Department of Agronomy, Pennsylvania State University,
116 Ag Science and Industry Building, University Park, PA, 16802, U.S.A.
Take Home Messages
Traditionally, nutrient management has been concerned with optimizing the economic return from nutrients used to produce a crop. Whereas, once agronomic science placed the greatest emphasis on determining the optimum yield response from fertilization, the development of animal agriculture demanded that evaluations of economic returns include animal as well as crop response. To these two concerns must now be added concerns about the impact of these nutrients on environmental quality. Leaching of nitrogen (N) through the soil can raise groundwater nitrate levels above the EPA drinking water limit adversely affecting the health of young children and livestock. Surface movement of N and phosphorous (P) in runoff increases levels of these nutrients in surface waters, such as the Chesapeake Bay, and there are other consequences of surface water pollution which can lead to eutrophication and fish kills.
Contrary to common belief, the problems that we have with nutrient pollution are not completely the result of mismanagement by farmers, but are more a result of our agricultural systems evolving with no direct costs associated with environmental quality. The intensive animal agriculture that many blame for the nutrient problems we face developed because of strong economic incentives for this type of agriculture. Thus, managing nutrients to address these additional concerns will mean more than just eliminating bad management it will mean changes in our agricultural systems. To meet this challenge will require innovative management approaches.
Nutrient Management Process
Nutrient management generally involves decision-making about a wide range of farm operations. The decisions in this process are made as frequently as several times a day to as seldomly as once every five years or more. Decisions may deal with the day-to-day details of farm operations, such as spreading manure on a specific field on a particular day, or with the long-range future of an entire farm, such as the decision to build a manure storage. Nutrient management is an integrated ongoing farm process with several key activities.
The process starts with an assessment of the overall nutrient balance on the farm. An initial assessment of the farm and the potential environmental impacts of the existing farm operations is an effective starting place in many situations. In the assessment the approximate nutrient balance of individual fields, groups of fields that are treated similarly, or even the whole farm can be determined depending on the purpose of the assessment. The outcome of the assessment can be used to determine what options should be considered for farm nutrient management to protect the environment while producing crops and animals. The extent of nutrient management assistance required to change the farm operation will also be influenced by the type and extent of the options to be incorporated.
Once an assessment has been accomplished, management options for dealing with the situation can be explored. If there is an overall nutrient imbalance on the farm, options for developing a plan to get rid of excess manure will be the main emphasis. If nutrients on the farm are near to balance, then a field by field plan for manure utilization will be the priority. If the farm is in a nutrient deficit situation, then the plan will emphasize maximizing the efficiency of manure use on the farm and on supplementing these nutrients with fertilizer or maybe manure from a farm with an excess. The nutrient management options can be specific practices, such as incorporating field-applied manure soon after application, identifying other landowners who may be interested in having manure spread on their fields, or more far-reaching possibilities, such as postponing a planned expansion of the livestock housing facilities on the farm. The assessment and the options selected can be the basis for many decisions that will be made in the development of a farm nutrient management plan to allocate the manure and to determine any supplemental fertilizer requirements.
These options are then developed into a nutrient management plan for the farm. Implementation of a nutrient management plan involves both the actual activities called for in the plan plus the appropriate recording of those activities so that the effectiveness of plan implementation can be assessed. The success of the management plan can be evaluated in a repeat of the assessment. It is important to recognize that this process must be comprehensive and continuous.
Farm Nutrient Cycles
Plant nutrient management decisions deal largely with farm nutrient cycles. Understanding various types of farm organization and the flow of plant nutrients to, from, and within farms, can be helpful in understanding the nutrient management problem and in practically all activities associated with the nutrient management process for crop production and environmental protection. This is especially true in evaluating the nutrient management situation on farms. A major goal of nutrient management for crop production and to protect the environment is the balance between agronomic crop requirements and the supply of nutrients available on the farm. Based on the evaluation of nutrient movement, approaches to nutrient management that are sensitive to specific farm situations and existing strategies of farm management can be developed.
