Final Report for SW06-028
Precision manure management is a multidimensional concept that converges the best manure management practices along with precision agricultural techniques. Our three years of project work indicate that the variable rate application of animal manure across productivity level management zones has potential to enhance grain yield of low producing areas of the spatially variable farm fields. There are also positive improvements in the soil quality parameters that are highly correlated to grain yield and productivity and sustainability. Extension workshops and meetings indicate significant interest among farmers and practitioners in learning about precision manure management.
1. Demonstrate and compare manure management strategies applied across site-specific management zones delineated on farmers' fields to assess impact on productivity and soil quality.
2. Demonstrate and compare the improvement in soil quality parameters (as measured by organic matter content, bulk density, aggregate stability, water retention, organic matter content, etc).
3. Demonstrate and make economic comparisons of site-specific management zone based manure management strategies to estimate differences in economic returns among the strategies.
4. Evaluate the impact of manure management strategy on water quality risk, through the use of P Index and the N Leaching Index.
5. Develop a network of farmers to conduct on-farm trials, provide hands-on training via field days, farm tours and extension workshops in the design, implementation and analysis of field-scale farm trials using innovative site-specific management zone techniques.
An innovative approach called “Site-Specific Management Zones” was developed by the Principal Investigator and his co-workers at Colorado State University (CSU). The approach classifies farm fields into (high, medium and low) productivity level management zones. Management zones are defined as homogeneous areas within a field that have similar productivity potential. On-going research in Colorado has clearly shown and documented that by applying variable rate fertilizer using site-specific management zones, i.e. applying higher rates of fertilizer on high management zones and lower rates on low management zones, results in overall increases in grain yield, net $ returns to farmer, nutrient use efficiency, nutrient uptake and reduction in environmentally sensitive nutrient loads into soil on a field-scale basis (Khosla et al., 2002, Hornung, et al., 2003; Koch et al., 2004; Inman et al., 2005).
The above approach, although scientifically logical, has faced resistance from farmers in adoption. Farmers' concern is that the approach recognizes variability in productivity potential, however it does nothing about the improvement of the yield limiting factors of low and medium management zones to enhance grain yields of those areas.
Several farmers at our focus group meetings have suggested application of manure on management zones with an inverse approach, i.e., higher amounts of manure application on low management zones and relatively much lower amounts of manure application on high management zones. Farmers have hypothesized that higher levels of manure on poor producing areas over time will enhance soil quality, water holding capacity, organic matter, other soil properties and will enhance grain yield and net $ return of such areas.
If the above approach of site-specific manure management as suggested by farmers works, then this creates an economically-sound method of disposing manure for enhancing soil quality and sustaining productivity. However, it raises the question that applying high amounts of nutrient-rich manure on low management zones may increase potential for environmental pollution; conversely, water quality impacts could also be reduced due to reduction of erosion from the low management zones.
The proposed project is unique in that it builds on a thoroughly tested innovative approach called “site-specific management zones” that is shown to work for variable rate fertilizer management. However, no data existed at the start of this project in western Great Plains that could provide recommendations on how much and when to apply manure on respective site-specific management zones.
Based on the above suggestions from the farmers in our focus group, we designed a three-year research and educational project that was outlined in the proposal. Our team (included interested farmers and grower associations, scientists from CSU and USDA-ARS unit, extension specialists and agents, and technology transfer agents from NRCS and cooperative extension) came together to test the hypothesis as suggested by the farmers. A combined research, education and extension effort was needed to achieve the objectives of this project.
Our target audience included, but was not limited, to growers and grower associations, technology transfer agents, NRCS personnel and cooperative extension agents, crop advisors and consultants and regulators.
There have been no SARE-funded projects that have previously addressed site-specific manure management in any region of the U.S. In addition, a search of the Agricola database at the beginning of this project found no publications at all with the term “site-specific manure” in the title or abstract. However, the CRIS database revealed two USDA-ARS projects related to site-specific manure management: one in Beltsville, MD, and one in Lincoln, NE. The Maryland project was terminated in 2000, and no relevant publications from it were found. The Nebraska research group completed a two-year study in an irrigated field from 1998-2000 that showed that site-specific manure application to low organic matter areas in a field increases soil P and organic C levels and reduces the variability of P and C in a field (Eghball et al., 2002; Eghball et al., 2003). Dr. Eghball, the leader of the Nebraska manure team, passed away in the summer of 2004. An email from Dr. John Gilley in Nebraska clearly stated “There are no plans to continue the site-specific manure management program…” The current research focus of the NE team is on manure management related to mico-organisms and odor.
