Calibration of the Pre-Sidedress Soil Nitrate Test to Improve Nitrogen Management on Dairy Farms
1. Calibrate the pre-sidedress soil nitrate test (PSNT) for use in predicting corn silage yield response to sidedress N fertilizer on manured and non-manured soils in the Willamette Valley of Oregon.
2. Derive N fertilizer response functions to be used in developing published guidelines for sidedress N fertilization of corn silage.
3. Conduct educational programs to assist dairy producers in making more efficient use of N fertilizers and on-farm manures.
Note: during the first year, we explored additional N tests (soil nitrate at planting, corn stalk nitrate at harvest, residual soil nitrate) and expanded our objectives to include the development of a N monitoring program for use by producers. After the first year, we found that the second objective could not be met because of a lack of N responsive sites from which to derive N fertilizer response functions.
Abstract of Results
Many dairies in Oregon’s Willamette Valley grow silage corn for fed. Commercial nitrogen (N) fertilizers and dairy manure are often applied to the corn crop, though the fertilizer value of manure is often not considered. As a result, nitrogen is often supplied in excess of crop uptake, creating the potential for leaching of residual soil nitrate. The purpose of this project was to calibrate soil and plant analyses and develop a program whereby producers can monitor crop nitrogen status and evaluate nutrient management. Grower adoption of a nitrogen monitoring program will likely increase grower awareness of nutrient dynamics on the farm, result in significant economic savings from reduced fertilizer purchases, and reduce risk of nitrate movement to ground water.
Twenty-six experiments were perforrned on 17 farms in 1994 and 1995. Two predictive tests, soil nitrate at planting (SNAP) and the pre-sidedress soil nitrate test (PSNT), and two evaluative tests, corn stalk nitrate at harvest and residual soil nitrate, were calibrated. Sites were identified as N-responsive if yield from unfertilized plots were less than 94 percent of yield from plots receiving 200 kg N ha(-1) sidedressed at the V5 or V6 growth stage.
The PSNT correctly identified 88 percent of the sites as having either sufficient or insufficient N for maximum yield. When PSNT concentrations were above 21 mg NO3-N kg(-1) soil, additional N was unlikely to improve yields. Twenty-two of 26 sites tested above the critical value. SNAP concentrations of 22 mg M)s-N kg (-1) soil or higher indicated that N was sufficient for maximum yield. SNAP concentrations below 22 mg NO3-N kg (-1) did not necessarily indicate N deficiency, and the SNAP test needed to be followed by a PSNT to assess the need for N fertilization.
Corn stalk nitrate concentrations at harvest were useful for identifying sites where insufficient, adequate, or excessive N had been supplied to the crop. A critical range of 3500 to 5500 mg NO3-N kg (-1) indicated an adequate N supply during the growing season. Residual soil nitrate concentrations above 16 mg NO3-N kg (-1) in the surface 30 cm (65 kg NO3-N ha(-1)) indicated N had been supplied in excess of crop demand. Comparison of at-planting and post-harvest concentrations of soil NO3-N to a depth of 150 cm (5 ft.) showed that N applied as manure or N fertilizer was often in excess of crop requirements.
The small number of N-responsive sites in this study suggests that N from manure can replace most or all of the fertilizer presently applied to silage corn on many Willamette Valley dairies.
Geographic area: The 17 cooperating farms were located in the Willamette Valley of western Oregon. The sites were spread along a 100 mile length of the valley. Climate: Mediterranean. Average annual rainfall is 40 inches, occurring primarily from Oct. through May. Soils: Alluvial. 26 experiments: 23 silty clay, 3 silty clay loam. Farm size: Dairy herd sizes ranged from 60 to 1200 animals. Acreage in silage corn production ranged from approximately 60 to 300 acres.
Data suggests that many dairy producers can reduce or eliminate N fertilizer purchases. Elimination of N applications of 100 to 200 lb N/acre will save producers $35 to $70 per acre of fertilizer purchases plus application costs. A typical producer growing 100 acres of silage corn can reduce input costs by about $5000 while maintaining yields. One project cooperator reported fertilizer savings of $20,000 the first year of adoption on N monitoring techniques on 300 acres of corn. Elimination of fertilizer N applications on half of the valley’s 16,900 acres of silage corn would reduce N use by 845,000 to 1,690,000 lb N per year, saving growers up to $500,000 annually.
This project demonstrates that dairy producers can reduce production costs, protect water quality, and maintain corn silage yield by using SNAP, PSNT and corn stalk nitrate, and residual soil nitrate tests to refine management of nitrogen applied as manure or commercial fertilizer. The most immediate impact will be the financial savings realized by the farmers. Society will benefit from grower adoption of this technology because ground water quality will be protected. By minimizing nitrate-nitrogen remaining in the soil profile at harvest, growers will be able to reduce over-winter leaching loss of nitrate.
Producers can make use of the plant and soil analyses calibrated in this project as components of a nitrogen monitoring program for silage corn production. While there is no need to make use of all methods in a single year, having an array of options should increase the likelihood that a given producer can develop a program that fits into his/her overall farm management program.
By monitoring nitrogen supply during corn production, growers can refine management over a period of years. Currently, manure application rates are calculated by balancing estimates of N supplied by manure with projected crop N removal. Monitoring N can improve management efficiency by detecting N sufficiency or deficiency on a site specific basis.
Evaluative tests such as corn stalk nitrate at harvest can aid growers working to reduce N fertilizer inputs. When growers reduce N fertilizer use and yields are lower than expected, reduced N rates may be the suspected cause. The stalk nitrate test at harvest can help determine if N was, in fact, the yield limiting factor. If the stalk nitrate test indicates N was not limiting, a grower may avoid returning to higher N rates unnecessarily. Similarly, a grower who has been successful at lowering N rates may use the stalk nitrate test to decide if further N reductions are advisable in future years.
We expect that most dairy producers in western Oregon will eliminate or sharply curtail the use of N fertilizers on silage corn once these results have been widely disseminated. Reductions of N fertilizer application from 30 to 200 lb N/acre are highly probable. Reported in 1996.