On a cash grain farm the nutrient flow is a fairly simple, straight through flow (Figure 2a). On such farms nutrients leave the farm in the crop produced and fertilizer or other sources of nutrients are brought onto the farm to replace this removal. On this type of farm there is a very direct linkage between field nutrient management and the expected economic returns to the farming activities. Anything that improves the efficiency of the nutrient management should increase the economic returns to the farm. Thus there is a strong economic incentive for good nutrient management.
On a primarily feed-self-sufficient livestock farm, nutrients are harvested from the farm fields with the crops. The crops are then used as feed in the animal enterprise resulting in some nutrients (usually less than 25%) leaving the farm in the animal products and the rest of the nutrients being returned to the farm fields in the manure. Nutrients may be added to this cycle as fertilizer on the farm fields and as nutrients contained in feed purchased for the livestock operation, but the primary nutrient flow is from the farm field to the barn and back (Figure 2b). The most important consideration in managing nutrients on this type of farm is accounting for all sources of nutrients, especially as purchased feed increases, and effectively recycling them in the cropping program. This type of system has been the common one on dairy farms where we have historically had two acres or more for every cow and her replacement. There are two very important differences in nutrient management on this type of farm compared to the cash crop farm. First is the considerable recycling of nutrients. On the dairy farm only a very small proportion of the nutrients needed by the crops must come from external sources. The second difference is that on a dairy farm neither crop production nor fertilizer use are directly connected to the output of such farms. The linkage between nutrient management on the farm fields and economic returns to the operation is just as important, but much less direct than on the cash crop farm. On a dairy farm the main consideration in crop management is to produce adequate high quality feed for the cows because the money is made from the dairy. Farm performance depends on the animal husbandry skills of the farmer, not just success in crop production. On a farm organized this way the decisions about plant nutrient use in the fields are not as sensitive to the economic or agronomic criteria in crop production as on the cash-crop farm. Often agronomic and economic principles of crop production are compromised in the effort to ensure that there will be adequate, high quality feed for the cows.
There is an accelerating trend toward greater intensification in the dairy industry which has resulted in a third system which is a combination of the first two (Figure 2c). With larger numbers of animals on farms, the on-farm cropland can no longer support the feed requirements of the dairy. Therefore, much larger proportions of the feed requirements of the animals must come from off the farm. Usually this comes from other cropland or a cash crop farm either locally or at some distance. Even if additional land is acquired to meet the feed needs of the cows, the distance and amount of manure to be hauled can increase substantially if the nutrients are to be spread uniformly over potentially suitable crop areas. This often results in more manure in limited areas. In reality, the cash-crop farm and the intensive, modern livestock farm are connected by the flow of feed. However, as was discussed previously, only about 25% of the nutrients in the feed the animals eat leaves the farm in milk and cull cows, the rest remains in the manure and is applied to the cropland.
Thus, nutrients usually do not cycle back to their original locations. The application of nutrients to the fields on these farms is not closely related to the major production activity of the farm; selling animals or animal products. This often will result in an excess of nutrients on the farm and a high potential for environmental problems. The field-based economic and agronomic incentives that can be effective incentives to manage nutrients on a cash-crop farm, and that will also minimize negative environmental impacts, are not as significant on the intensive livestock production-oriented farm. Further, field-based agronomic practices may be of limited effectiveness in treating the total quantity of nutrients on the farm because of the small land area on the farm relative to the number of animals. It is very likely that plant nutrient management to protect environmental quality can not be accomplished solely on the farm where the animals are housed. Successful management of nutrients to protect the environment will depend on support from off-farm people and organizations. Neighbors with land for manure application could cooperate by providing land for manure distribution. Off-farm organizations may deal with manure hauling to locations where the manure can be used directly or transformed into another product such as compost.
Finally, since dairy animals often spend part of their time outside of buildings, the number of animals in barnyards and holding areas can be greater than when the number of animals supported on the farm was less. The result is that the areas around farmyard facilities can be degraded and become sources of nutrient losses from the farm directly to the environment. This is an area that is often overlooked in nutrient management discussions.
