Objective: To evaluate the influence of a simultaneous injection of GnRH- and PGF2α-analogues at different time points of a cycle on cycle length and on the concentrations of progesterone and oestradiol-17β. Additionally, the length of the following cycles was investigated.
Material and methods: In six cows the cycle length was determined in three cycles that were influenced by the simultaneous injection of GnRH and PGF2α on days 7, 14 or 17 of the cycle (treatment cycles 1, 2, and 3) as well as in three uninfluenced control cycles. A daily sonographical examination was conducted to confirm ovulation from day 18 onwards in the control cycles and from the point of hormonal treatment onwards in the treatment cycles. The concentration of oestradiol-17β and progesterone were determined on the first day of every cycle, on the day of hormonal treatment and on the following 5 days to validate sonographical results.
Results: Treatment cycle 1 with the hormone application on day 7 of the cycle was 12.2 ± 4.6 days long, treatment cycle 2 with the hormone application on day 14 lasted 15.5 ± 1.2 days and treatment cycle 3 with the hormone application on day 17 was 21.0 ± 3.3 days long. Control cycle 1 lasted 21.8 ± 1.9 days, control cycle 2 was 21.0 ± 3.2 days long and control cycle 3 lasted 18.5 ± 4.6 days. Only one of a total of 18 control cycles was shortened (10 days). The length of all three control cycles was statistically significantly increased compared with treatment cycle 1. Control cycles 1 and 2 were statistically significantly longer than treatment cycle 2. Hormone injection induced a highly significant decrease in the progesterone concentration in all treatment cycles. The oestradiol-17β concentration rose in parallel with the growth of a follicle after induced luteolysis, but remained uninfluenced by the treatment itself.
Conclusion and clinical relevance: A simultaneous injection of an analogue of GnRH and PGF2α analogues had no effect on PGF2α-induced luteolysis or oestradiol-17β concentration. An increased rate of shortened oestrous cycles was not observed.