210 FACTORS AFFECTING SUPEROVULATION RESPONSE IN CATTLE: A RETROSPECTIVE STUDY

Superovulation, embryo collection, and transfer allow for genetic improvement resulting in more rapid enhancement of production traits. Despite these advantages, variability in the response to superovulation has been observed between and within individual animals, becoming a costly inconvenience of these techniques. The purpose of this study was to evaluate the factors that influenced the superovulatory response and the resulting quantity and quality of the ova/embryos collected during embryo flushing in the study group, with cow as the experimental unit. A retrospective study was conducted, evaluating superovulation records from 794 embryo donors collected for the first time. Individual information was classified as breed (Angus or Holstein), parity (heifers or dry cow), body condition score (BCS; 5, 6, or 7 was considered optimal and BCS 7 was considered not optimal; in a 1 to 9 scale), hormonal dose used (375 or 500 IU of FSH), ovarian response (presence of more than one corpus luteum), collection (abnormal: blood or 80% fluid recovered) and season at collection (summer, fall, winter, spring) to determine their predictive value for the total number of ova/embryos collected: number of transferable embryos, degenerated embryos, and unfertilized oocytes (UFOs) according to the IETS standards for classification. Variables were evaluated and transformed when needed to establish normality. Binary logistic regression was used to identify predictors of ovarian response or nonresponse. A model was built for each of these dependent variables using a general linear model (GLM). Tukey's post hoc test was used to compare adjusted means between groups when there were significant factors identified by the GLM. The study found a significant difference (P = 0.024) in the percentage of cows responding to superovulation treatment in summer compared with spring (91.5% v. 98.4%). From the 755 donors that responded, the mean number (±s.e.m.) of ova/embryos collected from donors with optimal BCS was higher (P = 0.052) than those from donors with nonoptimal BCS (9.8 ± 0.37 v. 8.4 ± 0.42). The mean numbers of total ova/embryos obtained from normal collections was higher (P < 0.001) than the mean from those collections classified as abnormal (10.25 ± 0.30 v. 6.60 ± 0.62). The mean number of viable embryos from responding donors was 5.66 ± 0.18 per collection. Donors with BCS classified as optimal had significantly higher (P = 0.034) mean number (±s.e.m.) of viable embryos compared with donors with nonoptimal BCS (5.95 ± 0.23 v. 5.18 ± 0.29). For the present study, the percentage of cows treated that responded and were collected decreased in summer. The effect of ambient temperature may be reduced by evaporative cooling of donors or by avoiding collections during the hottest days of the summer. Complications during the collection are rare and can be partially avoided by using experienced technicians only. Results may improve when donors are in an appropriate nutrition status, either by supplementing those animals with low body scores or by restricting diets for those that are overweight, to improve the efficiency of the technique.