The bottom line conclusions from analysis of these nutrient cycles is that nutrient management varies on different types of farms and is directly related to the intensity of the dairy operations and thus the external feed requirements. Even though many people think about fertilizer when we talk about nutrient management, feed is a much more important component of the system especially as dairies intensify.
Selecting Appropriate Nutrient Management Strategies
Nutrient management plans will vary depending on the situation on each farm. As noted above, the intensity of the dairy operation is a key factor in the nutrient cycle on a farm. We have attempted to categorize farms and target nutrient management efforts on the basis of intensity of the livestock operation. Two indicators have been identified to be useful for this purpose. The first is the percentage of total feed that comes from off farm sources. Alternatively, the number of animal equivalent units (AEUs) per acre is also a good indicator of livestock intensity. An AEU, as used here, is 450 kg of live weight. Thus a 600 kg dairy cow would be 1.3 AEUs. A common rule of thumb is that a cow and her replacement equal about two animal units. Either indicator will work, but the animal density per acre generally seems to be the easier one to use. The following categories are helpful in selecting appropriate nutrient management strategies. Note that these categories are not hard and fast rules, but rather they are general guidelines to help farmers make management decisions.
Low Intensity Farms
Low intensity farms are those where there is usually more cropland than is needed to meet the feed requirements of the animals and consequently there is not enough manure produced to meet total crop nutrient needs. These farms are characterized by animal densities less than 1.25 AEUs per manured acre and less than 60% of the feed for the animals comes from off the farm. In this group the strategy will be to utilize soil tests and manure analysis to assure distribution and timing of manure applications to maximize nutrient utilization from the manure and minimize purchase of commercial fertilizer. In short, maximize efficiency of nutrient use. Many of the standard best management practices for manure management will be applicable. Practices such as storing manure, applying it in the spring to non-legumes, incorporating it immediately, etc. will be used. The environmental impact of these operations should be nominal except where there is gross mismanagement. Changes in these operations are likely to have only small beneficial effects on the environment. The bottom line is that there is the possibility for substantial economic benefits to the farmer for developing and implementing an improved manure management plan on this category of farm.
Medium Intensity Farms
Medium intensity farms are those that generally produce almost enough feed for the dairy and where there is likely to be enough manure to meet total crop nutrient needs. Usually these farms have animal densities between 1.25 and 2.75 AEUs per manured acre and 60 to 90% of the feed for the animals comes from off the farm. In this group the strategy will be to utilize soil tests and manure analysis in conjunction with appropriate management practices to match as closely as possible nutrients available in manure with crop needs over the entire rotation. In most cases the emphasis will be on maximizing the safe utilization of the manure nutrients. Where there is a slight excess of nutrients efficiency may be intentionally compromised to increase the utilization of the manure as long as it does not create an environmental threat. An example of this is applying manure to legumes. Even though the legumes do not need the N in the manure, they will utilize it. Intense management will be needed to provide the most favorable economic situation while protecting the environment. There is good potential for environmental benefits from improved management on these farms. However, generally the economic impact on these farms will be small. A detailed manure management plan will probably be necessary on these farms. Also, other changes in the overall farm management, such as altering the cropping system, may be necessary on this group of farms. Most farmers in this group will probably want to take advantage of technical assistance from public agencies and/or private consultants in developing an implementing a manure management plan.
