The objective of this study was to determine whether a second injection of gonadotropin-releasing hormone (GnRH) at 54-h timed AI (TAI) of a CO-Synch/ CIDR (controlled internal device release) protocol improves fertility in replacement beef heifers. Heifers (n = 375) at three locations (Colorado, Wyoming, and South Dakota) were stratified by BW within body condition score (BCS) and randomly allotted to one of two treatments. All heifers received 100 µg of GnRH and a CIDR insert (d–7), followed by CIDR removal and 25 mg of prostaglandin F2α (PG; d 0). On d 2, control and treatment heifers underwent TAI at 54 h post-PG, and treatment heifers received a second 100-µg injection of GnRH. Blood samples were collected from all heifers at Colorado and Wyoming (d–17 and –7) to determine pubertal status. Ultrasonography was used to determine ovulation rate after TAI from a subsample of heifers (Colorado, n = 19; Wyoming, n = 49). No treatment x location interaction (P>0.10) occurred and pooled TAI pregnancy rates were similar (P>0.01) for control (46%) vs treatment (55%) heifers. Pubertal rates were greater (P<0.01) for heifers at Colorado (97.4%) than for heifers at Wyoming (46.4%); however, TAI pregnancy rates were similar (P> 0.10) for pubertal and prepubertal heifers. Ovulation rates tended to be different (P=0.10) for treatment (81.3%) than for control (62.5%) heifers. We conclude that the second injection of GnRH at TAI in the CO-Synch/CIDR protocol does not increase pregnancy rates to TAI in beef heifers, but that it may be economically viable and may guard against reduced fertility.

A self-determining control circuit is configured to assume one of several states depending upon the interconnections of its input and output terminals. The self-determining circuit is enabled to sense which, if any, of its input and output have been interconnected and assume its proper state of operation. For example, a self-determining integrated circuit may have four input terminals (e.g., terminals 1, 2, 3, 4 ) and four output terminals (e.g., terminals A, B, C, D). The circuit may sense that a particular input terminal is directly connected with a particular output terminal (e.g., input terminal 2 is shorted to output terminal D) and operate under one set of parameters. If, however, the control circuit senses that a different set of terminals are interconnected (e.g., input terminal 4 with output terminal B), then the control circuit may operation under a different set of parameters. Similarly, if the control circuit senses that none of the terminals are interconnected (or connected to a supply potential), then it may operate under yet a different set of parameters.

1 SUMMARY Results from a 5-year study to evaluate early (March) and late (May) Spring calving in Southwest Colorado indicate no affect on pre-weaning growth traits. Other factors such as age of dam, sex and age of calf had the usual impact on birth weight, weaning weight and weight per day of age, regardless of calving season. These results will allow us to more accurately evaluate reproductive and economic viability of the two calving seasons.