To better understand the state of the science and to lessen the present risk of winter manure application, a literature review was conducted that examined a wealth of studies dating back to 1945. Interest in winter manure application has increased, in part, due to the high-profile nutrient impacts to the Great Lakes and the several resulting new policies that have been implemented within the surrounding states. Based on this literature review, research needs and future directions were identified.
What did we do?
A comprehensive literature review was conducted that included scientific, technology transfer, and regulatory documents. Contaminants of concern, emerging pollutants, case studies, existing best management practices, state level policy, and key data gaps were identified.
What we have learned?
The US Environmental Protection Agency (EPA) and the Natural Resource Conservation Service (NRCS) discourages the application of manure in winter because of the elevated risk of nutrient loss to the environment as demonstrated by several field, laboratory, and modeling studies (Srinivasan et al., 2006). The emergence of environmental issues downstream of livestock operations such as algae blooms and fish kills has led some States to ban winter manure application all together, although some states still allow emergency applications and exempt smaller farms from the regulations. Additionally, the loss of nutrients during spring thaws means a loss of soil productivity for farmers and added expense to purchase soil amendments.
There are several parameters that ultimately determine the impact winter manure spreading will have on the environment and the nutrient content that remains in the soil after application. Included, but not limited to, are slope, soil type, depth of freeze, rate of thaw, depth of snow, presence of cover crops, tilling practices, manure moisture content, and timing of application. Several are interdependent, often resulting in difficulty isolating the relative effects of any particular parameter compared to another and, in some cases, contradictory research results are found. However, several general findings may still be derived, as discussed below.
Runoff from winter-applied manure can be an important source of annual nutrient loadings to water bodies, with nitrogen and phosphorous being the most often reported. In a 1985 study, Moore and Madison (1985) estimated that 25% of annual phosphorus load to a Wisconsin lake was directly attributable to winter spreading of animal wastes. Brown et al. (1989) investigated the Cannonsville Reservoir in New York and determined that snowmelt runoff from winter manured cropland contributed more phosphorus to the reservoir than runoff from barnyards. Clausen and Meals (1989) estimated that 40% of Vermont’s streams and lakes experienced significant water quality impairments from the addition of just two winter-spread fields in their watersheds. Plot studies of winter-applied manure found 23.5 to 1,086 mg/L of total Kjeldahl nitrogen (TKN) and 1.6 to 15.4 mg/L of phosphorus in runoff (Lorimor and Melvin, 1996; Thompson et al., 1979). In two Vermont field studies, Clausen (1990; 1991) reported 165 to 224% increases in total phosphorus concentration, 246 to 1,480% increases in soluble phosphorus, 114% increases in TKN, and up to a 576% increase in NH3-N following winter application of dairy manure. Mass losses of nutrients are highly variable across studies. Several studies have noted elevated, though moderate, mass losses of nitrogen ranging from 10-22% of applied nitrogen (Converse et al., 1976; Hensler et al., 1970; Klausner et al., 1976; Lorimor and Melvin, 1996; Midgley and Dunklee, 1945; Phillips et al., 1981). However, Owens et al. (2011) reported total nitrogen losses of 35-94%, by mass. These numbers are highly variable due the extreme variance in weather conditions, with flash events contributing more nutrient loss than slower melt events. Authors noted that it is possible for nearly all loss to occur in a single storm event (Klausner et al., 1976; Owens et al., 2011).
Steenhuis et al., (1979) reported decreases in ammonia volatilization rates for winter spread manure relative to spring due to lower temperatures. Lauer et al. (1976) showed that manure covered by snow had no signs of ammonia volatilization. These results suggested that limiting ammonia volatilization may be critical to nutrient retention in soil. However, Williams et al. (2010) showed that manure applied under snow did not truly maintain this ammonia but lost it through runoff. No case studies have quantified the reduction of other odor causing compounds such as di-hydrogen sulfide in winter applied manure relative to other seasonal applications.
Losses are contingent upon fields exhibiting certain risk factors (Klausner et al., 1976; Young and Holt, 1977; Young and Mutchler, 1976). Important are variations in local weather conditions, depth and type of soil freeze, the position of manure relative to the snowpack, and the timing of application relative to snow melt. Because of the large number of unconstrained variables in the natural environment, there continue to be disagreements on best management practices to limit nutrient movement. Additionally, the form of nutrient is critical. All of these factors impact the mechanisms of nutrient loss: plant uptake, sorption, polymerization, microbial degradation, volatilization, advective movement, and dispersive transport. Consequently, the fate of particulate forms may be very different than soluble, depending on the site and management-specific conditions.
