Nutrition-induced advanced puberty in dairy heifers does not affect ovarian response or oocyte in vitro competence...

While inducing early puberty in dairy heifers may seem a straightforward strategy to reduce rearing costs, it also raises concerns about potential adverse effects on reproductive function. The scientific question addressed in this study is whether advancing puberty may alter ovarian responsiveness to stimulation or compromise oocyte quality shortly after puberty. This study aimed to evaluate the effects of an early-life optimised nutritional plan designed to advance puberty in Holstein heifers on these parameters. From birth, Holstein calves were assigned to either a control diet or an optimised diet that increased average daily gain by 232 g/day and advanced puberty by 2.6 months (11.0 ± 0.21 vs 8.4 ± 0.14 months, mean ± SEM in control vs optimised nutritional groups, respectively, here and throughout the abstract). Circulating levels of anti-Mullerian hormone were monitored from birth to 66 weeks of age. Within weeks after reaching puberty, the heifers were synchronised and underwent repeated ovarian stimulation and ovum pick-up starting at 12.6 ± 0.17 vs 10.2 ± 0.19 months. Follicle number and size, cumulus-oocyte complex morphology, and oocyte diameter were assessed. Oocytes were matured and fertilised in vitro. Expression of the mitochondrial genome was analysed in oocytes and cumulus cells. The nutritional plan did not significantly affect anti-Mullerian hormone levels, the number of visible follicles (23.9 ± 3.38 vs 22.0 ± 2.09) or their size, the number of retrieved cumulus-oocyte complexes (10.8 ± 1.48 vs 9.5 ± 1.00) or their morphological grade, or the oocyte diameter (121 ± 2.6 µm vs 124 ± 1.3 µm). Following in vitro maturation and fertilisation (day 1), the rate of embryo cleavage on day 2 (76 ± 3.6 vs 81 ± 2.9%) and the rate of blastocyst formation on day 7 (43 ± 5.6 vs 39 ± 5.2%) did not differ between the two nutritional groups. However, reduced transcript levels were observed in oocytes from animals in the optimised nutritional group for seven of the 11 investigated genes (ND1, ND2, ND3, ND4L, ND6, ATP6, ATP8), but not in cumulus cells. Overall, the optimised nutritional plan and the younger age of the corresponding heifers did not seem to impair the ovarian response, the number, morphological quality or in vitro developmental competence of their oocytes. However, reduced mitochondrial transcript levels in oocytes suggest cell-type-specific regulatory responses to nutritional cues, with potential implications for mitochondrial function.