Mounting evidence suggests that the optimal rate of nitrogen (N) application for an individual cornfield varies greatly with percentage of the N lost between application and early June. This evidence prompts questions about optimal rates of application when the fertilizer is applied in June.

Precision farming technologies are being used to learn more about optimal rates of fertilization when a 28 percent N solution is knifed into soil midway between rows in June. Table 1 lists the yields obtained in 22 trials having N applied at rates of 50, 100, and 150 lb/acre.

The studies involved application of N at constant rates in 6- or 8-row strips usually more than 2,000 ft in length. The treatments were replicated at least four times and harvested with yield-monitoring combines. All of the fields were planted to corn following soybean.

The studies were conducted in fields having average to above-average variability in soil type. The strips were positioned to capture as much variability in soil type as possible. The strips often included knolls and poorly drained areas with relatively low yield potential.

A major objective was to assess the potential benefits of variable-rate fertilization, but only whole-strip averages are presented in this article. Whole-strip averages indicate optimal rates when variable-rate applicators are not used.

The results can be interpreted by considering the amounts of grain needed to purchase the fertilizer. It often takes about 4 bu of corn (priced before drying and storage) to purchase 50 lb of fertilizer N. The following interpretations are based on these costs.

If all fields received the same rate of N, then it would have been profitable to increase rates from 50 to 100 lb/acre. Increasing rates from 100 to 150 lb/acre, however, would have resulted in economic losses.

If each field received the best rate, eight fields would have received 50 lb, nine fields would have received 100 lb, and five fields would have received 150 lb of N/acre. The average rate across all fields would have been 93 lb of N/acre.

Most corn producers have learned that fertilizer N usually must be applied at rates greater than 100 lb of N/acre to avoid clear visual symptoms of N deficiency when N is applied in the fall or early spring before planting. Extra N seems to be needed to compensate for fertilizer N that escapes from the field before it can be used by the crop.

Delaying all but starter applications until June offers clear economic and environmental benefits. These benefits could be important as profit margins narrow and environmental concerns increase.

Table 1. Results from 22 corn-after-soybean strip trials.

	Yields obtained with various rates of N applied in June
Year-site	50 lb/acre	100 lb/acre	150 lb/acre
	bu/acre
1995-2	135	151	154
1995-3	101	124	114
1995-4	141	140	135
1995-5	151	152	151
1995-6	141	150	156
1995-7	104	129	129
1995-8	91	120	138
1996-1	126	128	135
1996-2	153	157	158
1996-3	146	150	149
1996-4	134	150	151
1996-5	133	142	143
1996-6	150	161	163
1996-7	137	137	137
1997-1	121	161	177
1997-2	120	139	150
1997-3	128	153	155
1997-4	114	126	127
1997-5	118	134	129
1997-6	147	151	150
1997-7	160	165	171
Mean	131	143	146