This project was conducted across several site-years, under both irrigated and dryland cropping systems. Site-specific management zones were developed for each site (farm fields) using soil color reflectance from bare-soil aerial photographs, field topography and farmers' knowledge of the production levels that occur within the field(s). This technique of delineating management zones (into high, medium and low productivity areas within a field) is well documented to capture the inherent productivity potential of field areas.
Demonstration plots for each manure management strategy were about six meters wide by the length of the field (at least 350 meters). Plot length varied on different sites depending upon the size of farm field and the equipment available for the project at that location. Large-scale field plots allowed farmers to use their own farm equipment for planting, harvesting with yield monitors and application of manure. The following cattle-manure management strategies were used across the three (high, medium and low) site-specific management zones (MZs).
The management strategies evaluated in this study were:
(i) Variable manure applications based on MZs using a Constant Yield Goal (CYG) strategy. The CYG manure management strategy was suggested by the cooperating farmers in this project. In this strategy, it is assumed that grain yield can be increased in low producing areas (low MZs) of the field to the same levels as that of high MZs by additional applications of manure to the low MZs. Hence the expected yield is kept at a constant for the entire field or across all three MZs.
(ii) Variable manure applications based on MZs using a Variable Yield Goal (VYG) strategy. In the VYG management strategy, manure applications are based on the productivity potential of the MZs, high, medium and low. Hence higher rate of manure are applied on high MZs and low rate of manure are applied on low MZs.
(iii) Variable commercial N fertilizer applications based on MZs using VYG N management strategy. The N fertilizer rate was determined using the N-recommendation equation based on the soil samples acquired from within each MZ, with a unique yield goal for each MZ. Variable yield goals for VYG N fertilizer management strategy expected for the three MZs were 11.3, 10.0 and 8.8 Mg ha-1 for high, medium and low MZs, respectively.
Manure applications were made and incorporated in the spring of each year. The soil samples were taken prior to planting and after harvest for nutrient analysis and to assess various soil quality parameters. Field days were organized during every growing season on demonstration sites.
Potential impacts on water quality was evaluated through the use of two risk assessment tools (the P Index and the N Index) developed by NRCS in partnership with CSU. The P Index utilizes soil P levels, slope, permeability and manure application rates and methods to assess the relative risk of P runoff to surface water. The N Leaching Index utilizes soil texture, irrigation efficiency and N application rates and timing to evaluate the risk of N leaching to groundwater.
Grain yield data was collected using GPS equipped yield monitoring combine. Soil samples were collected from geo-referenced locations each year in each field. Data and statistical analysis were performed using SPLUS 6.1 and SAS statistical software. Grain yield and soil quality differences between management zones and across treatments were analyzed for significant differences among the treatments. Mean separation was performed using least squares difference (LSD) at P<0.05.
For economic analysis of this project, each manure management strategy will be treated as a separate enterprise. The estimated net returns to the land and management will be compared to establish the most profitable manure management strategy. Crop enterprise budgets were constructed to assess the economics of uniform versus variable-rate manure management strategy.
Results and Discussion/Milestones listed below are for the period of 2006 to 2010 of the project.
Grain Yield Analysis:
Variable Yield Goal (VYG) manure management strategy
Grain yield for Variable Yield Goal (VYG) manure management strategies was significantly different (P?0.05) between low and high MZs in two out of three Site Years with high MZ producing higher yields than low MZ. Across all MZs, there was a substantial increase in grain yield in Site Year II and Site Year III as compared to the drought-affected growing season of Site Year I (Fig. 1). Severe drought in the months of June, July and August during the growing season of Site Year I could have had negative impact on grain yield; for example, 2.5 Mg ha-1 on low MZ in Site Year I within VYG manure management strategy. The months of June, July and August are considered the critical months for N mineralization from manure in Colorado (Marx, 2008). Koelsch (2005) and Bacon (1995) reported that soils that are dry throughout most of the growing season have low mineralization and nitrification rates. However, microbial activities in manure amended soils that are dry during most of the growing season are reported to be greatest immediately after rainfall or irrigation events, as may be the case in our study (Koelsch, 2005). This observation can be linked to increased grain yield from manure application in Site Year II when weather and precipitation were close to normal.
