Treating cattle with progesterone as well as a GnRH analogue affects oestrous cycle length and fertility

Initiating the chronic administration of progesterone to cattle during meto estrus will produce shortened oestrous cycles containing one or two wave-li ke sequences of ovarian follicle development. Conception rates are reduced to inseminations at the oestrus preceding these shortened cycles. In contra st, a single injection of the GnRH analogue, buserelin, around mid-dioestru s can lengthen the oestrous cycle by increasing the proportion of cycles wi th three waves of follicular development and may also increase conception r ates. A series of trials was conducted to test the hypothesis that the adve rse effects of progesterone on oestrous cycle length and conception rate co uld be: prevented with a strategic injection of GnRH, In Trial 1, progester one was administered per vaginum to heifers for 10 days from Day 2 or 3 (Oe strus = Day 0) and with (n = 42) or without (n = 46) an injection of a GnRH analogue (10 mu g buserelin) on Day 12 or 13, Other heifers (n = 44) serve d as an untreated control group. The average inter-oestrous interval (TOP) for those heifers treated only with progesterone was 17.0 days and was less (p < 0.05) than the average intervals for those also receiving GnRH (20.2 days) or in the control group (20.0 days). In Trial 2, 45 heifers were inse minated following a synchronised oestrus. Progesterone was administered as in Trial 1 to 22 of the heifers, Their conception rate was 45.4% and this w as less (p < 0.05) than the 73.9% obtained with their 23 untreated contempo raries. Trial 3 was completed using 530 cows in commercial dairyherds. The 259 cows receiving progesterone and GnRH (buserelin) after their first inse minations had a conception rate of 68.3% compared to 56.1% for their 271 un treated herdmates (p < 0.05%). Heifer calves born to treated cows had heavi er birthweights (33.4 vs. 31.1 kg; p < 0.05), but birthweights of bull calv es were unaffected (35.5 vs. 35.8 kg). Gestation lengths for cows conceivin g to first inseminations were similar for treated and control groups (280.9 vs. 280.5 days). The results of these trials confirmed the hypothesis that a strategic injection of the GnRH analogue, buserelin, could prevent the r eductions in oestrous cycle length and conception rate associated with the chronic metoestrous administration of progesterone. (C) 1999 Elsevier Scien ce B.V. All rights reserved.