Epsilon Open Archive

Abstract

Reproductive efficiency is essential for profitable dairy farming. Some heifers require several artificial inseminations (AIs) to conceive, and after repeated pregnancy failures, they are usually culled. The aim of this thesis is to contribute to the characterisation of repeat-breeder heifers (RBH), focusing on the oestrous period. Oestrous behaviour, follicular dynamics, and hormone patterns were studied in RBH (≥ AIs) and virgin heifers (VH, controls). RBH displayed endocrine aberrations, with progesterone concentrations of ≥ 0.5 nmol/L (suprabasal levels) at ovulation, whereas concentrations during the luteal phase were lower than normal. The preovulatory LH surge was delayed, standing oestrus was prolonged, and the preovulatory follicle persisted longer than in VH. In a field survey, the occurrence of suprabasal progesterone levels at insemination was confirmed and determined as a risk factor for non-pregnancy. Conception rates declined with increasing numbers of AIs, indicating the involvement of intrinsic factors in RBH. Pharmacological stress was induced in ovariectomised heifers, resulting in similar adrenal progesterone production (exceeding the suprabasal progesterone concentrations) in all heifers but higher cortisol-producing capacity in RBH, indicating that stress may contribute to repeat breeding. The question of whether in-built endocrine aberrations affected follicular and oocyte quality was studied. Results from light and electron microscopy (TEM) and the degree of apoptosis in the follicular wall (evaluated by the TUNEL technique) indicate that RBH enter standing oestrus with prerequisites similar to those of VH. Ability for in vitro maturation (IVM) was studied in oocytes collected by ovum pick-up (OPU), and effects of OPU on RBH characteristics were evaluated. Immature RBH oocytes were scored to be of lower quality at retrieval but not at evaluation by confocal laser scanning microscopy or TEM. After IVM, spatial organelle reorganisation was, however, less advanced in RBH oocytes, revealing cytoplasmic maturation asynchrony. Performing AI subsequent to OPU resulted in pregnancy rates similar to field fertility in RBH and VH, thus apparently not impaired by OPU. Multiple deviations (caused by intrinsic factors) occur along the course of events toward ovulation, which may affect oocyte developmental capacity and conditions for fertilisation and early embryonic development, thereby to some extent explaining the sub-fertility in RBH.