A 5-month study was conducted to evaluate the relationship between soil available phosphorus (P) and forage P concentrations in May, July, and September 2011. Four cool-season pastures were sampled at the Iowa State University McNay Research Farm near Chariton, Iowa. Forage samples were hand-clipped to ground level from a 0.25-m² square and soil samples were collected to a depth of 5 inches within (400 ft or less) and outside (700 ft) of previously established congregation areas along one transect within each pasture. Forage P concentration increased with increasing soil available P content and decreased late in the grazing season. On average, the P concentrations of the forages exceeded the P requirement of lactating beef cows throughout the grazing season. A model of monthly forage P concentration predicted that the probability that P supplementation of cows would be necessary wouldn’t increase until October.