As such, the industry will benefit from continued experiment and field research in an effort to account for very specific, definable variables and nutrient form. Further, because of the extensive list of relevant variables, the development of precise and accurate mathematical models is essential as experimentally modeling the infinite number of site and management-specific conditions is impossible.
Several varieties of pathogens are common in livestock excrement, though not all pose human health risks. Pathogens of concern include the following (USEPA 2004; Rogers and Haines 2005; Sobsey et al. 2006; Pappas et al. 2008; Bowman 2009).
- Bacteria: Escherichia coli (E. coli) O157:H7 and other shiga-toxin producing strains, Salmonella spp., Campylobacter jejuni, Yersinia enterocolitica, Shigella sp., Listeria monocytogenes, Leptospira spp., Aeromonas hydrophila, Clostridium perfringens, Bacillus anthraxis (in endemic area) in mortality carcasses.
- Parasites: Giardia lamblia, Cryptosporidium parvum, Balantidium coli, Toxoplasma gondii, Ascaris suum and lumbricoides, Trichuris trichuria.
- Viruses: Rotavirus, hepatitis E virus, influenza A (avian influenza virus), enteroviruses, adenoviruses, caliciviruses (e.g., norovirus).
As with nutrients, application of animal manure to impervious surfaces such as frozen ground can increase the risk of pathogen loss through runoff events relative to application in other seasons (Reddy, et al., 1981). Cool temperatures have been shown to improve the survival of fecal bacteria (Reddy et al., 1981; Kibbey, et al., 1978). However, field studies found that freezing conditions can be lethal to fecal bacteria (Kibbey, et al., 1978). While these reports hint at fecal bacteria being able to survive cool but not freezing conditions, Kudva, et al. (1998) reported E. coli surviving more than 100 days in manure frozen at minus 20°C. Conversely, freezing and thawing of a soil manure mixture was found to reduce E. coli levels by about 90% (Bicudo, 2003).
More research on this topic is needed to identify conflicting results. Of particular interest is the impact of warming soil temperatures. Slight variations can result in substantial microbial ecological changes. Further, it is well understood that the use of fecal coliform as a pathogen indicator is flawed. New microbial genetics techniques enable the identification of pathogens of greatest risk. Research should monitor for these specific, likely pathogens and their fate during freeze-thaw cycles.
Land application of both solid and slurry excrement has been cited as a vector for introduction of antimicrobials into the environment (Boxall 2008; Klein et al. 2008). In the early 2000s, it was estimated that approximately 60% to 80% of livestock and poultry routinely received antimicrobials through feed or water, injections, or external application (NRC 1999; Carmosini and Lee 2008). Though new best management practices involving non-therapeutic use of antibiotics in livestock are likely to decrease these percentages, estimated changes are not available. Livestock animals are estimated to discharge 70-90% of antibiotics administered through excrement (Massé et al., 2014). Approximately 55% of antimicrobial compounds administered to livestock and poultry are also used to treat human infections (Benbrook 2001; Kumar et al. 2005; Lee et al. 2007). The utilization of such overlapping antibiotics has been cited as a potential cause of antimicrobial resistance (Sapkota et al. 2007), a grave concern in modern medicine (Levy and Marshall 2004; Sapkota et al. 2007).
Antimicrobials are hydrophilic and do not readily break down in the environment and are, consequently, at high risk of introduction into water bodies through runoff events (Chee-Sanford et al. 2009; Zounková et al. 2011). Critically, these compounds show high adsorptive tendencies in soils and clays (Chee-Sanford et al. 2009), thus providing a potential for interception by soil.
Because antibiotics are highly hydrophilic, movement with melt water results, similar to soluble nutrients. Although this mechanism seems clear, movement during winter application is poorly understood. The mechanisms that determine their fate are the same as those listed for nutrients. However, this fate is poorly understood, especially regarding the amount that will reach the field and streams when comparing different seasonal applications. Further, some studies suggest prolonged storage in aerobic manure environments helps facilitate breakdown particularly at higher temperatures (Kumar et al. 2005; Lee et al. 2007; Boxall et al. 2008). However, the question remains whether these effects are present in winter storage.
Fate studies under diverse farm field conditions are essential. Further, the original compound may be broken down into metabolites, some of which may be even more dangerous. All original and breakdown products should be reviewed.