The grain yield increased from Site Year I to Site Year II and then decreased in Site Year III. Decrease in grain yield may partially be attributed to a hail storm that occurred at the V17 maize crop growth in Site Year III, which perhaps negatively impacted the grain yield across all MZs and may be the cause of observed low grain yields as compared to previous year of Site Year II. However, a decrease in grain yield was not observed on VYG N fertilizer management strategy that was on the same field, showing that hail was not a primary cause of significant decrease (P?0.05) in grain yields on manure management strategies. A complete assessment of nutrient balance and review of literature indicates that repeated application of manure can negatively impact crop grain yield due to increased level of salinity.
Constant Yield Goal (CYG) manure management strategy
Under the CYG manure management strategy, grain yield for the low MZ was significantly different and higher than medium and high MZs in Site Year I (Fig. 1). Historically, areas classified as low MZs generally produce lower grain yields compared to high MZs when nutrients are applied uniformly across an agricultural field (Inman et al., 2005; Hornung et al., 2006), but with CYG manure management strategy, low MZ produced 2 Mg ha-1 grain yield higher than that of the high MZ in the first year of this study (Fig. 1). In two out of three site-years (i.e. Site Year I and Site Year III), higher applications of manure on low MZs increased grain yield levels to a level higher than those of high MZs (P?0.05). The higher grain yields on low MZ than that of high MZ in Site Year III can be associated with increased precipitations in this limited irrigation study that potentially catalyzed mineralization and nitrification.
Variable Yield Goal N (VYG-N) fertilizer management strategy
In Site Year I of VYG commercial N fertilizer management strategy, grain yields were higher than the yields of CYG and VYG manure management strategies (Fig. 1). This was not a surprise because animal manure was applied in the spring prior to planting and manure needs time to mineralize before nutrients are released for crop availability. Adequate environmental conditions such as adequate soil moisture, necessary for effective N mineralization, did not prevail in Site Year I during the crop growing season. In Site Year II, all manure management strategies produced grain yields that were equivalent to that of VYG N fertilizer management strategy. However, in Site Year III grain yields declined for manure management strategies while the VYG N fertilizer management strategy produced higher grain yields than VYG and CYG manure management strategies.
Comparing the three management strategies, while manure application may have potential to improve grain yields of low producing areas of the field, it does have agronomic and environmental limitations. While manure management strategies failed to positively impact grain yield under limited irrigation, N fertilizer management strategy succeeded to continuously improve grain yield. Perhaps there is a two year threshold for using weight-based manure management strategies for maize grain yield across MZs, beyond which a positive impact on grain yield will not be realized. Therefore, it may be logical to combine manure management with N fertilizer management such that manure applications slowly but continuously impact soil properties/quality over time, while in-season N fertilizer management provides enough impetus to boost the grain yield of crops each year without negatively impacting the environment and grain yield. One source of nutrient, either manure or N fertilizer alone, may not optimize environmental and agronomic goals needed for sustainability of crop production.
Top Soil Quality:
Topsoil quality parameters evaluated included organic matter, bulk density, water holding capacity, electric conductivity and particle size analysis. The improvements of soil quality due to variable rate applications of animal manure were compared across manure application rates and MZs. Changes in selected soil properties findings are presented below.
Two repeated variable rates of animal manure applied under irrigated conditions significantly increased topsoil organic matter of low and medium MZs. A previous study conducted in the state of Pennsylvania showed that more than 44 Mg ha-1 of animal manure is required to maintain soil organic matter levels, and higher manure rates are necessary to increase soil organic matter content (Duiker, 2001). While Duiker (2001) made observations under uniform fields, accounting for spatial variability in our study has proved different on low and medium MZs but not on the high MZ. In the high MZ under irrigated conditions, only the highest manure rate of 67 Mg ha-1 increased topsoil organic matter as compared to a control treatment of no manure application.
