Effect Of Body Condition At Initiation Of Synchronization On Estrus Expression, Pregnancy Rates To AI And Breeding Season In Beef Cows

Abstract

The objective was to examine the influence of body condition score (BCS) at synchronization initiation on estrus and pregnancy rates. Data were retrospectively collected from beef cattle breedings that occurred between fall 2003 and spring 2008 on 12 beef farms. The cows (N=5510) included in the analysis were synchronized with progesterone based CO-Synch or Ovsynch protocols. The BCS (1- emaciated; 9-obese) of all cows were recorded on Day 0 of synchronization at the time of insertion of a controlled internal drug release device (CIDR; Eazi-Breed™ CIDR®, Pfizer Animal Health, New York, NY). At the time of CIDR removal, pressure sensitive mount detectors (Kamar Heatmount® detector, Kamar Products, Inc., Zionsville, IN) were placed on all cows to aid in identification of cows displaying estrus until artificial insemination (AI). Cows were inseminated at observed estrus or at a fixed time and bulls (approximately 1:40 bull:cow ratio) were introduced 14 days after AI and maintained for a 45 to 50 d breeding period to impregnate cows that failed to conceive to AI. Cows were examined for pregnancy at 55 to 70 days and again at 120 days after AI. The BCS ranged from 3 to 8. The expression of estrus, AI and breeding season pregnancy rates were influenced by BCS (P<0.05). The estrus expression rates were 41.8%, 40.5%, 50.5%, 53.0%, 56.4% and 40.4% for BCS 3 to 8, respectively. The fixed-time AI pregnancy rates were 36.7%, 47.4%, 51.8%, 52.9%, 50.9% and 44.9% for BCS from 3 to 8, respectively. Breeding season pregnancy rates were 74.7%, 78.2%, 86.4%, 90.2%, 89.9%, and 87.9% for BCS from 3 to 8, respectively. In conclusion, a minimum BCS of 5 should be achieved prior to the breeding season to ensure acceptable reproductive performance in beef cows managed on forage.