Estrual rise in body temperature in the bovine II. The temporal relationship with ovulation

Abstract

Increases in internal body temperature have been observed near estrus in cattle. In our companion paper, an estrual rise in vaginal temperature occurred within a consistent interval from the LH surge. However, the relationship of this rise in temperature to ovulation was not determined. The objectives of this study were to: (1) characterize the temporal relationship between the estrual increase in body temperature and ovulation in the heifer, and (2) assess the consistency of this temporal relationship among individuals by comparing it to the consistency of intervals between other periovulatory events. Body temperature was monitored automatically and recorded every 15 min with intravaginal radiotransmitters in post-puberal heifers. Temperatures were averaged hourly. To monitor the demise of the corpus luteum and anticipate estrus, progesterone (P) concentrations were monitored. Blood samples were collected daily, beginning on day 16, until serum P fell to ≤ 0.5 ng/ml. Samples were then collected every 4 h until ovulation. Serum concentrations of estradiol-17β (E) and luteinizing hormone (LH) were measured in these samples. Serum concentrations of P, E and LH were determined by radioimmunoassay. Hourly temperature averages were compared to the previous 3 days' hourly temperature averages for each hour of the day. A significant ( P < 0.05) rise of ≥ 0.3°C that continued for 3 consecutive hours was considered the onset of a temperature spike (TS). To identify the time of ovulation (TO), laparoscopy was performed 2 h before the anticipated ovulation and every 4 h afterward until ovulation had occurred. The variances among heifers (as indicated by the s.d.) of the following intervals between periovulatory events considered in this study did not differ ( P > 0.10, F-test): E peak to onset of a TS (1.00 ± 5.90 h, mean ± s.d., n = 6), LH peak to onset of a TS (0.29 ± 5.22 h, n = 7), E peak to TO (22.30 ± 3.92 h, n = 6), LH peak to TO (21.43 ± 4.31 h, n = 7), and onset of a TS to TO (21.14 ± 6.07 h, n = 7). In conclusion, intervals between periovulatory events had similar variances among heifers. Since the interval between the LH peak to ovulation is considered a repeatable periovulatory relationship, all periovulatory intervals in the current study can be considered repeatable. As such, relative to other periovulatory intervals, the interval between the onset of a TS to TO is consistent. Therefore, the onset of a TS, as monitored by radiotelemetry, may be as reliable a predictor of ovulation as the LH surge or other periovulatory events.