Under dryland conditions, two repeated variable rates of animal manure maintained topsoil organic matter across MZs. The 44 Mg ha-1 manure application rate, which is the manure rate commonly applied by farmers on Colorado agricultural land, significantly increased topsoil organic matter.
Applications of variable rates of animal manure across MZs under irrigated conditions significantly (P?0.05) decreased topsoil bulk density in the low and medium zones. The decrease in soil bulk density is due to improved soil structure resulting from increased porosity and a dilution effect resulting from the mixing of added organic matter with the denser mineral fraction of the soil (Powers et al., 1975). Interestingly, variable rate applications of animal manure treatments had no significant impact on topsoil bulk density under irrigation of the high MZ (Table 4). The top soil of the high MZ included higher clay content, organic matter content and lower bulk density as compared to the low MZ prior to manure application. The results of this study are supported by previously published work of Zacharias (2005) that animal manure is useful in improving quality of soils that are low in productivity.
Under dryland conditions, variable rate applications of animal manure significantly (P?0.05) decreased topsoil bulk density on low, medium and high MZs at all application rates. Most manure apparently remained in the soil unmineralized and consequently influenced topsoil bulk density. The significant decrease in topsoil bulk density due to unmineralized animal manure caused by drought conditions under dryland conditions can potentially be temporary in nature, and topsoil bulk density may increase again under favorable conditions for manure mineralization.
Soil water holding capacity
Soil volumetric water content at field capacity (qfc) and wilting point (qwp) have been reported to be affected by the addition of animal manure. The increase in soil volumetric water content is known to be driven by the increase in soil organic matter content and reduction in soil bulk density (Gupta et al., 1977; Unger and Stewart, 1974). The results of this precision manure management study support these observations. Animal manure induced increases in soil volumetric water content under both dryland and irrigated conditions. Significant increases (P?0.05) in qfc and qwp were observed between the control treatment (no manure application) and manure treatments on each MZ.
Nitrogen leaching and phosphorus runoff risk assessment
The Colorado N leaching risk assessment index indicated that NO3-N leaching from three years of variable manure application rates across MZs pose a “medium” risk of ground water contamination at the rates of manure used under irrigated conditions only. The NO3-N leaching index score of 10 was observed across MZs. According to Shaffer and Delgado (2002) and Sharkoff et al. (2006), a N leaching index score of 8 to 11 indicates that the field is at medium risk of ground water contamination.
The Colorado N leaching risk assessment index failed to detect differences between low, medium and high MZs as all zones showed a net score of 10 for the 22, 44 and 67 Mg ha-1 application rates. This indicates that the Colorado N leaching risk assessment index was insensitive to spatial variability across low, medium and high MZs for this location. This verifies that N leaching index does not estimate the actual presence or leaching of NO3-N (Shaffer and Delgado, 2002), hence all three MZs were classified as having the same environmental risk of N leaching.
The Colorado P index risk assessment recommends that the tool for P risk assessment not be used if there are no water bodies that can be impacted. However, runoff from this field is discharged into a local creek that eventually connects to the ditch that enters a larger irrigation ditch. The P runoff risk assessment index scores of 9 and 10 were calculated across MZs under irrigated conditions. While all MZs were certified to have medium risk of P runoff into surface water by a net score of 8 to 11, it was interesting to observe that the Colorado P runoff risk assessment index was able to differentiate between low and high MZs. In this study, soil test P was classified within medium and high environmental risk (Sharkoff et al., 2008). The furrow irrigated field was classified as having medium potential for off-site P movement. As a result, the best management practice suggested by Sharkoff et al. (2008) for fields with medium potential for off-site P movement was that manure application rates be calculated and applied according to crop N requirements.
All MZs were under moderate environmental risk of N leaching into ground water and a moderate environmental risk was associated with P runoff into surface water bodies. While the N and P risk assessment indices provided a general estimate of the status of the field in relation to leaching and runoff, respectively, there is a need for more detailed N leaching and P runoff indices that would consider the variability across MZs.
