A Systems Approach to Improving
Phosphorus Management on Dairy Farms

J. Mark Powell
USDA/ARS Dairy Forage Research Center, University of Wisconsin, Madison
Larry D. Satter
James C. Converse
Douglas B. Jackson-Smith

Over the past decade many dairy farms have been undergoing great change to remain economically viable. Many are increasing herd size and importing more and more feed. Excessive soil nutrient accumulation, runoff and the pollution of surface and ground waters are the most pressing environmental challenges facing these farming systems. The survival of many dairies will depend increasingly on farmers' ability to comply with ever-stricter environmental regulations, especially those associated with proper manure management. However, the added costs and large labor input required to properly handle, store, transport and land-spread manure with little confidence of an economic return deters many from managing manure more effectively. There are no easy solutions to the manure problem. Reduced environmental impact require solutions to an array of system problems, such as excessive feeding of nutrients; poor manure handling, storage, and land-spreading techniques; inadequate land and in some cases inappropriate cropping systems to properly recycle manure nutrients; farmers' unwillingness and/or inability to adopt improved practices, etc. The "manure problem" is taking on greater significance. Public concerns over the negative impact of agriculture on the environment are heightening. Legislation aimed at controlling farmer practices that potentially pollute surface and ground waters is becoming more strict. Whereas nutrient losses from agricultural (as well as natural) systems are inevitable, a continuous challenge facing agriculture is to minimize nutrient losses through good management.

Improper manure management can contaminate the groundwater with nitrates. The primary concern with phosphorus (P) mismanagement is runoff into lakes and streams and enhanced eutrophication. Agricultural P management options are targeted at minimizing P imports onto farms while controlling runoff and erosion. However, even with proper manure credits and appropriate use of commercial fertilizer, many dairies are consistently accumulating P. Imports of P to the farm in the form of dietary supplements and other feeds simply exceed exports in the form of milk, cattle, and surplus grain or hay. Increasing levels of soil P in excess of plant requirement increases the risk of environmental damage (Sharpley, 1996). Options for efficient utilization of manure include basing application rates on site susceptibility to runoff (P-based strategy) or leaching (N-based strategy); linking the number of animals to the area of land available for manure utilization; formation of cooperatives that can more economically compost or concentrate manure to increase the economic distance it can be transported from its source, thus increasing the area of land for application; establishment of cost sharing programs so that both the consumer and producer share the economic burden of environmental sustainability; and expansion of education and extension programs highlighting the nutritive and mulching values of manure to non-producing farmers (Sharpley and Withers, 1994).

Balancing nutrient inputs and outputs, and proper manure management are a few of the many challenges facing dairy producers. Producers seek to optimize a range of goals (only some of which consider environmental impacts) and they face a number of institutional, economic, informational and social-psychological constraints that limit their ability to utilize practices aimed at improving nutrient management (Nowak et al., 1997). As a result, efforts to improve P management on dairy farms need to be based not only on a sound scientific understanding of key interactions between plants, animals and soils, but also on how farmer management affects agricultural productivity, farm profitability and the environment. Collaborative assessment of feeding recommendations and practices are essential to achieving a mutual understanding of factors affecting feed management and how this impacts the types and amounts of manure nutrients that need to be recycled through cropland.

The National Research Council (NRC) recommends that the typical dairy cow diet contain between .34 and .42% P, and early lactation diets should contain .49% P. However, it is common to see dietary P levels between .5-.6% of dietary dry matter for high producing herds (Howard and Shaver, 1992). There is evidence that the current feeding standards overestimate the amount of P required in the dairy cow diet. While more recent studies are very supportive of the view that diets containing approximately .35% P are adequate for lactating dairy cows, critical studies of P requirements with high producing dairy cows (9000-12,000 kg milk/yr) are sorely needed.

The uncertainty about what amount of P should be fed to lactating cows is illustrated by the different feeding standards used in Europe and North America (Table 1). The major reasons for these differences stem from the difficulty in determining endogenous losses of P and P availability in the gut (true absorption). These feeding standards assume that only 50 to 70% of dietary P is potentially available. Feeding standards are underestimating availability of dietary P (particularly in the US), leading to serious over supplementation of P. Also, it appears that the actual P content of feedstuffs may be higher than the NRC tabular values that are widely used in formulating dairy diets (Table 2). This appears particularly true for alfalfa, a major component of dairy rations. While it is generally considered that alfalfa does not accumulate significant P under luxurious P supply, this group of over 4000 samples suggests that alfalfa, for some reason, can have substantially higher P content than indicated by the NRC feed composition tables.

Since almost all P is excreted in the feces, it may be possible to extract P from manure by settling and or by solid/liquid separation. A preliminary study was conducted to determine the P concentration in various fractions of liquid dairy manure. Manure was separated into an effluent and fiber fraction using a screw press. Additionally, the effluent was centrifuged into a decant and particulate fraction. Approximately 75% (50.5% in fiber and 24.3% in particulate) of the solids in the manure were removed from the waste stream. These two fractions contained approximately 71% (17.5 % in the fiber and 54.1% in the particulate) of the total P in manure. There appears to be potential for extracting P from the manure stream. Additional P may be extracted via flocculation of the remaining solids in the waste stream. The fiberous solid stream, with the addition of a small amount of bulking material, could be composted and utilized in off farm locations. The effluent stream will have a much lower solids content, making it easier to pump without adding dilution water. Pumping liquid manure to fields is often more economical than hauling it.

Environmentally sound manure application strategies depend on the land type (slope, texture, nutrient attenuation potential), amounts and method of manure application (surface applied or incorporated), timing of application (months before or just prior to planting) and the nutrient demands of the subsequent crop to be grown. Therefore, strategies for reducing nutrient losses from manure must be site specific. For example, if the potential for nitrate leaching is high, then N should be a priority management consideration. If runoff and erosion potential exceed leaching potential, then P should be the main element driving application rates. This strategy will mitigate the excessive build up and loss of soil P and at the same time lower the risk for nitrate leaching to ground water. However, many farmers have no recourse than to base manure recycling plans on crop-N needs. They do not have sufficient land for recycling all manure-P through plants. Nutrient management plans based on application of sufficient manure to meet crop N demands results in P application in excess of crop P demands and create P pollution on soils having very high soil-P levels, especially on sloping land (Sharpley, 1996).

The USDA/ARS Dairy Forage Research Center and University of Wisconsin were awarded a National Research Initiative Grant to develop improved feed, manure and land management strategies that will reduce P accumulation and loss from dairy farms. The project aims to create a unique forum for interdisciplinary research in partnership with producers and agribusiness; elucidate production risks and environmental hazards associated with dietary-P recommendations and practices; provide information on the relationship between dietary-P inputs and manure-P outputs; investigate the feasibility of processing manure to reduce its P content; and recommend loading rates for different manure fractions on sloping soils under different tillage systems. By linking investigations on feed P management, manure P separation and land application of manure fractions, the project seeks to systematically evaluate how improved P management in one production component (e.g., feed) affects P cycling in other production components (e.g., P excretions, soil P levels) and the relative impact of each component's management on P losses to the environment. Results of each activity will be assessed in relation to the primary goal of reduced environmental impact. It is anticipated that positive environmental impacts will be derived from improvements in P feeding which could result in 20% less P imported and excreted; manure P separation which can increase manure P use efficiency by 30%; and manure/land management techniques which can reduce long-term soil P accumulation and runoff by 15%. Not all practices will be adaptable to all dairy systems, but one or more practices should be suitable for reducing environmental impacts under most conditions where feeds are imported, dairy are fed in confinement, and producers rely on their land base for recycling manure nutrients.

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