Component analysis of alfalfa (Medicago sativa L.) forage yield can reveal how plant density, shoot density, and shoot mass interact resulting in variation in herbage production. Critical shoot density values of 440 to 590 shoots/m² (40 to 55 shoots/ft²) have been proposed recently. Our objective was to determine the relationship between shoot density and forage yield of alfalfa as influenced by P and K fertilizer application between 1998 and 2004. While mass per shoot was consistently and positively associated with herbage yield at each harvest of every year, the relationship of shoot density to forage yield varied both with year and harvest within year. From 1998 to 2001, there was a generally no relationship or a negative linear relationship between forage yield and shoot density over a range 350 to 800 shoots/m². Beginning with Harvest 2 of 2002 a positive linear relationship between shoot density and forage yield was oftern observed. Poor fertility management reduced shoot numbers and with it, forage yield.