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a Dep. of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061
b Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695
* Corresponding author (rmaguire{at}vt.edu).
Received for publication December 20, 2006.
ABSTRACT
Diet modification to reduce phosphorus (P) concentrations in manures has been developed in response to environmental concerns over P losses from animal agriculture to surface waters. We used USDA-NASS statistics on animal numbers and crop production to calculate county scale mass balances for manure P production, P removed in harvested portion of crops, and the potential effects of diet modification. Although spreading manure evenly over all crop acreage within a county is unlikely to occur, these calculations give a good indication as to the impact diet modification to reduce P can have at a regional or national scale. There was a high degree of regional variability in manure P surpluses (e.g., with the large crop acreages in the grain belt leading to large P offtake in crops preventing most P surpluses). In 89% of counties, there was a deficit of manure P relative to crop P removal; therefore there was a manure P surplus in 11% of counties. Diet modification decreased the percentage of states with a manure P surplus from 11 to 8%, a decrease of approximately 27%. Diet modification decreased the percentage of counties with the greatest surpluses of manure P (>30 kg ha–1) from 3% of all counties to 1%. Diet modification to decrease manure P is an important part of strategies to alleviate environmental concerns associated with surplus manure P in many areas, but additional strategies to deal with manure P surpluses are needed in some areas.
MORE phosphorus (P) is often produced in manure in areas with intensive animal production than local crops require, leading to a surplus of manure P in many areas of developed countries, such as the USA (Kellogg et al., 2000). The surplus manure is commonly applied to land, which can cause increases in soil test P and P losses to surface waters, leading to decreased water quality (Sharpley, 1995; Sims et al., 2000). With intensification of animal production set to continue in developed countries and meat consumption set to double in developing countries over the next 20 yr, innovative manure management is required if water quality is to be protected (Sims and Maguire, 2004).
One strategy to address the excess manure P produced in areas with intensive animal production is to reduce the P concentration of animal diets. Feeding to animal P requirements, increased use of phase feedings, more accurate prediction of P availability in feeds, feed additives such as phytase (that can increase the digestibility of P fed), and novel feed ingredients (such as high available P grains) can reduce the P concentration in feed ingredients (Council of Agricultural Science and Technology, 2002). There are several factors that lead to P overfeeding, such as unknown digestibility of P in feed ingredients and uncertainty over exact animal requirements. The most beneficial feeding strategy depends on the animal species because monogastrics (swine and poultry) are unable to digest phytate-P, whereas ruminants (dairy and beef) have the ability to digest phytate P (Council of Agricultural Science and Technology, 2002). Therefore, using high available P corn and soybeans and phytase in monogastric diets can reduce total P in manure and reduce P fed (Maguire et al., 2005a). However, because ruminants can digest phytate-P, the main strategy is reducing P fed to more closely match animal requirements. Most animal diets are supplemented with calcium phosphate, so reducing P fed is straightforward once the requirement is identified.
Several studies have demonstrated large decreases in total P excreted when these dietary strategies are used. For example, Maguire et al. (2004) reported reductions in total P of up to 31% in broiler litter and 38% in turkey litter when P was fed closer to animal requirements and phytase was used. By feeding high available P (low phytate-P) corn and phytase, Baxter et al. (2003) were able to reduce the total P concentration in swine manure by 40% relative to manure from a normal diet. In the Netherlands, the excretion of P per growing-finisher pig has been more than halved in the last 30 yr due to reductions in P fed and improved genetics (Jongbloed and Valk, 2004). For dairy cattle, total P in manure was reduced from 12.65 g kg–1 dry matter to 5.21 g kg–1 by decreasing the concentration of P fed from 6.7 g/kg to 3.4 g kg–1 (Dou et al., 2002).
Although work remains to be done on diet modification to maximize reductions in manure P while maintaining animal performance, sufficient literature exits to show the approximate scale of P reductions that can be achieved. There is a need to understand how these reductions in manure P can address surpluses of manure P in areas of intensive animal production and how this varies regionally. Therefore, our objectives were to look at existing surpluses of manure P and compare these with surpluses that would exist if diet modification to reduce P were implemented. This will give a guide to how much of the problems associated with P surpluses can be remediated through diet modification and identify areas where additional approaches are needed.
Materials and Methods
Mass Balance Calculations
The mass balance of manure P was calculated using several sources of available data. The crop production and animal inventory were obtained from the 2002 Census of Agriculture (USDA-NASS, 2006) completed every 5 yr by USDA. The animal categories used by the USDA are listed in Table 1. The USDA data were available in English units, so calculations were done in English units before being converted to SI units. These data were generally available on a per-county basis, but in a few cases more than one county was combined by the USDA for reporting purposes. Manure P production was calculated using manure P production per animal values from Kellogg et al. (2000). Where necessary for some species, such as poultry, flocks per year were taken into account. Manure P production was calculated by:
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 | [2] |
Net manure P on a county basis was calculated by combining manure P production and the assimilative capacity (all units in kg):
 | [3] |
This gave a total P mass balance at the county scale at the most accurate level possible. Counties in the USA vary in size and in the proportion or land area that is available for manure applications. Therefore, we divided the net manure P by the cumulative harvested area of crops included in the balance to convert the mass balance into a per hectare basis as follows:
 | [4] |
Data were collected, aggregated, and analyzed using an open-source relational database (MySQL 4.0, MySQL.org). Upon completion of calculations performed at the census area level, tabular data were linked by census area identification to the spatial data representing the census area boundaries. Maps were developed using a commercial geographic information system (ArcGIS 9.1; ESRI, Redlands, CA).
