Management of fertilizer nitrogen (N) is one of the most important components in producing maximum yield and profits in corn and cotton. Corn is inherently inefficient in fertilizer N uptake, typically using less than half of that applied. Cotton, on the other hand, has extremely high fertilizer uptake efficiency. When the two crops are grown in rotation, fertilizer not used by one crop may affect the optimal fertilizer rate for the next. The weather is probably the one factor that most affects yield potential and plant N use. During growing seasons with adequate rainfall and moderate temperatures, N uptake increases and maximum yields are produced. But, when growing conditions are less than ideal, yields are lower and N not removed by the crop is left in the field.

A cotton-corn rotation study was initiated in 1996 on Commerce silt loam at the Northeast Research Station near St. Joseph and on irrigated Gigger silt loam at the Macon Ridge Research Station near Winnsboro to evaluate fertilizer N rates and N carried over from year to year. Two blocks, corn and cotton, were planted adjacent to each other each year, and crops were switched in alternating years to maintain a cotton/corn rotation. Nitrogen rates were 0, 150, 200 and 250 pounds N per acre for corn and 0, 25, 50, 75, 100 and 125 pounds N per acre for cotton. Nitrogen fertilizer, ammonium nitrate, was broadcast each year. In the corn test, four corn N rates were superimposed over the previous year’s six cotton N rates. Similarly, each cotton N rate was superimposed over the previous year’s four corn N rates. Effects of carry-over N were determined from response of the current year’s crop.

Optimal N rate for corn yield was 150 pounds per acre in four of five years at St. Joseph and all five years at Winnsboro. Averaged across five years, there was little difference in yields among fertilizer N rates of 150 to 250 pounds per acre (Figures 1 and 2). Responses to N rate in this study are consistent with previous research and with current recommendations. With application of optimal fertilizer N, average yields for the five-year test were 151 bushels per acre at St. Joseph and 146 bushels per acre at Winnsboro.

Carry-over N from cotton had little effect on corn yield (Figures 1 and 2) and, in fact, increased yield of the following corn crop only when no fertilizer N was applied to the corn. When no fertilizer N was applied to corn, yield increases from residual cotton N were about 16 bushels per acre at both St. Joseph and Winnsboro. When fertilizer N was applied, there were no yield responses to residual cotton fertilizer N, even at St. Joseph in 1998, the only time when attainment of maximum yield production required more than 150 pounds per acre of N. These results illustrate that corn would respond to residual N when fertilizer N was insufficient for plant needs; however, in this study, the lowest fertilizer N rate of 150 pounds per acre was usually adequate for maximum yield.

The number of N rates (main plots) for corn had to be limited in the experiment to four because of land availability. Previous research had established that the optimal N rate on silt loam soil was 150 pounds per acre for corn and 75 pounds per acre for cotton. Corn is not as sensitive as cotton to overfertilization, and corn producers usually apply more than 150 pounds N per acre to ensure maximum yields. Thus, in this study, we chose to evaluate N rates for corn of 150 pounds per acre and higher that are commonly used rather than lower rates that are unlikely to be used.

This study substantiates that the optimal N rate for corn production on both Commerce silt loam and Gigger silt loam is about 150 pounds per acre. Results also demonstrate that cotton N rates of 75 to 100 pounds per acre are not likely to affect the quantity of fertilizer N required by the following corn crop. The average corn yield in these tests was 150 bushels per acre. Cotton lint yields were excellent during this study, ranging from 1,100 to 1,400 pounds per acre at St. Joseph and 1,100 to 1,300 pounds per acre at Winnsboro. Fields with yield potential exceeding those in this study may require higher N rates for maximum yield. Residual N effects on corn may be greater when cotton yields are reduced by poor growing conditions, which may result in higher residual N levels.

Acknowledgment
The authors thank Cotton Incorporated and the Louisiana Cotton Support Committee for funding for this research.

(This article appeared in the summer 2000 issue of Louisiana Agriculture.)