A nitrogen fertilizer study was conducted in order to develop tissue testing guidelines for fertilizer application to forage sorghum. The study was conducted at the University of Arizona Maricopa Agricultural center on a sandy clay loam soil irrigated using surface flood methods. Forage sorghum was planted on 8 July 09 and fertilized with eight N rates varying from 0 to 350 lbs N/acre in 50 lb N/acre increments. The plants were sampled six times during the growing season and the lower stem, most recently developed leaf, and whole plant were analyzed for nitrogen content. Maximum yield at final harvest was obtained at 150 lbs N/acre and plant growth was highly affected by N rate. Before the initiation of rapid growth, the relationship between plant growth and N content in the various tissues was weak (R2 < 0.20), but was very strong (R2>0.50) from the initiation of rapid growth through the pre-boot stage at the time when post-plant nitrogen fertilizer application may be considered. Stem nitrate was most strongly related to yield for the tissues tested, but the relationships between plant growth and total N in the newest leaf and whole plant were also very strong. Preliminary tissue testing guidelines are suggested for nitrate in the stem tissue. The lower stem, newest leaf, and whole plant are all potential candidates for development of tissue testing guidelines for forage sorghum.