductive performance of purebred (H and L) and reciprocal crosses (HxL and LxH) were evaluated using least squares analyses. Levels of heterosis were also assessed. The mean survival of the two crossbred lines was notably superior to the midparent value in absolute terms, although the contrast did not reach statistical significance (P = 0.098). Further research is required to establish whether this within breed heterosis for lamb survival can be exploited to decrease lamb losses. Reproduction, number of lambs born (NLB) and number of lambs weaned (NLW) in the H line was higher than in the L line (P < 0.05) while the two crossbred lines were intermediate and different from both the H line and the L line (P < 0.05) from the analyses of annual reproduction and overall “lifetime” reproduction across three lambing opportunities. Individual heterosis for annual reproduction was estimated at 2.2% for NLB, 13.8% for NLW and 8.5% for corrected weight of lamb weaned (TWW), with the estimate for NLW reaching significance (P < 0.05). Corresponding estimates for total production over three lambing opportunities were 8.7% for TNLB, 19.1% for TNLW and 13.8% for TTWW, with the estimate for NLW reaching significance (P < 0.05). Ten RAPD markers were used to study molecular divergence between the H and L lines. Phenotypic data on the lifetime reproduction of ewes born in 1999 and 2000 indicated that reproduction in the H line ewes was markedly higher than that of L line contemporaries (P < 0.01). The RAPD assay, conducted on 15 ewes from each line, used eight primers and produced 87% polymorphic loci. The mean coefficient of genetic differentiation between lines (Gst) was estimated to be 0.25. In conclusion, the H and L lines were shown to be divergent for genetic trends and levels of inbreeding. The derived estimates of heterosis may also be used to infer divergence between the lines and significant molecular divergence proven using RAPD assays. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

No description available.

[Truncated abstract] Reproductive activity is affected by external factors such as photoperiod, social cues, stress and nutrition, all of which can alter the pulsatile activity of the GnRH neurons, which is the major neuroendocrine system used by the brain to control gonadal function. In the male Merino sheep, nutrition is one of the most powerful factors that affect pulsatile LH secretion, used commonly to bioassay GnRH neuronal activity. More accurately, the reproductive system responds to “metabolic status”, rather than “nutrition”, and the three factors that contribute to metabolic status are food intake, the amount of body reserves and the rate of energy expenditure ... In this thesis, I tested the general hypothesis that the metabolic hormones and hypothalamic neuropeptides that are known to control food intake also mediate the effect of metabolic status on the activity of the GnRH neurons ... In conclusion, the results from my experiments provide some insight into the mechanisms by which metabolic status affects reproductive activity in male sheep. Plasma insulin, which changes with alterations in metabolic status, appears to play a critical role in the regulation of GnRH neuronal activity. The level of leptin seems to have a permissive role only in lean animals. Orexins acting via OX2 receptors could be involved in the activation of reproductive function following an acute increase in nutrition. However, the neuropeptidergic systems can not be ruled out because they might be involved in very early steps of responses to nutrition.

Thesis (Ph. D.)--University of Adelaide, Dept. of Crop Protection, 1998? / Includes bibliographical references (leaves 120-145).

Thesis (Ph.D.)--University of Adelaide, Dept. of Animal Sciences, Waite Agricultural Research Institute, 1989. / Includes bibliographical references (leaves 143-165).