Manure following beans: How much N contribution to take?
by Gina Hanson, manure management specialist, NEIDP

With more farmers shifting toward corn-bean rotations in northeast Iowa, the application of manure following beans for the next year's corn crop has become a common practice. Northeast Iowa Demonstration Project staff have conducted manure demonstrations looking at this scenario four of the last five years.

Demonstrations were conducted for three years on the Tom Hayes farm, Lamont, in Fayette County, and one year on the Jule Brown farm, Strawberry Point, in southern Clayton County. Both farmers realized there was contribution to be taken from the manure applications on their farms, but they wondered how much additional nitrogen (N) was needed after both manure and legume contributions were taken into account.

Iowa State University Extension recommends taking a one-pound nitrogen contribution for the next year's crop for each bushel of beans produced. This resulted in a legume contribution of approximately 60 pounds per acre for each of the demonstrations. Manure contributions were estimated by calibrating the manure spreaders to determine application rate and taking a manure sample to determine the nutrient content. In both cases manure was surface-applied on soybean stubble in the fall.

In 1994, 1995, and 1996, 3,000 gallons of swine finishing manure were applied followed by no-till corn. The first-year available N contribution was 96 pounds per acre. The firest three years' estimated available N contribution was 96 pounds per acre. At the 1998 site, dairy freestall manure was fall-applied with first-year crop-available N rate of 65 pounds per acre.

The replicated plot design included plots without manure. Other treatments included ammonium nitrate was applied at rates of 0, 50 or 100 pounds per acre of actual N in addition to the fall-applied manure. To evaluate N management at the end of the season, eight-inch sections of cornstalk were cut from plants selected at random within each plot and a residual nitrate N analysis was completed for each manure/nitrogen treatment.

Cornstalk residual nitrate nitrogen levels are an indicator of whether or not the nitrogen supply was adequate for corn development. Low stalk nitrate levels (less than 250 parts per million, or ppm) indicate that additional nitrogen would have been likely to increase yields. The marginal range (250-700 ppm) indicates a level very close to the minimal amount needed by the crop, but grain yield may not be reduced. The optimal range (700-2,000 ppm) indicates that N availability was close to the rate needed by the plant, while levels in excess of 2,000 ppm indicate a high probability that there was more N than needed.

The corn yields and nitrate nitrogen results are reported in the accompanying table. In looking at the demonstration results, remember there is a 60-pound legume contribution from the soybeans. The impact of 50 pounds of N added to the manure treatments is six bushels of corn per acre. This is about $14.40 more corn (if corn is valued at $2.40) at a cost of $10 for applied nitrogen (at 20 cents per pound of N). The next 50 pounds of N only nets a three bushel increase and lowers the overall profit per acre. Also note that the cornstalk nitrate N levels are excessive for the nitrogen applications, indicating nitrogen was not a limiting factor where nitrogen was applied. The increase in yield from the no manure, no nitrogen treatment to the manure-only treatment clearly shows a response to manure. The amount of response will depend on the manure application rate, method of application and nutrient content of the manure. These factors should be considered before deciding how much nitrogen if any should be applied following a manure application.

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