This study was conducted to evaluate changes in milk profiles of oxidative stress (OS) biomarkers in dairy cows with ovulatory and an-ovulatory oestrous cycles. Thirty healthy, cycling Holstein cows averaging 60±17 days in milk, and producing 33±6kg of milk per day (the week before commencing the study) were enrolled in this study. Composite milk samples were collected thrice weekly and assayed for the following OS biomarkers: lipoperoxides (LPO), biological advanced potential, superoxide dismutase (SOD), advanced oxidation protein products (AOPP), ceruloplasmin, glutathione (GSH), β-carotene and glutathione peroxidase (GSH-Px). Milk samples were also tested for fat and protein composition and the fat:protein ratio (FPR) was categorized as low (≤1.31), medium (1.32-1.56) and high (>1.57) to evaluate their main effect and the interaction effect of FPR and the week of study on OS using linear mixed models with cow identification being a random factor. Cows with ovulatory oestrous cycles (n=20) presented significantly greater SOD levels than cows that did not ovulate ((n=10; P<0.05). On the other hand, LPO, GSH-Px and GSH concentrations were lower in ovulated cows compared to the an-ovulated cows (P<0.05). The highest level of LPO and AOPP were noted at prooestrus phase while β-carotene presented the lowest value at that phase of oestrous cycle. It could be postulated that the elevated level of milk SOD and the observed lower level of LPO, GSH-Px and GSH in ovulating cows may be an essential event preceding the ovulatory response.
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