The objective of this collaborative study between Colorado State Univ. and Intervet, Inc. was to determine whether the administration of human chorionic gonadotropin (hGC) at 5 days post-AI improves fertility in heifers by increasing blood serum progesterone concentration. A total of 96 crossbred heifers were allotted to one of two treatments: 1) Controls (no hCG); or 2) hCG (3300 IU injected intramuscular at 5-days post-AI). Serum progesterone concentrations were similar on days 4, 5, and 6 post-AI for Controls and hCG heifers, but were greater for hCG heifers on days 7 and 8. Nevertheless, AI pregnancy rates did not improve for hCG heifers vs. Controls. This led the authors to conclude that elevating serum progesterone concentration does not improve fertility in beef heifers (Walker et al. 2005. Prof. Anim. Sci. 21:361).