Mechanisms behind the alteration of lamb sex ratios when ewes are fed a diet high in omega-6 fatty acids prior to joining

Manipulation of progeny sex ratios to suit livestock production systems has the potential to greatly increase efficiency and profitability. Researchers have linked maternal nutrition to changes in offspring sex ratios; however, variability exists. This has led to investigations into specific dietary components, with increased dietary omega-6 (n-6) fatty acids (FA) identified as increasing the proportion of female offspring. The aim of this thesis was to investigate potential mechanisms for dietary-induced changes in sex ratios as a basis for future research. High dietary n-6 is associated with increased prostaglandin-F2α (PGF2α) production. Additionally, several studies linking high dietary n-6 with a higher proportion of female lambs have also observed early onset of oestrus. Combining this evidence, the preliminary theory tested in this thesis was that increased PGF2α, resulting from high dietary n-6 causes early corpus luteum (CL) luteolysis. This has a two-fold effect, early onset of oestrus and lengthened oestrus to ovulation interval, thus increased time between insemination and fertilisation. A longer period between mating/insemination and fertilisation has previously been associated with a higher proportion of female offspring; with evidence of slower female spermatozoa passage through the cervix and physiological mechanisms within the dam for differentiation between X- and Y-bearing spermatozoa. Two animal house experiments were conducted, with ewes fed experimental rations for approximately 40 d before oestrus detection. The aim of the first experiment was to identify if a shorter time of oestrus, associated with high dietary n-6, caused a longer interval between oestrus and ovulation via early CL luteolysis. An oat based diet, high in n-6 and low in omega-3 (n-3), was used as the experimental ration, while a silage diet, high in n-3 (low n-6), was used as a control, simulating a grazing diet. In the second experiment, three diets were employed, which consisted of a low-fat ration with added sunflower oil (high n-6 diet), linseed oil (high n-3 diet; control diet) or palm oil (high saturated FA (SFA) diet; neutral diet). The aim of using such diets was to test the hypothesis that changes in sex ratios were due to changes in dietary n-6 and no other dietary parameters. In both studies, high dietary n-6 increased plasma n-6 proportion (P < 0.05). No differences (P > 0.05) in timing of oestrus to ovulation interval were observed in either study. Without further evidence, these results do not provide evidence for further investigation into this proposed mechanism for change in offspring sex ratios due to changes to maternal dietary n-6. Identifying and refining a mechanism for manipulation of offspring sex ratios has the potential to allow producers to take advantage of sex-related or sex-specific traits to suit a target market and increase efficiency.