«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
The Central Valley Regional Water Quality Control Board (CVRWQCB) asked the University of California to establish a committee to review the relevant issues. Dr. Reg Gomes, Vice President of the UC Division of Agriculture and Natural Resources convened a Dairy Manure Management Committee of University experts to address technical questions posed by the staff
of CVRWQCB. Committee members were:
Andrew Chang, Associate Director, Center for Water Resources, University of California Thomas Harter, Associate CE Groundwater Hydrology Specialist, Department of Land, Air, and Water Sciences, University of California, Davis John Letey, Director, Center for Water Resources, University of California Deanne Meyer, CE Livestock Waste Management Specialist, Department of Animal Science, University of California, Davis Roland D. Meyer, CE Soil Fertility Specialist, Department of Land, Air, and Water Sciences, University of California, Davis Marsha Campbell Mathews, Farm Advisor, UC Cooperative Extension Stanislaus County Frank Mitloehner, Assistant CE Air Quality Specialist, Department of Animal Science, University of California, Davis Stu Pettygrove, CE Soils Specialist, Department of Land, Air, and Water Resources, University of California, Davis Peter Robinson, CE Dairy Nutritionist, Department of Animal Science, University of California, Davis Ruihong Zhang, Associate Professor, Department of Agricultural and Biological Engineering, University of California, Davis
The committee met from September 2001 through June 2005 to deliberate these issues:
Amounts of manure and N excreted by dairy cows Distribution of excreted manure and N in dairies Loss of N during storage Fate and transport of manure N and salts following application on cropland Committee members conducted field monitoring and experiments to collect relevant data.
Models were developed to simulate crop yields and nitrate leaching when dairy manure is applied on cropland for forage production.
In the course of the investigation, the Committee was assisted by many technical support staff of
the University of California, including:
David Birkle, Staff Research Associate, Center for Water Resources, University of California Gounglon Feng, Post Doctoral Research Scientist, Department of Environmental Sciences, University of California, Riverside We also acknowledge Chris Amrhein, Ken Tanji, and Alejandro Castillo for their critical comments on portions of this report.
1.5 The Role of this Report This report summarizes the current scientific literature and research results and provides – to the degree possible - answers to the original questions posed.
In addressing the questions, the committee needed to acknowledge that dairies and their environment are inherently complex systems. At a minimum, we distinguish the following
systems within the dairy-environment complex:
animal system animal feed system animal housing system animal health system milk production and food safety system manure collection, treatment, recycling, and storage system manure application system forage crop production system irrigation system the atmospheric environment system soil environment system surface water and groundwater environment systems These systems are tightly linked and closely interact with each other. Yet, each has classically been described, investigated and researched by specific disciplinary researchers in animal science, agronomy and engineering. Only recently have multi-disciplinary approaches been implemented to better understand and characterize the interactions of dairy systems with their land, air and water environments. Addressing the RWQCB questions took efforts of a multidisciplinary team, and most of the research that has been reviewed has been implemented in a disciplinary (i.e., single system) fashion. However, we attempted to interpret and summarize literature relevant to the questions that were posed. In the process, we found that our scientific and technical language, as well as the multitude of methods used among the various disciplines involved, were not always compatible thereby forcing careful interpretation of available information. Needless to say, such interpretation is not unique. In this document, we have attempted to leave behind disciplinary bias as much as possible and provide the best possible answers given the background and experience of the committee members. Clearly, the answers may not be the only possible answers – depending on the point of view; and some can be formulated in either a much narrower or in a more general way. Hence, we expect both, constructive critique and disagreement with the results provided.
Most importantly, we find that these questions touched on an interdisciplinary field of research around animal farming and environmental issues that is only now being established. New results are being published almost daily. This document reflects our knowledge as of early 2005. We expect that some of the questions will need to be re-examined within less than 5 years, while the answer to others may not change for a decade or more. We fully understand that this poses a critical challenge to the regulatory agencies and to the dairy industry, who urgently seek longterm solutions (rather than intermediate fixes) to addressing environmental issues on dairies.
Chapter 2 - Manure and Nitrogen Excretion
2.1 Introduction Amounts of manure and nitrogen (N) excreted by dairy cows have been reported in the literature since the 1960s, although the values have not been consistent. These discrepancies create uncertainties in establishing and implementing dairy manure management plans. In a 1973 correspondence, the Water Quality Task Force of the UC Committee of Consultants concluded that an average dairy cow in California produced manure N at the rate of 0.4 lb/head/day (182 g N/head/day). This conclusion was reached by conducting feed-milk-manure N mass balance calculations, and by chemical analyses of feed and milk samples collected in the state, thereby accounting for the N content of feeds that are in dairy rations and the N content of milk produced. The N excreted by a dairy cow was the net difference of N input (i.e., amount of N in feed consumed) and N output (amount of N in milk produced), and mass balance estimates were used to derive estimated total manure and N excretion for dairy cows in the Santa Ana Basin, Central Valley and North Coast regions (Table 2-1).
Table 2-1: 1973 Water Quality Task Force of the UC Committee of Consultants Estimate of Total Manure and N Excretion of Dairy Cows in California. N excretion was obtained from the difference between intake and output. Although physiologically inaccurate (some of the N goes to animal growth), it is a reasonable approximation for adult animals.
The 1973 Water Quality Task Force of the UC Committee of Consultants found that the N excretion values estimated by their mass balance method “exceed the actual amount excreted by about 0.4 lb/cow/day as determined by chemical analyses of the wastes” and reconciled the discrepancy between the two estimates by concluding that “the difference is believed to be due to denitrification, within the ruminant animal, of a substantial portion of the input N”.