Impact of Diet Modification on Mass Balance Calculations
In a review of existing literature, the Council of Agricultural Science and Technology (2002) summarized reductions in dietary and hence manure P that should be possible using existing technologies. Existing technologies include techniques such as formulating diets closer to animal requirements and feed additives such as phytase. These reductions in manure P were reported to be 40% for poultry, 50% for swine, and 25% for dairy cows and were used to reduce manure P production for the USDA animal categories as detailed in Table 1. Reductions in P in beef diets and manure have been reported. For example, feed-lot cattle generally have mineral P supplements added to their diets, but Erickson et al. (2002) reported that these P supplements were unnecessary. We were not able to incorporate the reductions in beef cattle manure P into the mass balance calculations. Metric groups reported by USDA-NASS groups certain age and weight classes of beef cattle, and those groupings could not be reliably fractionated into classes that correlated with P reductions reported by Erickson et al. (2002) or other authors. It was also not possible from the USDA beef categories to determine which animals were grazed versus confined and grain fed, which would have a huge effect on reductions in manure P that were possible. Therefore, we did not include reductions in manure P for nondairy cattle. The reductions in manure P for poultry, swine, and dairy above were fed back into Eq. [3], and the mass balance P in Eq. [4] was recalculated for manure from modified diets.
Inherent Errors in the Mass Balance Calculations
Manure is often produced in intensive farming systems in part of a county, or there may be a large production of manure on one or more farms within a county. The cost of transport and manure ownership issues normally makes the even spreading of manure over all of the agricultural land in one county impossible. However, the mass balance of manure at a county scale is the smallest scale possible with available data. It gives a strong indication of regional variabilities and the importance or potential impact of diet modification at this scale. Some of the "cropland used as pasture" defined in the census may rarely be used as cropland, and this would lead to an overestimation of assimilative capacity where this is the case (Kellogg et al., 2000). The inclusion of rangeland as permanent pasture may lead to an overestimation of assimilative capacity. The census combines rangeland with permanent pasture, so we used the Kellogg et al. (2000) values for this category.
Results and Discussion
Crop Acreage and Phosphorus Assimilative Capacity
Twenty-eight percent of counties had crop P removal in harvested portion of the crop of less than 15 kg P ha–1, and more than half of all counties had P removal of less than 20 kg P ha–1 (Fig. 1). Phosphorus-based manure management closely matches manure P applications with crop requirements, whereas N-based manure management generally overapplies P relative to crop removal. For example, Maguire et al. (2005b) applied turkey litter from a traditional high-P diet at a rate to meet corn (Zea mays) N requirements, which led to an application of 126 kg P ha–1. At a crop removal rate of 20 kg P ha–1, it would take approximately 6 yr to remove this amount of P. For the same N rate, diet modification was able to reduce the P application in turkey litter to 78 kg P ha–1 (Maguire et al., 2005b). A crop removal rate of 20 kg P ha–1 would take 4 yr to remove this P.
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The crop acreage per county varied widely, from no significant acreage in the Juneau area of Alaska to 881 633 ha in Elko County, Nevada. The mean crop acreage per county across the USA was 58 377 ha, and the median was 38 390 ha (Table 2). The removal of P in the harvested portion of crops per county varied with crop acreage from zero in the Juneau Area, due to no crop acreage, to 11 362 Mg P in Cherry County, Nebraska. Clear trends in crop acreage and hence P removal in the harvested portion of crops could be seen across the USA (Fig. 1). There is a great assimilative capacity of counties in the midwest Corn Belt on a ton per county basis (Kellogg et al., 2000). This is due to the large proportion of the counties in this region that are in crop production. West of the Corn Belt, extending into Montana, Wyoming, eastern Colorado, and south into Texas, there are also substantial crop acreages in small grains, cotton, and hay, leading to large amounts of P removed in harvested portion of crops (USDA-NASS, 2006). In California, the largest agricultural producer in the USA by value, diverse production ranging from grapes and almonds to hay led to several counties with great P removal in crops (USDA-NASS, 2006). Potatoes, wheat, and hay were the main crops contributing to P removal from counties in the northwestern states of Oregon and Washington (USDA-NASS, 2006).
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500 ha harvested), which is overwhelmed by its cattle production, presumably feed lots.