- Figure 1. Mean grain yield across low, medium and high management zones for variable yield goal manure, constant yield goal manure, and variable yield goal N management strategies under limited irrigation conditions. Within a site-year, bars with different letters are significantly different at P = 0.05 level of significance.
This project has provided several unique and innovative scientific information that would be very useful for scientific community for future research and also to the farming community to learn and adopt from the findings of this study.
The underlying hypothesis of this study was that maize yields could be increased in low producing areas of the field, i.e. low MZs. Maize grain yield of low MZs were enhanced with the constant yield goal manure management strategy; however, the right amount of manure to be applied needs to be found to maintain balance with the environment and not pose a potential threat for environmental contamination through nitrate leaching, etc.
Secondly, the constant yield goal manure management strategy must be scrutinized with other associated parameters such as costs associated with animal manure transportation (distance) and application on low MZs. Therefore, application of animal manure based on constant yield goal strategy at rates equivalent to the ones used in the study could not be an environmentally friendly strategy in continued over a long period of time, if the intent is to use manure to meet the crop N needs. Agriculture today is under pressure to meet environmental targets and, therefore, we must weigh agronomic and economic benefits against environmental burdens. We therefore, suggest that constant yield goal manure management strategy be used in conjunction with N fertilizers to meet crop N requirements at early maize growth stages. Manure application would improve soil quality while N fertilizer application would meet the peak N requirement of the crop.
Given the law of diminishing marginal returns and its application to the manure management strategies, we further suggest not to exceed two consecutive repeated manure applications when N fertilizer is not used in conjunction with constant yield goal manure management strategy for profitable crop production and environmental quality under irrigated conditions. The key to precision manure management is to find a balance between economical and environmentally sound manure management strategy which is capable of improving soil fertility status of low producing areas of the field and consequently enhancing grain yield across MZs.
Varying animal manure application rates across MZs is potentially a good approach for maintenance and improvement of bulk density, soil water holding capacity and organic matter of low producing areas; however, the approach was not shown to benefit high productivity MZs. While the soils remained non-saline after two repeated manure applications, soil electrical conductivity under dryland and irrigation conditions were increased. Phosphorus runoff index risk assessment indicated that there are no environmental risks associated with manure applications across MZs. However, the study suggests that the irrigated field could be at medium risk of nitrate leaching. Based on the results of this study, one can only surmise that continued applications of animal manure may further improve topsoil quality in low productivity areas of the field; however, it may not be environmentally suitable for other soil parameters such as salt accumulation and nutrient overloading.
Educational & Outreach Activities
As per our plan, during the project implementation, we did a number of extension activities such as on-farm demonstrations, field days, farm tours, extension workshops, presentations, etc.
Moshia, M.E., R. Khosla, J. Davis, and D.G. Westfall. 2010. Precision Manure Management: It matters where you put your manure. In the CD-Rom Proceedings of the 10th International Conference on Precision Agriculture, Denver, CO.
Moshia, M.E., R. Khosla, D.G. Westfall, J. Davis, and R. Reich. 2010. Precision Manure Management Strategies Site Specific Management Zones for Enhancing Corn Grain Yield. In Abstracts of the Annual meetings of the American Society of Agronomy, Long Beach, CA. Nov., 1-5th.
Khosla, R. and M.E. Moshia. 2010. Precision Manure Management: It matters where you put your manure. In the Abstracts of the annual meetings of the Western Society of Soil Science, Las Vegas, NV, June 2010.
Moshia, M.E., R. Khosla, D.G. Westfall, J.G. Davis, and R. Reich. 2009. Precision Manure Management across Site-Specific Management Zones in the Western Great Plains of the USA. In the CD-ROM Proceedings of the European Conference on Precision Agriculture, Wageningen, July 2009.
Khosla, R., W.M. Frasier, D.G. Westfall, B. Koch. 2009. Economics of Fertilization Under Site-Specific Management Zones. In the Proceedings of the 2009 Western Nutrient Management Conference. Salt Lake City, UT.
Moshia, M.E, R. Khosla, D.G. Westfall, J.G. Davis, and R. Reich. Precision Manure Management across Site-Specific Management Zones in the Western Great Plains of the USA. In proceedings of the Joint International Agriculture Conference, Waginengen, Netherlands. July 6th-8th, 2009.