Benefits of Winter Manure Application
The soil health benefits of winter manure application appear to be limited. However, the literature suggests that soil compaction and nitrogen volatilization can be reduced when applying to frozen soil, but at the potential expense of nutrient runoff. There are also many benefits to agriculturalists, as Fleming and Fraser (2000) noted:
- Reducing size and number of manure storage structures.
- Spreading the manure when logistics suite the farmer.
- Reducing soil compaction by avoiding equipment use during compressible soil conditions.
There is little standardization in regard to winter manure application and most states cite the NRCS conservation practice standard 590 for nutrient management (NRCS, 2013). In regard to winter manure application, this standard states the following. “Nutrients must not be surface-applied if nutrient losses offsite are likely. This precludes spreading on: frozen and/or snow-covered soils, and when the top two inches of soil are saturated from rainfall or snow melt. Exceptions for the above criteria can be made for surface-applied manure when specified conditions are met and adequate conservation measures are installed to prevent the offsite delivery of nutrients” (NRCS, 2013). As a continuation of standard 590, the NRCS states that at a minimum the following factors should be considered before winter manure application (NRCS, 2013):
- Field slope
- Organic residue and living covers
- Amount and form of nutrients to be applied
- Setback distances to protect local water quality
- Application timing
The ambiguity in standard practices for winter manure application has led to several different State policies. States with winter manure application guidelines include Ohio, Pennsylvania, Michigan, and Illinois. States that have some form of bans include Vermont, Iowa, Maryland, Indiana, Minnesota, and Wisconsin. States not listed have policies that are identical to the NRCS standard 590.
Based on this literature review, needed research has been identified:
- Review the incidences of emergency spreading on frozen ground versus incorporation during cold weather. Understanding the frequency and timing of emergency spread events is critical to crafting policy and best management practices.
- Evaluate compliance with new rules and if intended impacts are realized, including comparing watershed level of target pollutants across state lines and time lines to view the impacts of this policy change.
- Determine if application in early spring, when soil is saturated and precipitation events are frequent, is more desirable than in winter application before a deep freeze allows for incorporation. Related is the impact of soil moisture content on the fate of target pollutants during thaw events.
- Determine the economic impact on producers and the potential loss of small to medium sized farms. One of the most often cited criticisms of unconditional winter manure application bans is that it can disproportionately disadvantage smaller producers. In a Michigan survey of small producers, 27% of non-CAFO dairy farmers suggested that they would need to suspend operations if such a ban were instituted (Miller et al., 2017). This same survey found that a total ban on winter application in Michigan would collectively cost small farms in that state an estimated $30 million dollars (Miller et al., 2017). An important task is to survey, with time, states that have banned winter manure application to determine if significant shift with regard to average producer size occurred. If so, it is important to consider the resulting economics of the environmental benefits and if national biosecurity decreased with a reduction in producers.
- Verify the effectiveness of risk indices such as the Manure Application Risk Index (MARI), Wisconsin’s Online Manure Advisory System, and other individual states’ P-indices. Many of these indices were developed based on recommendations from research and the practical experience of experts, but literature verifying this is scarce.
- Determine the impact of climate change on winter manure application policies. Climate change effects the duration and intensity of winter temperatures and the frequency and intensity of precipitation events. Such conditions may require more adaptable metrics such as frost depth, depth of snow, ability to incorporate, and forecasted thaw events.
Steven I. Safferman1, Jason S. Smith2, and Rachelle L. Crow3
1Associate Professor; Michigan State University, Biosystems and Agricultural Engineering; Corresponding Author: SteveS@msu.edu
2Teaching Specialist, Michigan State University, Engineering CoRe
3Undergraduate Research Assistant, Michigan State University, Biosystems and Agricultural Engineering
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This project was funded by the North Central Regional Water Network Manure and Soil Heath Working Group and the Soil Health Institute.
The references for the original reports follow:
- Smith, J.S., Safferman, S.I. 2018. Winter Manure Application: Research Needs and Future Direction. North Central Region Water Network, Soil Health Nexus.
- Smith, J.S., Crow, R., Safferman, S.I. 2018. Winter Manure Application: Management Practices and Environmental Impact. North Central Region Water Network, Soil Health Nexus.
- Smith, J.S., Crow, R., Safferman, S.I. 2018. Winter Manure Literature Review – Document Summaries. North Central Regional Water Network, Soil Health Nexus.
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