Qualidade de oócitos de vacas leiteiras e produção de embriões a partir de oócitos maturados in vitro com melatonina

Abstract

The objective of this study was to evaluate the effect of the addition of melatonin to the in vitro maturation (IVM) medium of bovine oocytes aspirated from dairy cows on cleavage rate (CR), embryo production and quality. Additionally, we aimed to establish an association between oocyte quality and genetic groups, considering the physiological and environmental variables recorded throughout the trial. Oocytes were obtained in 52 ovum pick-up (OPU) sections performed during summer and spring in three farms. In each OPU section, environmental variables (temperature and relative humidity) and physiological parameters of cows (respiratory frequency, rectal and superficial temperature, milk production and days in milk – DIM) were obtained. Overall, 29 OPU sections were performed in Holstein cows (H; n = 15, farm 1) and 23 aspirations were performed in crossbred Holstein x Zebu - HZ (n = 16, farm 2; n = 7, farm 3). Oocytes obtained were classified according to its quality in excellent, good, regular, denuded and degenerate. Oocytes obtained during summer, except degenerate, were distributed to one of three treatments: 0 mol/L (control), 10-6 mol/L or 10-4 mol/L of melatonin in IVM. CR was evaluated on day 3 (D3) after in vitro fertilization (IVF), and embryo production on day 8 (D8). Quality of blastocysts was evaluated through count of embryo cell number and apoptotic cell number. The temperature-humidity index (THI) was 79.0 ± 1.4 on farm 1; 77.7 ± 3.2 on farm 2; and 68.5 ± 1.8 on farm 3. Physiological variables did no differ (P > 0.05) between genetic groups (respiratory frequency – movements/minute: H = 74.2 ± 10.5; HZ = 53.4 ± 6.9; rectal temperature -°C: H = 38.3 ± 0.3; HZ = 38.3 ± 0.2; superficial temperature -°C: H = 36.6 ± 1.4; HZ = 36.9 ± 0.9). Genetic group influenced oocyte quality (P < 0.05). The proportions of grade 1 (H = 3.2 ± 1.2; HZ = 27.6 ± 4.6; P < 0.0001) and grade 2 (H = 1.6 ± 0.6; HZ = 12.9 ± 2.2; P < 0.0001) oocytes were lower and the proportion of degenerate (H = 5.2 ± 1.5; HZ = 1.7 ± 0.6; P = 0.05) was greater in Holstein cows. The covariate respiratory frequency was significative (P < 0.05) and negatively associated to grade 1 oocytes and positively associated to grade 2 oocytes. DIM was significative (P < 0.05) and positively associated to grade 1 oocytes and negatively associated to degenerate oocytes. The proportion of viable oocytes (grade 1 and 2) was lower (P < 0.0001) in Holstein than in HZ-crossbred. Relative humidity and respiratory frequency were significative (P < 0.01) and negatively associated to good oocytes. An interaction (P = 0.03) between the effects of farm and treatment regarding CR was observed. Only on farm 1, the concentration 10-4 mol/L increased (P <0.09) the CR (70.0 ± 17.0%) compared to control (22.0 ± 15.6%), and only on farm 2, CR was increased (P < 0.09) by melatonin in the concentration 10-6 mol/L (50.0 ± 20.3%) compared to control (20.0 ± 13.5%). In conclusion, physiological parameters did not differ between genetic groups, but HZ crossbreds’ cows produced greater quality oocytes in comparison to Holstein. The addition of melatonin to the IVM medium may be a strategy to increase CR of embryos produced from lactating dairy cows during summer. However, the dose to be added differs according to genetic group, and it is greater from Holstein than crossbred HZ cows.