Moshia, M.E., R. Khosla, D. Westdfall, J.G. Davis, and R. Reich. 2009. Precision Manure Management on Site-Specific Management Zones: Surface soil quality and environmental impact. In the abstract book of the Western Society of Soil Science Annual Meetings. Fort Collins, CO June 22-23rd, 2009.
Khosla, R., and J. Davis. 2009. Precision Manure Management: Enhancing Soil Quality and Productivity. Newsletter of the Colorado Livestock Association.
Khosla, R., and J. Davis. 2009. Precision Manure Management: It matters where you put your manure. Extension Newsletter From the Ground Up. Vol 28. Issue 3. pg 24-25.
Moshia, M.E., R. Khosla, D.G. Westfall, J.G. Davis, and R. Reich. 2008. Nitrogen Mineralization Rate of Animal Manure across Productivity Level Management Zones. In Proceedings of the 9th International Conference on Precision Agriculture, Denver, CO July 21-23, 2008.
Moshia, M.E., R. Khosla, D.G. Westfall, J.G. Davis, and R. Reich. 2008. Precision Manure Management Strategies across Site Specific Management Zones for Enhancing Corn (Zea Mays L) Grain Yield. In Proceedings of the 9th International Conference on Precision Agriculture, Denver, CO July 21-23, 2008.
Moshia, M.E., R. Khosla, D.G. Westfall, J. Davis, R. Reich. 2008. Nitrogen mineralization rate of animal manure across productivity level management zones. In Proceedings of the Great Plains Soil Fertility Meetings, March 4th – 5th, Denver, CO.
Bacon, P.E. (ed.). 1995. Nitrogen fertilization in the environment. New York. Marcel Dekker, Inc.
Duiker, S.W. 2001. Manure and soil organic matter. Penn State Field Crop News. 1 (12): 1-11.
Eghball, B., D. Ginting, C.A. Shapiro, J.S. Schepers, and C.J. Bauer. 2002. Manure as carbon source for soil improvement and crop production: site-specific application. pp. 22-28 In Great Plains Soil Fertility Conference Proc. March 5-6, 2002; Denver, CO.
Eghball, B., C. Bauer, and C.A. Shapiro. 2003. Reducing spatial variability of soil carbon and phosphorus by site-specific manure application. Manure Matters 9(5):1-3. University of Nebraska; Lincoln, NE.
Gupta, S. C., R.H. Dowdy, and W.E. Larson. 1977. Hydraulic and thermal properties of a sandy soil as influenced by incorporation of sewage sludge. Soil Sci. Soc. Am. Proc. 41:601–605.
Hornung, A., R. Khosla, R. Reich, and D.G. Westfall. 2003. Evaluation of site-specific management zones: Grain yield and nitrogen use efficiency. p. 297–302. In J. Stafford and A. Werner (ed.) Precision agriculture. Wageningen Academic Publ., Wageningen, the Netherlands.
Hornung, A., R. Khosla, R. Reich, D. Inman, and D. Westfall. 2006. Comparison of site-specific management zones. Soil color based and yield based. Agron. J. 98:405–417.
Inman, D., R. Khosla, D.G. Westfall, and R. Reich. 2005. Nitrogen uptake across site-specific management zones in irrigated maize production systems. Agron. J. 97:169–176.
Khosla, R., K. Fleming, J.A. Delgado, T. Shaver, and D.G. Westfall. 2002. Use of site-specific management zones to improve nitrogen management for precision agriculture. J. Soil Water Conserv. 57:513–518.
Koch, B. 2003. Economic Feasibility of Variable-Rate Nitrogen Application Utilizing Site-Specific Management Zones. Masters thesis. Dept. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO.
Koch, B., R. Khosla, M. Frasier, D.G. Westfall, and D. Inman. 2004. Economic feasibility of variable rate nitrogen application utilizing site-specific management zones. Agron. J. 96:1572–1580.
Koelsch, R. 2005. Livestock and poultry environmental stewardship curriculum. Lesson 30 No. 2: Nutrient management. Iowa State Univ. Ames, IA.