High Intensity Farms
High intensity farms are those where the feed demands greatly exceed the capacity of the cropland and thus where livestock manure production significantly exceeds total crop nutrient needs. Usually these farms have animal densities greater than 2.75 AEUs per manured acre and over 90% of the feed for the animals comes from off the farm. In this group the objective will be to utilize every available means to remove all excess manure not needed for crop production. Alternative off-farm uses for the manure will need to be explored. In most instances this will mean locating a market for the manure and arranging the logistics of transportation and appropriate application. The on-farm plans for this group of farms will involve determining the maximum amount of manure that can be safely disposed of on the farm. However, in most cases the available land and the high residual nutrient levels in the soil may severely restrict on-farm use of the manure. Detailed nutrient management plans will be important for the farms where the manure is ultimately utilized. This group of farms has the highest potential to negatively impact the environment. In many cases, unless a favorable marketing arrangement can developed, implementing improved nutrient management on this group of farms will have a negative economic impact on the farm. Assistance from public agencies and private consultants, manure brokers, and manure haulers will be critical to improving nutrient management. Unfortunately, this is not currently well developed in many regions.
Field Management of Crop Nutrients
The first step in developing a plan for manure management is to collect information on the manure production for the farm, nutrient content, and application system(s). Manure production can best be determined from the amount of manure in manure storage. However, not all farms have a manure storage system and it is not always possible to determine the amount of manure in a storage even if there is one on a farm. In this case manure production can be estimated from animal numbers, animal weights, and time of confinement. As a rule of thumb, a dairy cow produces about 20 tons of manure per year. The actual amount to be handled and spread will vary from this depending on how much of the manure is actually collected and the amount of bedding or dilution water added.
Manure nutrient content should be determined by manure analysis. "Book" values for manure nutrient content are good as averages for a type of manure on many farms, but because of farm to farm variability they are of no value for making decisions on an individual farm. Results of analysis of many dairy manure samples in Pennsylvania has shown that there is a very wide range in nutrient content for nominally similar samples of manure from different farms. Generally we must assume 100% error in the manure nutrient content if we use book values to develop a manure management plan. On the other hand a single manure sample may not be much better. Even in agitated storages there can be considerable variation in the manure analysis. A manure storage should be intensively sampled at least once to characterize the variability. This could be done by sampling every so many loads as the storage is emptied or by taking a sample every time there is an obvious change in the manure consistency. An example of the variation in percent solids and N analysis of dairy manure as a storage is unloaded is shown in Figure 3.
The behavior of the nutrients in manure in terms of how they can contribute to the nutrition of a crop must be known if the true fertilizer value of the manure is to be determined. You cannot determine the fertilizer value of manure simply by multiplying the nutrient content by the current fertilizer nutrient price. Such a calculation will give an indication of the potential value of the nutrients in the manure, but the actual fertilizer value realized will depend on how the manure is handled and used. Approximately 50% of the total N remaining in dairy manure after storage is potentially available to the crop during the year the manure is applied. How much of that N is actually taken up by the crop, however, is significantly affected by the method of application. If the manure is not incorporated only about 20% of the total N will be available. The N that is not immediately available will become available over time as the organic material in the manure decays and releases the N. The amount of the residual N that will become available in any given year is very difficult to predict. However, it can be estimated from the history of previous manure applications using a decay series and should be taken into account. For corn, the Pre-Sidedress Soil Nitrate Test has been used successfully in the humid regions of the east to help estimate residual N from previous manure applications.
Another important consideration in managing manure nutrients is the effect of the cropping system. Different crops have very different nutrient requirements. Manure nutrients, especially N, are used more efficiently by corn and cereal grains than by legumes. But, in general, if manure is applied to meet the N needs of a continuous grain crop, P, and potassium (K) will likely be applied in excess of crop needs and eventually build up to excessive levels in the soil. Forage crops, to which manure is not applied, planted in rotation with grain crops receiving manure will help remove the excess P and K and keep the three nutrients in balance over the rotation. This is illustrated in Figure 4. In each example in Figure 4 manure was applied to totally meet the N needs of the corn crop. With continuous corn (Figure 4a), note the large excess of P and K that are applied. In the rotation example (Figure 4b), when manure is applied to meet the N needs of the corn, the unmanured forage crop in the rotation uses the excess P and K and some fertilizer P and K will probably be required to meet the needs of the forage crop. This effect will vary with different rotations, but the concept will be the same.
