Reproductively normal crossbred beef heifers were individually offered a diet of barley straw and concentrate supplemented
with one of four levels of a fish oil (FO) enriched supplement. Following oestrous cycle synchronisation, blood samples were
collected at appropriate intervals for the measurement of progesterone (P4), oestradiol (E2), fatty acids, insulin-like growth factor 1
(IGF-1) and metabolites. On days 15 and 16 of the cycle, oxytocin was administered intravenously and the prostaglandin F2a
(PGF2a) response was measured as venous concentrations of 13,14-dihydro-15-keto PGF2a (PGFM). The heifers were slaughtered
on days 17 or 18 of the oestrous cycle and endometrial tissue, rumen fluid and follicular fluid were collected for determination of
fatty acid concentrations. In general there was no effect (P > 0.05) of diet on plasma P4 or E2 concentrations. Increasing FO
supplementation increased CL diameter on day 7 post-oestrus (P < 0.0001) but had no effect on diameter on day of slaughter
(P > 0.05). On day 15, PGFM concentration was greater on the highest level of FO supplementation compared to controls
(P < 0.05), however, there were no differences between other diet comparisons (P > 0.05). There was no effect of diet on PGFM
concentration on day 16 (P > 0.05). There was a strong positive relationship between plasma and uterine endometrial
concentrations of both EPA (R2 = 0.86; P < 0.0001) and total n-3 PUFA (R2 = 0.77; P < 0.0001). IGF-1 concentrations increased
on all diets and were greatest at the highest level of n-3 PUFA supplementation (P < 0.05).
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