Thus the N excretion value recommended by the 1973 Water Quality Task Force of the UC Committee of Consultants was approximately 50% of the mass balance calculated N excretion.
The assumption that much of the ingested N was lost through ammonia volatilization from, or denitrification within, the intestinal tract of the cattle could not be confirmed based upon the technical literature. Indeed, the rumen of dairy cows is not a conducive chemical environment for oxidation of organic N compounds in feed to nitrate, the precursor of denitrification, and unless the N was already in an oxidized form, precursors for denitrification (i.e., nitrate and nitrite) would have been absent.
Since 1973, the nutritional management of dairy cows has improved in California. Advances in dairy nutrition, genetics and management have resulted in slightly larger cows and sharply increased milk production. With the changes in rations, the quantity and chemical composition of manure were expected to change. Tomlinson et al. (1996) demonstrated that N excretion by dairy cows could be predicted from dietary intakes of dry matter and its N content. Based on a mass balance of dietary N intake and the N content of the milk, Van Horn et al. (1998) estimated that N excretion by lactating cows varied from 0.73 to 0.85 lb/head/day (330 to 384 g/head/day), figures that are in general agreement with values frequently reported in the literature (e.g., Arogo et al., 2001).
In this chapter, we review currently accepted methods for estimating nutrient excretion from dairy cows (section 2.2), apply these methods to develop best estimates of average excretion rates at California dairy farms (section 2.3), discuss regional differences in excretion rates (section 2.4), and summarize the role of diet on excretion rates (2.5).
2.2 Methods to Estimate Manure Nutrient Excretion by Cows At least three methods can be used to estimate manure and nutrient excretion by groups of dairy cows, although the choice of the most appropriate method depends on how the results will be used. For example, calculations made from newly revised Standard Table D384.2 (American Society of Agricultural Engineers - ASAE) are most useful when planning a new dairy (i.e., when no cows actually exist), whole farm mass balance models document nutrients entering and exiting the entire dairy operation (i.e., document the whole farm balance but provide no information on nutrient balances in any sector of the facility), and estimates based on farm sector mass balance models1 that use production, diet and physiological status of defined groups of cattle within the dairy are useful in documenting nutrients entering and exiting the entire dairy operation while allowing evaluation of the nutrient balances for any defined sub-group of cows (i.e., are useful in identifying sectors of the dairy where efficiencies may be low and so allow dairy operators a means to improve overall dairy operation efficiency). The following discussion provides brief reviews of these approaches.
2.2.1 Method 1. Calculations made Standard Table D384.2 of ASAE
Historically, manure excretion has been reported relative to body weight. However for lactating cows, it is far more appropriate to report excretion on per unit of milk produced as milk production of cows varies to a much greater extent than does body weight, and because cows eat Note that, in the context of estimating N excretion, the terms “loss” and “mass balance” refer to the animal system itself, not to other environmental systems such as atmospheric or groundwater losses from lagoons or N fluxes from the production area to the land application area. These are discussed in later chapters of this report.
more feed as their milk production increases. In addition, breed differences are much smaller when manure and nutrient excretion are expressed per unit milk produced.
The ASAE has revised its standards for manure production based on data collected from total manure collection studies with cows at numerous US research facilities. Regression analyses were conducted for manure and N excretion for lactating cows, dry cows, replacement heifers and young calves, in order to create tabular values.
As an example, use of the new ASAE equations estimates of dry manure and N excretion for an ‘average’ California lactating dairy cow weighing 1375 lb and producing 88 lbs of milk/day while consuming 1.37 lbs of N/head/day, were 19.5 lbs and 0.97 lbs/head/day. Similarly, using the new ASAE equations for a California ‘average’ dry dairy cow weighing 1660 lb and consuming 0.56 lb of N/head/day, dry manure and N excretion was estimated to be 10.8 lbs and
0.49 lb of N/head/day, respectively.
The ASAE approach is extremely useful as a means to estimate future N balances when planning a new dairy (i.e., when no cows actually exist). Once the dairy has been built, actual production levels may vary and result in substantially higher or lower excretion levels than projected from the ASAE method.
2.2.2 Method 2. Estimates based on whole farm mass balance models
Whole farm mass balance models (with respect to the animals) are most useful in documenting manure nutrient outputs from entire commercial dairies. These mass balances are done by documenting total N inputs and exports from the animals in the animal operation (Koelsch, 2001). The approach requires the dairy facility operator to measure and quantify all sources of N entering and leaving the animal operation. Data can be obtained from records of feed purchases as well as sale of cattle and milk.
From a regulatory perspective, such animal-based mass balance models (e.g., Meyer and Robinson, 2002) offer a site specific method to quantify the N excretion in a dairy operation, but provide no information on the nutrient balances in any animal sector of the facility since the evaluation is based upon a complete input/output balance of all nutrients to/from an entire dairy herd.
The whole farm mass balance approach offers little insight as to why a particular facility may be out of compliance, as it provides no information on the efficiency of the definable and measureable components of the facility, and none at all on how to regain compliance.
2.2.3 Method 3. Estimates based on farm sector mass balance models Farm sector mass balance models (individual mass balances on each identifiable group of cattle on the dairy) are useful in documenting manure nutrient, including N, outputs from entire commercial dairies as the sum of its identifiable cattle groups. This approach requires the dairy facility operator to measure and quantify feed characteristics, physiological conditions of the cattle, and the conditions of the cattle’s environment for all identifiable groups of cattle on the dairy. Data can be obtained from herd records, observations of cows, climatic information databases and feed analyses. Animal mass balance outputs, by cattle group, are based on consumption of dry matter, energy, N and minerals. Estimates of the nutrient content of feces and urine are calculated by subtracting nutrients partitioned into product, growth and reproduction from nutrients consumed.