Seasonal changes in bovine fertility: relation to developmental competenceof oocytes, membrane properties and fatty acid composition of follicles

Follicle dynamics and oocyte viability in Holstein primiparous and multipar ous cows and the relationships between fertility and the biochemical and ph ysical properties of oocyte membranes with season were examined. The concep tion rates of primiparous (n = 70 885) and multiparous (n = 143 490) cows d iffered, peaking in the winter and decreasing in the summer. The number of follicles 3-8 mm in diameter per ovary was higher in winter (19.6) compared with summer (12.0). However, in winter the percentage of ovaries with fewe r than ten follicles per ovary was 16%, in contrast to 50% in summer. After aspiration of follicles, 7.5 oocytes per ovary were found in winter and 5. 0 oocytes per ovary in summer. Cleavage to the two- to four-cell stage afte r chemical activation was greater in winter than in summer; this was enhanc ed at the morula stage and embryo development to the blastocyst stage was s ignificantly higher in winter than in summer. Determination of the lipid ph ase transition in oocyte membranes revealed a shift of 6 degreesC between s ummer and winter. Fatty acid composition of phospholipids from follicular f luid, granulosa cells and oocytes indicated that there was a higher percent age of saturated fatty acids during the summer and that the percentages of mono-unsaturated and polyunsaturated fatty acids were higher in oocytes and granulosa cells during the winter. Oocytes and granulosa cells had similar fatty acid compositions, in contrast to follicular fluid. These results ma y explain the differences in the ability of oocytes to develop to the blast ocyst stage at different seasons. Thus, temperature changes may lead to cha nges in membrane properties, which, in turn, can influence oocyte function and fertility.