Water Watch Issue No. 77, December 1998Corn-following-soybean trials yield over 200 bushels per acre
by Eric Palas, NPM coordinator, NEIDP


Since 1995, the number of acres planted to soybeans in northeast Iowa has increased substantially. Many growers have added soy beans to their crop rotation in response to commodity prices, while others have attempted to break weed, insect and disease cycles associated with continuous corn production. An added benefit is the reduced amount of nitrogen needed for corn fcilowing soybeans in the rotation.

Since 1992, staff of the Northeast Iowa Demonstration Project (NEIDP) in Postville have conducted a series of corn demonstration plots designed to help answer questions about nitrogen (N) management. Corn following soybean demonstrations were conducted on three sites in 1998. Yields exceeded 200 bushels per acre at two of the locations.

Results are consistent with the general recommendation that N use for corn following soybeans can be reduced by at least one pound for every bushel of soy beans produced the previous year. In most cases, a 90 pound per acre application rate has proven ad equate in the demonstrations. Results for 1998 and for nine demonstrations conducted since 1993 are shown in the table 1. The 1997 soybean yields on the demonstration sites were 57 bushels per acre at site 1, 40 bushels per acre at site 2, and 56 bushels per acre at site 3.

Table 1. Corn yields for 1998 and 1993-98 average, corn-following-soybeans response to nitrogen.
Site 1 Site 2 Site 3 1993-98
Treatment
lb/A N		 Yield
bu/ A		 Cornstalk
NO3, ppm		 Yield
bu/ A		 Cornstalk
NO3, ppm		 Yield
bu/ A		 Cornstalk
NO3, ppm		 Yield
bu/A		 Profit
$/A
0 157 147 144 28 170 106 150 0
30 167 - 160 - 188 - 159 16
60 205 137 171 1,829 189 680 167 29
90 201 699 174 1,579 201 800 169 28
120 216 - 176 - 191 - 172 29
150 220 3,006 178 3,359 195 2,676 172 23

The profit calculations for the demonstration compare the amount of nitrogen used and crop response to the zero N check. For these calculations, the value of N fertilizer is 20 cents a pound and the value of corn is $2.40 a bushel.

End-of-season cornstalk nitrate nitrogen data is also included for the three sites. (See "Refined nitrogen rates put to test in high- yielding season", for an explanation of the numbers.)

The test provides a valuable source of information for site-specific refinement of N use. For example, yields for the 60 pounds N per acre treatment at site I were very good, while the cornstalk nitrate N test was low. When used over a number of years, tests like this can provide information that might not be visible from the seat of a combine. With the increasing amount of soy bean acres there is also an increas mg need for local N management information. The demonstrations provide a solid management re source for northeast Iowa producers.

An Allamakee County site was used for a demonstration of N rates for corn following an alfalfa crop. Alfalfa can be a valuable part of a crop rotation, since there is no need for additional N for corn following alfalfa. The site was on a Downs soil and the final alfalfa stand count averaged only one plant per square foot. Results from this demonstration and from seven similar demonstrations conducted since 1992 are shown in table 2.

Table 2. First-year corn following alfalfa.
Treatment 1998 yield 1992-98 averages
lb/A N bu/A Yield, bu/A Profit, $/A
0 156 152 0
30 150 150 -11
60 150 152 -12
90 157 154 -13
120 154 154 -19
150 160 153 -28

Most farmers recognize that they need very little N for corn following alfalfa, however, they are often reluctant to eliminate the N application based on the quality of the alfalfa stand they are rotating. Because there was no response to the added N, there is only an increasing loss in profit as the amount of applied N is raised.

For more information on 1998 field demonstrations, contact the NEIDP staff, Postville, at 319-864-3999.

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