By Jerry Shurson, University of Minnesota

The greatest amount of phosphorus fertilizer use in the U.S. (56%) is attributed to feed production for livestock production, including traded commodities¹. Of the total P used in the U.S., about 25% is embedded in the production of crops and animal products that are exported, resulting in a large amount of P leaving the country¹.

In contrast, excess application of P from fertilizer and animal manure relative to crop removal rates, has resulted in inefficient P use and management in crop and livestock production and has contributed to serious water pollution problems in watersheds in the Midwest.

Recently, the National Corn Growers Association and 62 other agricultural groups sent a letter to the U.S. Commerce Secretary requesting reconsideration of the duties (nearly 20%) placed on phosphate fertilizer imported from Morocco because high prices and volatility, along with limited supply are causing a significant financial burden on farmers. While this may initially seem to be a reasonable request to address an immediate phosphate fertilizer supply and cost problem, it does not address the more serious long-term P supply, cost, usage and management problem in U.S. agriculture.

There is a tremendous need to build greater resiliency in global agriculture and food supply chains because of various disruptors including wars and ongoing geopolitical conflicts, climate change induced destructive weather events, and labor strikes around the world. In addition, for some natural resources like P, mineable reserves are located in only a few countries.

About 85% of global P reserves are located in Morocco², which means that the U.S. competes with numerous countries for access to these reserves, which will eventually be depleted over time. Israel is also a significant supplier of phosphate, and contributes about 3% of total phosphate exports globally, but increasing concerns that the recent Hamas-Israel war may cause additional price increases and shipping delays in the coming months.

However, many of these challenges can be overcome if more responsible management of P application from fertilizer and manure was practiced on farms to meet crop nutrient needs without excess P application³. This would reduce cost of P application for optimal crop production, reduce surplus soil P and subsequent surface water pollution from runoff that has been occurring across the U.S., and improve future P security in U.S. agriculture.

Seeing the big picture

P is an essential nutrient for life, but the one-way flow of P from mineral reserves to farms (soils) to freshwater sources and oceans has already exceeded the safe operating space (Earth’s carrying capacity) for sustainable life⁴. Phosphorus is a finite natural resource that will be depleted within the next 40 years in the U.S. if conservation and recovery practices are not implemented³. Reliance on imported P from Morocco and a few other countries is not sustainable in the long-term because these mineable reserves will also be depleted in the next 70 to 140 years without proper management³.

Animal manure is another valuable source of P for crop production, but it is routinely overapplied due to greater application rates to meet the nitrogen needs of crops, which contributes to P accumulation in soil because crop uptake of P is less than that for N.

Crop and livestock production systems are the greatest contributors to the disrupted global P cycle. Global soil P surplus increased by 5.5-fold compared with the 3.8-fold increase in soil N surplus during the past 50 years⁵. Most of the surplus soil P is lost through runoff into surface water or accumulates in soil due to fertilizer application rates exceeding rates of crop uptake of P. In fact, P losses in agricultural soils due to erosion by water have been projected to represent more than 50% of total P losses worldwide, which will ultimately limit food and feed production in the future⁶.

Unfortunately, U.S. regulations are minimal regarding P use, management and losses to the environment, in which agricultural nonpoint P sources are allowed to use voluntary P reduction methods despite being the main contributors to P pollution.

Animal manure is a primary source of P in surface and groundwater in the U.S.⁷. Currently, about 58% of rivers and streams in the U.S. are rated poor for P pollution by the U.S. Environmental Protection Agency. Because P is a limiting nutrient in freshwater ecosystems, extensive algae blooms occur in lakes and rivers located in agriculturally intensive Midwestern states. These algae blooms deplete oxygen levels in water which can lead to fish kills and cause other detrimental effects in the ecosystem. Continued loading of excess P in soil and watersheds will likely continue unless effective management practices are implemented to conserve P and avoid the many detrimental environmental consequences resulting from its inefficient use.

Impact of corn distillers grains co-products

P is the third most expensive component of animal diets, but its estimated efficiency of utilization to produce edible meat, milk and eggs is highly variable and generally less than 60% depending on animal species⁸ (Table 1). Unlike beef and dairy cattle, P is not very digestible in common feed ingredients such as corn and soybean meal for swine and poultry. A high proportion of P in most plant-based ingredients is in the form of phytic acid (phytate), which is indigestible for pigs and poultry. As a result, P utilization efficiency in pork production is low, with only about 34% of dietary P being recovered in edible lean pork⁸, and the remaining 66% of dietary P consumed is excreted in manure.

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The U.S. ethanol industry produces about 37 million metric tonnes of distillers grains co-products annually. About one-third of annual production is exported for use in animal feeds in other countries, and the remaining 25 million metric tonnes are consumed by beef cattle (45%), dairy cattle (33%), swine (15%) and poultry (6%) domestically. In the U.S., swine diets may contain up to 30% corn dried distillers grains with solubles (DDGS), which has a greater total and digestible P concentration than in corn and soybean meal (Table 2). As a result, adding DDGS to swine diets can reduce diet cost by reducing the need for inorganic phosphate supplements when diets are formulated on a digestible P basis.

