Utilization of Dairy Manure in Low-input, Conservation Tillage Animal Feed Production Systems
Manures are often viewed as waste products that must be disposed of, rather than the nutrient-rich resources that they really are. All too often, manure nutrients are not taken into account after application to cropland. Producers often spread manure to dispose of it and follow that application with full rates of inorganic fertilizers. While this usually results in good crop yields, environmental problems can occur. Excess nitrogen can leach toward groundwater resources and both nitrogen and phosphorus can be lost to surface waters in runoff, causing potential eutrophication problems. This practice also represents a waste of resources and capital, in the form of purchased fertilizer inputs, for the farmer.
The objectives of this research project were to:
1) Monitor the long-term residual impacts of manure applications on corn silage growth and soil properties.
2) Evaluate the potential impact of manure applications on off-site surface and subsurface water quality.
The information from this research is helping us to refine manure application recommendations for Tennessee and surrounding regions. Current manure application recommendations in the state of Tennessee are based on information from other regions of the USA.
The research was conducted on plots at two University of Tennessee agricultural experiment stations and two cooperator farmer locations. The Experiment Station sites were the Martin Agricultural Experiment Station in Martin (northwest Tennessee) and the Dairy Experiment Station in Lewisburg (south-central Tennessee). The plots at Martin had not received previous applications of manure, while the Dairy site has been manured frequently for nearly 40 years. The farm sites are both working dairies, one near Martin, and the other south of Lewisburg. Plots were established in 1993 at the university experiment station sites and in 1994 at the farmer sites. The experiment at the experiment stations consisted of 17 treatments: a 0-fertilizer check, three rates of N as NH4NO3 (75, 150, and 225 lb N/acre), and three rates of manure N (100, 200, and 300 lb manure-N/acre) applied for either 1) three consecutive years, 2) two consecutive years + no application in year three, or 3) the first year with no application for years two and three. To provide a tillage comparison, the 150 lb ammonium nitrate-N/ac and the 200 lb manure N/ac treatments applied for one, two, or three years were established with conventional tillage (chisel + disk + harrow) and no tillage.
In the fourth year, no manure was applied to evaluate any residual effects that the manure applications may provide. Manure rates were derived using the assumption that 75% of the N applied as liquid manure would be available for plant use in year one. This resulted in an estimated availability of 75, 150, or 225 lb N/acre which matches the inorganic rates. The 150 lb/acre rate corresponds to the high end of N recommendation rates for corn in Tennessee. Inorganic P2O5 and K2O were applied to the inorganic fertilizer N plots according to soil test recommendations. Manures and fertilizers were applied prior to tillage and planting operations. On no-till plots, the manure was surface applied with no further incorporation. Smaller versions of this experiment were established on two cooperator farm sites, one in each region. The treatments were scaled down to the following 11 no-tillage treatments: the 0-fertilizer check; the 150 lb NH4NO3 N/acre treatment; and the 100, 200, and 300 lb manure N/acre treatments for one, two or three years.
To evaluate manure impacts on water quality, a second series of plots were established at each station. Treatments at the Martin site include four rates of liquid dairy manure (113, 225, 338, and 450 lb N/acre; 2/3 applied each spring before corn, 1/3 in fall prior to hay crop), one NH4NO3 rate (195 lb N/acre; 150 lb prior to corn, 45 lb prior to hay crop) and a control (0 lb N/acre). At Lewisburg, the 338 lb N manure treatment was omitted due to lack of space. The applications range from deficient to excessive N rates; however, the high application rate is not uncommon for dairy operators in these areas. Corn for silage was no-till planted on all plots each spring, followed by an annual ryegrass-clover winter cover at Martin and orchard grass cover at the Dairy Station. These crops were harvested for haylage in the spring prior to corn planting. Instrumentation was installed beneath each plot (at a depth of three feet) to enable the collection of water leaching through the soil and out of the root zone. After every storm, leachate is analyzed for nitrate-nitrogen and other constituents, such as phosphate. Also, plots from the 225 and 450 lb manure-N and the 195 lb inorganic-N treatments at Martin were instrumented with surface water collection equipment for determining runoff volumes and N and P concentrations in runoff.
The data for the residual experiments indicate that prior manuring history will affect the availability of N. At Martin, yields of silage corn were much lower for second and third year corn grown on plots receiving manure only during 1993. For soils receiving manure for two years, the yields in the third year were still well below those for normal rates of inorganic-N applications (168 kg N ha-1). Manure applications of 224 or 336 kg N ha-1 for three years resulted in yields equal to those from the inorganic control. However, there was little impact of three years of manuring on silage production in the fourth year. Fourth year yields were significantly lower from plots receiving 300 lbs manure-N for three years than from those receiving the inorganic N. However, the soils at the Dairy Station have received manure for many years and have shown no response to added fertilizer or manure-N for the past four years. Soils with no manure or fertilizer application have consistently produced silage yields equal to those from soils receiving 225 lb fertilizer N. This site has been manured for nearly 40 years, and illustrates the potential for over-application of nutrients when using both animal and inorganic sources of N simultaneously for a number of years.
Both on-farm locations observed good yields with applications of 200 lb manure-N/acre after three years. There were no statistically significant differences between the 150 lb fertilizer N and the 200 lb manure-N treatments in the third year. The results from the Martin Experiment Station and the two farm sites indicate that the repeated application of approximately 200 lbs N as manure per acre will result in yields equivalent to those from 150 lbs of ammonium nitrate nitrogen. There were no differences in yields between no-till and conventional-till treatments, indicating that nutrient availability from manures in no-till corn is not likely to be a problem.
Water quality monitoring at Martin has indicated that rates of manure nitrogen as high as 225 lbs per acre result in yields comparable to those using recommended rates of inorganic N fertilizers 150 lb/acre) with no significant impact on nitrate-N concentrations in the leachate. Cumulative nitrate-N losses are much higher from plots receiving 450 lb N per acre per year, and may pose a problem. Observations at the Dairy Station for the last few years indicate that there is appreciable N being made available from prior manure additions, and leaching losses have occasionally been unacceptable even from 0 N control plots. Again, these data indicate that we will need to closely evaluate the previous history of a farm when deciding how much N or P we will add in any form. Runoff water volumes are generally lowest from the high manure treatment. Large amounts of organic materials on the soil surface have apparently resulted in higher rates of infiltration into the soil. N and P concentrations are higher in the runoff from the 450 lb manure-N treatment, but total losses of N and P are not different than from the 225 manure-N and the 195 lb inorganic-N treatments, due to lower runoff volumes.
Impact of results
These results are already impacting informal recommendations on manure applications in Tennessee. Applications of manure to provide 200 to 250 lbs of manure-N/acre result in yields equivalent to those from recommended inorganic-N rates. In addition, these rates of manure have not resulted in adverse impacts on subsurface or surface water quality, as measured by total nitrogen or phosphorus export from the soil. Extension specialists are currently sharing this information with county agents by word of mouth. Extension bulletins will be released with this information in the near future.