Nitrogen rate studies have been conducted at Marianna, Arkansas for the last 17 years. Rates utilized in the experiment were of 0 to 150 lbs. per acre N in 25 lb. increments. General production practices have been those recommended for cotton production. Radiation data prior to 1988 was estimated utilizing published procedures.

The actual N response curve was generally one that gave higher yield increases per unit of fertilizers applied at low N rates. The GOSSYM/COMAX program resulted in an almost straight line yield increase with increasing N fertilizers or in a yield reduction at N rates greater than 25 lbs. per acre. Deviations in GOSSYM/COMAX predicted yield and actual measured yield ranged from 90% below to 150% above.

The average percent deviation varied from about 60% below to 20% above actual yield at 0 to 150 lbs. N per acre, respectively. These data indicated that GOSSYM/COMAX consistently estimates yields low at low N rates. On any given year the range in deviation from actual yield was about 30 to 80 below at 0 N per acre to about 90 below to 150 above at 150 lbs N per acre.