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Impact of Diet Modification on Manure Phosphorus Excretion
Many studies have demonstrated that diet modification can greatly reduce the concentration of P in animal diets and hence manure generated (Council for Agricultural Science and Technology, 2002; Maguire et al., 2005a) (Table 1). Figure 2 shows manure P production on a kg P ha–1 basis and uses five categories of manure P production. Full implementation of diet modification would decrease the percentage of counties in the manure P production categories 31 to 45, 46 to 60, and >60 kg P ha–1 to 3, 1, and 1%, respectively. Therefore, using no diet modification, 9% of counties had manure P production of >30 kg P ha–1, but if diet modification were fully implemented, this would drop to 5% of counties. This may be a change in only 4% of all USA counties but represents a 44% reduction in the number of counties with the greatest manure P production per crop area. In the lowest category (0–15 kg P ha–1), the converse is true, with the percentage of counties in this category rising from 78 to 81%. The impact of diet modification on manure P production depends on the animals being raised in a county (Table 1).
Reliable reports on how often diet modification technologies have been implemented are hard to find. In Delaware, the adoption of diet modification strategies (mainly dietary phytase and reductions in mineral P supplements in poultry diets) are credited with leading to a reduction of the P concentration of broiler litters from commercial houses by 30 to 40% in recent years (Hansen et al., 2005). Hansen et al. (2005) reported that two poultry integrators who used phytase in all their broiler diets reduced P being fed by 625 and 1905 Mg yr–1, so substantial changes are occurring in some areas. This was in part due to regulations in neighboring Maryland that required the use of feed additives to increase dietary P efficiency (generally the enzyme phytase) for poultry (Simpson, 1998). Phytase can increase digestion of phytate-P and hence decrease the need for inorganic P supplements, lowering total P in feeds and manure (Maguire et al., 2004). Anecdotal evidence from private conversations with poultry and swine producers in the mid-Atlantic region has also indicated that reduced P diets are being implemented to some extent. Efforts are ongoing in several states, such as Virginia, to decrease dietary P in dairy cattle. This is due to concerns about nutrient management regulations, economics (because P is expensive), and environmental protection/public relations.
Manure Phosphorus Surpluses with and without Diet Modification to Reduce Phosphorus Fed
When removal of P in the harvested portion of crops is subtracted from manure P production, 89% of counties are in a P deficit and therefore require the input of inorganic fertilizers to attain a P balance (Fig. 3). The presence of a deficit for a county does not mean there are not localized surpluses within a county because these mass balance calculations assume that manure P produced can be spread evenly over all crop land within that county. Surpluses of P may exist on certain farms or fields due to uneven spatial generation of manure, the expense of manure transport, and the use of inorganic P sources at some locations that do not have available manure.
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Phosphorus applications in excess of crop removal lead to the buildup of P in soils over a period of many years (Barber, 1979). This buildup of P has been shown to elevate P losses in leaching and runoff and is therefore undesirable (Sharpley, 1995). Phosphorus indices can tell producers where to avoid applying manure so that manure P is not placed in areas with the greatest connectivity to surface waters and hence at greatest risk for P loss (Sharpley et al., 2003). However, if there is an imbalance in manure nutrients relative to crop P removal, the implementation of a P index may result in P surpluses being applied to fields with a lesser connectivity to surface waters. This is not sustainable in the long term because soil test P in these soils eventually rise to an unacceptable concentration. Where there is insufficient crop land available for the sustainable use of manure, other possibilities, such as manure transport, alternate uses, or animal depopulation, could be investigated. For example, water quality programs in Florida led to a 26% reduction in dairy cows in the Lake Okeechobee basin (Boggess et al., 1997).
Counties with a manure P deficit dominate the USA, with or without diet modification (Fig. 3 and 4). If diet modification were fully implemented, the percentage of counties with a manure P surplus would decrease from about 11 to 8%. Therefore, diet modification was able to decrease the number of counties with a manure P surplus by approximately 27%. The reductions were more apparent in areas of swine, poultry, and dairy production because these are the species in which diet modification is thought to have the largest impact on reducing manure P.
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Economic considerations have driven the consolidation and intensification of animal agriculture in the USA for the past few decades, and this is unlikely to be reversed. Therefore, there is a great need to improve nutrient and manure management associated with these animal operations because research has shown that poor manure management can lead to nutrient losses that negatively affect surface and ground water quality. There are huge regional differences in crop and manure production, and problems with excess manure nutrients generally arise where crop production is low and manure production is high. Diet modification to decrease the P concentration in some animal feeds, and hence in manures produced, is generally a more economical option than manure transport and some other options. Our study showed that surpluses of manure P relative to crop off-take exist in approximately 11% of USA counties, and this could be decreased to approximately 8% if diet modification is fully implemented.
Therefore, diet modification will help decrease manure P surpluses, but in some counties or farms additional strategies will be required. Several alternate strategies exist, and the most appropriate strategy or combination of strategies is site specific. These alternatives include (i) manure transport off farm or out of county (but this is often cost prohibitive if transport distances are great); (ii) value-added products, such as compost or pellets, that make transport of manure nutrients economically feasible; (iii) combustion or co-combustion for energy production, which also results in concentrating up P in the ash and making transport more economical; and (iv) animal depopulation. Diet modification to reduce manure P will be one strategy that helps move intensive animal production toward sustainability by moving toward P balance.
NOTES
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REFERENCES
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