Regular soil testing is helpful to monitor the balance of P and K over the crop rotation. The ideal pattern of soil test levels for P or K is illustrated in Figure 5. Note the buildup of nutrients in the corn part of the rotation and then the subsequent draw-down in these levels in the unmanured forage part of the rotation. The bottom line is that over the rotation the soil test levels are in the optimum to high range.
If manure is also applied to the forage crop in the rotation, for example, to utilize excess N present on the farm, this rotational balance will be disturbed. If, in addition to the manure applied to the corn, manure is also applied to the forage crop at a rate to equal the N removal by the forage crop the N remains in balance, but now the rotation balance for P and K discussed earlier no longer hold. Large excesses of P and K will now be applied and soil test levels will increase. This raises serious environmental concerns because of the excess P. If manure is to be applied to the forage crops in the rotation it should be applied at a rate based on the P needs of the forage crop.
Developing a nutrient management plan requires soil tests for the entire farm or land where manure will be applied, cropping and manure application history, planned crop rotation, and other characteristics such as field location and soil characteristics. Using this information, the fields on the farm can be prioritized from high to low for manure applications as outlined in Figure 6.
After the fields have been prioritized and the manure amount, nutrient content, and nutrient availability have been established the available manure is allocated to the fields in the priority order. Rate calculations are based on the priority nutrient (usually N or P). If the rate is based on N the manure analysis must be adjusted for N availability and the crop requirement must be adjusted for residual N from previous manure applications and legume crops. The best approach to determining an environmentally sound manure application rate would be to use the rate that does not apply an excess of any nutrient over what is needed for the crop as indicated by a soil test. In our experience, P is usually the limiting nutrient when this approach is followed. In fields where there is a crop rotation this limit should be calculated on the basis of the nutrient needs of the entire rotation rather than just on the current crop. However, this can still be very restrictive. Currently in Pennsylvania, we usually base the manure application rate on N and try to limit the P and K applications based on the rotation requirements. In practice many farmers still apply excess P and K. Since the major loss pathways for P and K are runoff and erosion, good soil conservation practices are critical in minimizing the environmental effect of excess P applications in manure.
Once a manure rate has been established for each field on the farm, the rates are adjusted for practicality. This usually results in the fields being grouped into one or a few standard rates that the farmer is able and willing to apply. Using these standard rates the available manure is allocated to the fields in the priority order until all of the manure is allocated or all of the fields have been used up. If all of the fields are used up and there is still manure left then the emphasis shifts to developing a plan for dealing with the excess. This usually means finding ways to get the excess off of the farm. Sometimes, if the excess is small, the cropping program can be adjusted to utilize the excess manure nutrients. Note that this priority order has nothing to do with the order that the manure is spread on the fields. That is an operational management decision. The main point is that when the manure is spread, it is spread on the high priority fields rather than on the low priority fields. Manure should always be spread uniformly on fields at the planned rates. Nutrient losses, pollution, and odor are reduced if manure is spread as near as possible to the time when plants will use the nutrients and incorporated into the soil as soon as possible after it is spread.
Finally, the nutrients supplied in the manure must be compared to the needs of the crop to determine if additional nutrients are required or if there is a serious excess of any nutrient being applied. Deficiencies are taken care of by applying supplemental fertilizer nutrients or manure from other sources. Serious excesses must be evaluated in light of the nutrient needs of the crop rotation and may require a change in the manure application plan.
Nutrient management is an on-going process of continually evaluating and adjusting nutrient management plans based on the strategic goals of the farm and the practical operational realities of dairying. The unique interaction of crops, feed, fertilizer, and manure: where they come from, where they go, and transformations as they cycle through a farm will determine the nature of the nutrient management process on each individual farm. Understanding plant nutrient cycles and how they differ on different types of dairy farms is critical to developing a nutrient management plan that optimizes the feed production and economics of the farm while minimizing the potential environmental impacts. Nutrient management is not a one size fits all procedure. If it is to be a practical and successful management operation it must be tailored to fit a specific farm and farmer.