Marx, E. 2008. Developing a Nitrogen budget for organic soil fertility. Colorado agriculture conference and trade show. Island Grove Park, Greeley, CO. www.colostate.edu/Dept/CoopExt/Adams/cabas/pdf/NitrogenBudgetforOrganicFarming-Marx.pdf. (Accessed 20 October, 2009).
Massey, R., and Payne, J. 2008. Animal manure management: Costs of solid manure application and transport. University of Missouri Extension. http://www.extension.org/pages/Value_of_Manure_and_Economics_of_Manure_Management.
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Shaffer, M.J., and J.A. Delgado. 2002. Essentials of a national nitrate leaching index assessment tool. J. Soil Water Conserv. 57:327–335.
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Massey and Payne (2008) reported that dollars may not be the only metric for discussing costs. However, in this precision manure management study, all costs and revenues have been reported in dollars for simplicity. The net returns to the land and management for maize grain yield were based on current maize market price of $137.9 Mg-1 ($3.5 bu-1). The grain yield and prices associated with maize were directly proportional to the net return to the land and management. Over a three-year period, this study averaged maize grain yields of 6.5 Mg ha-1 (104 bu ac-1) on VYG manure management strategy. Based on the economic analysis, the study indicates that maize producers can realize a positive net return by marketing maize at $118.2 Mg-1 ($3.0 bu-1), which is below the current market price.
Variable rate application of manure based on constant and VYG nutrient management strategies under limited irrigation was advantageous over time based on net returns to the land and management. In Site Year I, under the VYG manure management strategy, there was an economic loss, and the year 2007 averaged a net return to land and management of $448 ha-1 ($181.4 ac-1). While high MZ within VYG manure management strategy produced higher grain yield than low and medium MZs, the net returns were negative because of the total costs of production associated manure transportation and application on high MZs. A similar situation happened to low MZ of CYG manure management strategy where an economic loss was reported while grain yield was higher than that of medium and high MZs in all three years. Nevertheless, CYG and VYG manure management strategies realized a positive net return to land and management in Site Year II.
The two dimensional sensitivity analysis performed for this study logically suggests that producers will benefit economically by producing and marketing maize at higher grain prices. While this is not surprising, the challenge is that the maize market prices are controlled by a broader economic market and not by farmers, but increased grain yield is part of farmers’ crop management production
The VYG N fertilizer management resulted in positive net return across MZs in all years. The net returns to land and management for VYG N fertilizer management strategies diverge from the results reported by Koch (2003) using similar management strategy. Koch (2003) reported negative net returns to land and management. This could be attributed to the fact that Koch (2003) performed economic analysis at the then prevailing maize price of $78.8 Mg-1 ($2.0 bu-1) versus $137.9 Mg-1 ($3.50 bu-1), the current maize price used to estimate net $ returns for this study.
As per our plan, during the project implementation, we did a number of extension activities such as on-farm demonstrations, field days, farm tours, extension workshops, presentations, etc. Our extension activities indicate significant interest among farmers and practitioners in learning about precision manure management. There is significant interest among farmers who are enthusiastically participating in our project to find out the soil quality benefits associated with utilization of manure on their fields and consequential increase in the yield of the low productive areas of the field. We received new requests from consultants, producers and extension agents to organize workshops to share the findings of our work.
Precision manure management shows potential; however there are concerns about the peak nutritional requirement of the crop which may be compromised because of non-synchronous mineralization of manure. So we are cautiously excited about the potential and at the same time are contemplating strategy that may perhaps include addition of fertilizer at peak nutrient uptake of crop while manure may meet the nutritional need of the crop at other times and also aid in improving soil quality.
Areas needing additional study
It is quite clear from this project that manure alone cannot meet the crop nutritional requirements. Moreover, continuous application of manure year after year may not be environmentally suitable. Hence future research needs to focus on studies that would investigate site-specific application of manure coupled with N-fertilizer application during the peak crop requirements. This can be achieved with using active crop canopy sensors that would measure the reflectance from canopies and would identify the crop N needs. Manure would improve soil quality and hence productivity over time in environmentally responsible manner, while fertilizer application would meet the peak crop demand and hence increase grain yield in short term and overall productivity in long term.