Effects of postpartum energy intake on pregnancy rates in beef cattle subjected to GnRH- or CIDR-based timed artificial insemination protocols

Wichtel. J. J., Charinley. E., Richardson. G. F. Duynisveld. J. L. and Lofstedt. R, 2008. Effects of postpartum energy, intake on pregnancy rates in beef cattle subjected to GnRH- or CIDR-based timed artificial insemination protocols. Can. J. Anim. Sci. 88: 439-447. The objectives were to determine the effects of three levels of postpartum metabolisable energy (ME) intake on pregnancy rates in beef cattle subjected to either GnRH-based (OVS) or progestin-based (CIDR) protocols for fixed time artifical insemination (TAI). Hereford cross cows were assigned to ME and TAI treatments (within ME) on the basis of parity and predicted calving date. The postpartum grass-silage-based diet was formulated to provide either Low (93 MJ d(-1)). Medium (103 MJ d(-1)) or High (120 MJ d(1))] ME from calving (January to February) to turnout (May 25). Lactating cows [n = 175, 5.7 +/- 1.1 mean (+/- SD) body condition score at calving] were subjected to their assigned TAIprotocol: OVS [i.m. treatments of GnRH (100 mu g) on day 0, PGF(2 alpha) (25 mg) on day 7, a second GnRH on day 9 and TAI 16 to 18 h later]. or CIDR [i.m. treatment with 1 mg estratiol benzoate and 100 mg progesterone concurrent with CIDR (1.9g progesterone) insertion on day 0, PGF(2 alpha) treatment at CIDR removal on day 7, a second estradiol treatment on day 8 and TAI 28 to 30 h later). Cows were 60 +/- 13 d post-partum at the time of insemination. Lower ME intakes reduced (P < 0.05) maternal body weight and calf weight gain, but ME intake did not affect (P > 0.05) the proportion cycling (113 175 = 65%. based on serum progesterone concentrations on days -5 and -14), ovulation following TAI, or the TAI pregnancy rates (based on ultrasonography). Timed insemination pregnancy rates were greater for CIDR- than OVS-treatment (63 vs. 45%. respectively, P<0.05), regardless of ME intake.