In this article, we summarize partial results from a project that has been demonstrating crop utilization of liquid swine manure nutrients - from understanding the nutrient application rate to measuring crop response. General goals and details of methods such as manure sampling, analyses, and application rates being used were outlined in two previous issues of the Odor and Nutrient Management newsletter (Fall 2002 and Winter 2002 issues). Herein, we present results for crop response to applied manure nitrogen (N). The final article in this series, planned for the Summer 2003 newsletter, will summarize crop response to applied manure phosphorus (P). For the first 3 years of the project (2000-2002), we worked with 16 producer cooperators at 39 production/field sites located in 12 counties.

Harvesting replicated strip trails in the swine manure utilization project.

Corn response to manure N application. Low- and high-rate (the target rate at most sites was 75 and 150 lb total manure N/acre for corn rotated with soybean, and 100 and 200 lb total manure N/acre for continuous corn) liquid swine manure applications substantially increased average corn strip yields relative to the no manure check at 15 of 17 evaluation sites in 2000–2002 where manure was applied before the corn crop (Table 1). Of the total yield increase from manure application, the majority typically came with the low manure rate (average 27 bu/acre strip yield increase across sites with the low manure rate and an average additional 10 bu/acre increase with the high rate). At several sites, the low rate seemed to supply adequate plant-available N, because there was no additional yield response with the high rate. Two sites in 2000 (Hardin and Plymouth) were nonresponsive due to high manure application history or drought conditions. Strip yield increases were considered mainly due to manure-N at most sites, although part of the strip yield increases could be due to response to manure P or potassium (K) at some sites when soil tests were optimum or lower. When warm, drying conditions during broadcast application (Clay 2001) or excessively wet spring conditions (Washington 2001, Davis 2002, Washington 2002) resulted in apparent N losses, or where corn followed corn, then corn yield was increased with higher manure rates (Table 1). If yield was increased with the higher manure rate, it was due to a combination of specific manure-N rates applied and site conditions (corn N requirement and potential N loss). These results with liquid swine manure, and potential effects from loss conditions, are similar to those encountered with N fertilizer.

Cooperator Rob Stout during August 27, 2003, field day held at his farm.

Corn yield response to additional N fertilizer was most consistent in the strips that received no manure or the low manure rate. In 2000 and 2001, at only the most N-responsive sites did corn yield increase with additional fertilizer-N applied in addition to the half-rate manure application, and with only up to 40 lb fertilizer N/acre (Figure 1). At those field sites receiving excess rainfall after manure application (denitrification/leaching losses) or warm temperatures at manure application (N volatilization losses of surface applied manure) corn yield increased with additional fertilizer-N applied in addition to the high manure rate - no sites in 2000, one site in 2001, and two sites in 2002. These 3 years of yield data suggest that supplementing swine manure with additional fertilizer N is only necessary when the manure-N rate is inadequate to meet specific corn needs or losses reduce N supply. Corn yield response to fertilizer N in the residual manure year (manure applied before soybean and then corn grown the following year) was similar for all prior year manure rates (Table 2), indicating no second year crop-available manure N supply.

Figure 1. Effect of average liquid swine manure total-N rate (five corn rotated with soybean sites in 2000 and 2001) and additional fertilizer-N on corn grain yield, relative corn ear leaf SPAD chlorophyll meter reading, and cornstalk nitrate-N concentration.

Grain yield and relative leaf greenness indicated similar corn responsiveness to manure and fertilizer N (Figure 1 for five similar C-S rotation sites). Leaf greenness (Minolta SPAD chlorophyll meter readings) will not indicate excess N but will show deficiency (at approximately <95 percent relative SPAD - relative to adequately N fertilized corn greenness); therefore, those readings do not increase once maximum greenness is reached, even with more N. Corn yield responded to higher manure or fertilizer N rates when relative SPAD values were below 95 percent. Relative SPAD values above 95 percent generally indicated yield did not increase with more N. When manure N or manure plus fertilizer N application was greater than corn need (especially when the rate was excessive), stalk nitrate tests indicated high levels (well above 2,000 ppm). The average manure total-N rate of approximately 150 lb N/acre seemed to supply adequate plant-available N at these five sites. At an average 80 lb total manure N, approximately 40 lb additional N/acre was needed from fertilizer.

Corn was very responsive to liquid swine manure application, with large yield increases at responsive sites. Most yield increase was with the low manure rates, with further yield increase from high manure rates at the more N responsive sites. It was possible to meet corn N requirements solely with liquid swine manure. Although it is not possible to exactly discern first year crop availability, yield and plant N measurements suggest that N in liquid swine manure is highly available to corn in the year of application and seems to support the current recommendation that first year swine manure N availability is near 100 percent. The Winter 2002 newsletter article noted that the average ammonium-N in liquid swine manure samples collected at application was 83 percent of the total-N, indicating that crop availability should be high. Results from these 3 years also indicate that liquid swine manure should be applied following steps of known manure total N content (manure preapplication and at application laboratory analysis instead of book values); applied with equipment calibrated at rates to supply corn N fertilization recommendations; applied in a manner to minimize volatile loss (injection instead of broadcast); and applied at times to minimize conversion of manure ammonium to nitrate well before crop use.

Summary. The project is documenting the importance and value of liquid swine manure as a nutrient source for crop production in Iowa. Following a comprehensive approach of preapplication manure sampling and laboratory analyses, manure sampling during application, and calibrated rate applications, it is feasible to agronomically provide crop N nutrient needs from liquid swine manure. Results from these 3 years also confirm that best management of liquid swine manure should consider practices that enhance achieving desired manure rates for providing N, minimize potential for N loss, and closely estimate rates of needed N.

The ISU Swine Manure Nutrient Utilization Project, part of the Integrated Farm/Livestock Management (IFLM) Demonstration Program, receives funding from the Iowa Department of Agriculture and Land Stewardship, Division of Soil Conservation, USDA Natural Resources Conservation Service, and the Leopold Center for Sustainable Agriculture.

This is the third in a series of newsletter articles highlighting the ISU Swine Manure Nutrient Utilization Project. The final article will appear in the July 2003 ONM newsletter and will highlight crop yield response to manure phosphorus application.