January 2004
A Variable Rate Fertilization Program
Increases the Value of Manure
Dayton Lambert and J. Lowenberg-DeBoer, Purdue, and Gary Malzer, University of Minnesota
Introduction
Manure can be a valuable commodity, but it requires time and money for handling. Improper disposal of manure may result in environmental degradation, hence many of the proposed changes in feedlot and manure management regulations (Ribaudo et al., 2003). From a production standpoint, there are problems with manure as an alternative fertilizer input: (1) what is the nutrient value of the manure; (2) is the product homogenous; (3) what amount of credit should be given to nutrients in the organic portion of the manure; (4) is variable rate manure application feasible for the producer; (5) can a producer avoid variable rate application fees by varying the rates of other inputs; and (6) what are the economically optimal application rates for a field or part of a field.
Figure 1. Management zones using P-Bray soil test levels (ppm) as a proxy. Key: Z1 < 5 ppm; 5 ppm < Z2 < 10 ppm; 10 ppm < Z3 < 15 ppm; 15 ppm < Z4 < 20 ppm; Z5 > 20 ppm. The blank areas (999) indicate sections of the field not included in the experiment.
The net present value of returns to variable rate manure was compared with a whole-field manure management strategy in this report. Site-specific management zones were delineated based on phosphorous soil test levels (Figure 1). If the SSM objective is to identify input management zones, then a model based on this breakdown makes sense because: (1) the candidate management zones are supported by scientifically informed extension literature; (2) P is a convenient proxy since it correlates strongly with zinc (Zn), pH, and % organic matter (%OM) soil tests (table 1); and (3) P is a manageable input that has been well-studied in extension and agronomic literature.
In a sensitivity analysis, whole field management and variable rate manure strategies are combined with one of three soil fertility management strategies: (1) do nothing, (2) use soil test information to raise potassium (K), phosphorus (P), or lime levels to whole-field average levels recommended by extension, and (3) to use soil test information to vary P, K, or lime site-specifically. In total, there are six scenarios compared. Whole field management strategies are evaluated at extension recommendation rates (3500 gal acre-1) and the whole field optimal manure rate (4329 gal acre-1).
Experimental Design
The variable rate manure experiment was conducted in cooperation with Christensen farms, near Sleepy Eye Minnesota. Corn grown during the 1999 season was followed by soybean. Four rates of liquid swine manure, including a check strip (0, 2000, 4000, 6000, and 8000 gal acre-1) were applied over a 10.7-acre field in constant rate strips. Manure was only applied before the corn-growing season. No manure was applied prior to planting soybean. Manure was applied via surface broadcast and immediate incorporation with double discs attached to the applicator. Yield data was collected in 15-m segments for corn and soybean crops. Grain yield was measured from the center row of each treatment strip using a Massey Ferguson plot combine equipped with a ground distance monitor and computerized Harvest Master weigh-all (Harvest Master, Logon Utah). Every 15-m, the combine was stopped and the harvest grain weighed.
Results
Figure 2. Net present values (NPV) for manure management strategies. Key: VRM = Variable rate manure only; VRM-WFF = VRM combined with a whole-field P, K, and lime fertilization program; VRM-VRF = VRM combined with a variable rate fertilizer (VRF) program; WFM = whole-field manure management strategy only, evaluated at an extension recommendation rate of 3500 gal acre-1; WFM-WFF = WFM combined with a whole-field P, K, and lime fertilization program; WFM-VRF = WFM combined with a P, K, and lime VRF program; WFM-VRF* = WFM-VRF evaluated at the whole-field, optimal manure rate (4329 gal acre-1).
The maximum net present value of $598.44 acre-1 is achieved at the whole field optimal (4329 gal acre-1) manure rate (Figure 2) given based on the economics. The next highest value was achieved by the strategy which applies both fertilizer and manure using variable rate technology, followed by the strategy which applies a uniform rate of manure at the extension recommendation rate of 3500 gal acre-1 combined with variable rate fertilizer. The lowest amounts of manure applied on an average per acre basis were those resulting from variable rate manure recommendations (Figure 3). If the extension recommended rate of 3500 acre-1 is assumed to be a limit on the amount of liquid manure a producer should apply acre-1, then environmental benefits may be attributed to the variable rate manure strategies that use soil test information. The average economically optimal manure rate for the variable rate manure and fertilizer strategy is 2991 gal acre-1 (Figure 3). The average economically optimal rate for the variable rate manure strategy using soil test information and following the extension guidelines for variable rate fertilizer is 2958 gal acre-1.
Figure 3. Economically optimal manure rates for management zones.
Statistical analysis of the manure nutrient contents revealed that although the gallons applied manure per treatment was known, the manure N, P, and K content varied (Figure 4).
Figure 4. Nitrogen, phosphorous, and potassium variability in manure treatments by 2000, 4000, 6000, and 8000-gallons/acre applications. Bars show that applied N and K were different for all treatment levels, but P was not.
This
argues in favor of a whole field management strategy
as opposed to variable rate manure. With current technology (i.e. base data,
soil sampling intensity, good agitation, careful calibration, and a uniform
application of manure combined with variable rate fertilizer may be the best
management strategy. It is important to note that variable rate technology in
theory is designed to apply nutrients precisely by varying the spatial location
of inputs in addition to the correct rates identified for the specific field
conditions.
For more information:
Ribaudo, Marc O., Jean Agapoff, and Abdrea Cattaneo. Can EQIP be effective in helping farmers meet manure management codes? Paper presented at the annual meetings of the American Agricultural Economists Association, Montreal, Canada, July 26-31, 2003.
For additional information about this report, access Lambert, Dayton, Gary L. Malzer, and Jess Lowenberg-DeBoer. 2003. A SYSTEMS APPROACH INCORPORATING SOIL TEST INFORMATION INTO SITE-SPECIFIC MANURE MANAGEMENT RECOMMENDATIONS. Staff Paper #03-13, Purdue University Department of Agricultural Economics, West Lafayette, IN, 47906, http://agecon.lib.umn.edu/cgi-bin/view.pl