Performance testing Simmental heifers : the effects on puberty and superovulatory response
Genetic change in Simmental cattle is being accelerated using multiple ovulation and embryo transfer (MOET) techniques. Success of the project depends largely on the ability to generate grade 1 embryos from juvenile heifers following a performance test. Performance testing imposes specific nutritional, management and environmental conditions on heifers which may influence the onset of puberty and subsequent response to superovulation. The objectives of this study were to determine when puberty occurred in Simmental heifers on performance test and to investigate the effects of pubertal development and performance during test on superovulatory response. Heifers were performance tested between 23 and 49 weeks of age. Performance test measurements included: food intake, energy intake, liveweight, backfat depth, muscle depth and muscling score. The onset of puberty was determined in one generation of heifers (n=30) by detecting elevated plasma progesterone levels indicative of first ovulation. Following performance test heifers were superovulated, using ovine FSH, artificially inseminated and embryo recovery carried out at 52 and 61 weeks of age. Puberty occurred in 87% of heifers before the end of test. There was considerable variation in age, liveweight and withers height at puberty, however, daily liveweights gain appeared to be an important determinant of age at puberty. The yield of grade 1 embryos at the first embryo recovery was highest in prepubertal heifers, however, these heifers experienced a substantial decrease in yield at the second recovery apparently due to reduced recovery rates. Investigation of relationships between performance on test and superovulatory response in three generations of heifers (n=110) revealed no significant effect of performance during test on the yield of grade 1 embryos. It was concluded that several factors contributed to reduced embryo yields including: ineffective control of the corpus luteum in cyclic heifers; the incidence of short luteal phases in pre- and peri-pubertal heifers; luteinization of potential ovulatory follicles; and inappropriate timing of exogenous synchrony and superovulation treatments in relation to waves of follicular development.