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The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH, LH, or estradiol benzoate for fixed-time AI in beef heifers¹

M. F. Martinez^*, J. P. Kastelic, G. P. Adams^* and R. J. Mapletoft^*,2

^* Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4 and and Research Centre, Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada T1J 4B1

² Correspondence:
Dept. of Large Anim. Clinical Sci., 52 Campus Dr. (phone: 306-996-7149; fax: 306-966-7159; E-mail:
reuben.mapletoft{at}usask.ca).

The objective of this experiment was to compare two progestins and three treatments for synchronizing follicular wave emergence and ovulation in protocols for fixed-time AI in beef heifers. On d 0 (beginning of the experiment), Angus and Angus-Simmental cross beef heifers at random stages of the estrous cycle either received a CIDR-B device (n = 257) or were started on 0.5 mg•animal^-1•d^-1 melengestrol acetate (MGA; n = 246) and were randomly assigned to receive i.m. injections of 100 µg GnRH, 12.5 mg porcine LH (pLH), or 2 mg estradiol benzoate (EB) and 50 mg progesterone (P₄). The last feeding of MGA was given on d 6 and on d 7, CIDR-B devices were removed and all heifers received 500 µg cloprostenol (PG). Consistent with their treatment groups on d 0, heifers were given either 100 µg GnRH or 12.5 mg pLH 48 h after PG (and were concurrently inseminated) or 1 mg EB 24 h after PG and were inseminated 28 h later (52 h after PGF). Estrus rate (combined for both progestins) in heifers receiving EB (92.0%) was greater (P < 0.05) than that in heifers receiving GnRH and pLH (combined) and a CIDR-B device (62.9%) or MGA (34.3%). Although the mean interval from PG treatment to estrus did not differ among groups (overall, 47.8 h; P = 0.85), it was less variable (P < 0.01) in MGA-fed heifers (SD = 2.5 h) than in CIDR-B-treated heifers (SD = 8.1 h). Pregnancy rates (determined by ultrasonography approximately 30 d after AI) did not differ (P = 0.30) among the six treatment groups (average, 58.0%; range, 52.5 to 65.0%). Although fixed-time AI was done, pregnancy rates were greater in heifers detected in estrus than in those not detected in estrus (62.6 vs 51.9%; P < 0.05). In conclusion, GnRH, pLH, or EB treatment in combination with a CIDR-B device or MGA effectively synchronized ovulation for fixed-time AI, resulting in acceptable pregnancy rates in beef heifers.

Key Words: Estradiol • Fixed-Time AI • GnRH • LH • Progestins • Synchronization

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