Role of progesterone in maintaining pregnancy is well known in the bovine. Subtle differences exist between dairy (milked) and beef (suckled) cows because of differing concentrations of progesterone during recrudescence of postpartum estrous cycles, rate of follicular growth and maturation, proportions of 2- and 3-follicular wave cycles, and other effects on pregnancy outcomes per AI (P/AI). Because proportions of anovulatory cows at the onset of the AI period are greater and more variable in beef (usually ranging from 30 to 70%) than dairy (25%) cows, AI programs were developed to accommodate anovulatory and cycling beef cows enrolled therein. Incorporating a progestin as part of an AI program in beef cows improved P/AI by reducing the proportion of cows having premature luteal regression and short post-AI luteal phases. Two- vs. 3-follicle wave cycles are more common in beef cows (86 vs. 14%) than in dairy cows (68 vs. 32%), respectively, even though P/AI may not differ in 2- vs. 3-wave beef cows (82 vs. 100%), but differed in dairy cows (63 vs. 81%). When dominant follicles matured in subluteal-phase progesterone concentrations compared with those matured in luteal-phase concentrations, P/AI increased in beef cows, but were reduced in dairy cows when the first dominant follicle (matured in sub-luteal milieu) was induced to ovulate compared with cows ovulating a second-wave dominant follicle. Further, supplementing progesterone during growth of the first-wave dominant follicle improved fertility in dairy cows. Initiating timed AI programs in dairy cows in a greater progesterone environment and during cycle d 5 through 12 enhanced fertility. In contrast, progesterone status in beef cows at the onset of synchronization does not seem to be related to P/AI in multiparous cows, whereas P/AI were suppressed in primiparous cows that began a timed AI program in a low-progesterone environment. Pregnancy losses after AI between 35 and 60 to 70 d are less than 5% in beef cows and are not associated with pre-AI progesterone or cycling status, whereas losses in dairy cows (6 to 20%) are inversely related to progesterone and adversely affected in anovular dairy cows.