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To further reduce the use of phosphate supplementation in swine diets, phytase enzymes are routinely used to reduce cost and improve P digestibility of the corn and soybean meal portions of the diet, which also reduces P excretion in manure¹⁰. Results from a recent study (Table 3) conducted by our team at the University of Minnesota, showed that adding 30% DDGS and phytase to grower-finisher diets formulated on a digestible P basis, can reduce the amount of supplemental inorganic P added to the diet, reduce total P intake by 8 to 15%, and reduce P excretion in manure by about 14% without negatively affecting growth performance and carcass composition compared with feeding corn-soybean meal diets with supplemental phytase¹⁰. This is a major step for reducing the P footprint of U.S. pork production.

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In contrast to the reductions in manure P excretion from adding DDGS to swine diets, use of high amounts of distillers grains beef feedlot cattle diets dramatically increases manure P excretion. Finishing beef cattle are commonly fed high energy diets containing wet DG at levels up to 40% dry matter intake, which results in the consumption of much more protein (N) and P than the amount needed to meet the protein and P requirements for optimal growth and carcass composition. This results in the excretion of relatively high amounts of nitrogen and P excretion in manure. As the amount of corn wet distillers grains with solubles is increased from 0 to 30% of dry matter intake in feedlot cattle diets, P intake increases by 33%, P retention stays about the same, and P excretion in manure increases by about 39% (Table 4).

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As a result, an increase of about 1.7 kg (3.8 lbs) of P per feedlot steer is excreted in manure during a 167-day feeding period compared with feeding diets with no DG11. In fact, some studies have shown that manure P excretion can increase by 92% when feeding 40% wet DG compared with feeding an 85% corn-based diet to finishing beef cattle.

Therefore, this is a huge difference in dietary P utilization efficiency between growing-finishing pigs and beef feedlot cattle fed high DG diets, with feedlot cattle have six times greater P intake/day, and nearly 10 times greater P excretion/day in manure than growing-finishing pigs, which explains the difficulty in minimizing manure P from beef cattle feedlots fed high amounts of DG. These results, also explain why only about one-third of P in swine diets, and about one-fourth or less of P in beef cattle diets is recovered in edible lean pork and beef (Table 1).

These inefficiencies of dietary P utilization from feeding corn DG to livestock and poultry have major environmental consequences. One approach to reduce P loading in agricultural soils has been to introduce legislation to restrict the size and number of large animal production operations in counties and states with high livestock and poultry densities. Another approach to minimize manure P excretion is to reduce the amount of DG in animal diets, especially in beef feedlot diets. However, this has economic and other potential unintended environmental consequences if demand for corn DG co-products in animal feeds is reduced.

Perhaps a more prudent approach is to modify current manure application practices to reduce excess P application to agricultural soils. Animal manure is typically applied to cropland on a N basis, which results in excess application of P because the crop needs, and removal rate is less for P and for N. If manure from beef cattle feedlots is applied on a N basis to cropland once every year, the amount of P being applied is three to six times greater than needed for crop production¹¹. If manure is applied on a P basis, the amount of land required is about four times greater than the land needed for N-based application, and may require increased time and transport distances, which increases labor, fuel and equipment costs.

Therefore, to minimize P accumulation in soils, and the risk of surplus P runoff from erosion into surface waters, manure should be applied on a P-basis rather than a N-basis once every four-years rather than on an annual basis¹¹. This P-based manure application to the same soils every four years would reduce application costs and environmental risks from surplus soil P instead of annual application. Different on-farm and off-farm technologies for removing and recovering nitrogen and P from animal manure have been developed, but they have not been widely implemented mainly due to lack of regulatory requirements and economic incentives. Because of the abundant quantity of corn co-products produced, the relatively high concentration of P in DG, and the amount of excess P consumed relative to animal requirements, removing a portion of P before producing DG would be beneficial.

Reducing and recovering P from DDGS

Our recent collaborative study¹² estimated that feeding distillers grains co-products to livestock and poultry resulted in 1.1 million metric tonnes of total P excreted in manure per year. If a portion of the P is removed from DG to produce low P distillers grains co-products, containing 65% less P compared with conventional DG co-products, about 143,000 metric tonnes of P could be saved by using low P distillers grains co-products in animal feed.

Results from another recent study showed that P can be precipitated in the form of calcium phytate from the thin stillage fraction after distillation during the ethanol and corn co-product production process, and the total amount of P generated from the ethanol industry from using this technology could produce enough P to displace 12.5% of national P fertilizer consumption in the U.S¹³. In addition, about 91,900 metric tonnes of P embedded in DDGS leaves the U.S. in the form of DDGS exports every year, but 59,600 metric tonnes of P could be recovered prior to export if P removal technologies were used.

Therefore, multiple benefits could be achieved by removing a portion of P from DG co-products. First, high diet inclusion rates of DG in animal feeds could be maintained while reducing excess P excretion in manure. Second, the amount of renewable P generated from ethanol biorefineries would reduce P fertilizer cost and need to purchase additional fertilizer. In fact, our estimates indicate that 37% of P fertilizer consumption used in crop production in the North Raccoon Watershed in central Iowa could be displaced with this renewable P source¹². This is environmentally significant because the North Raccoon Watershed is a high priority watershed for nutrient pollution¹², and Iowa is a main contributor of P loading in the Gulf of Mexico¹⁴. Lastly, recovering excess P in DG co-products would contribute to greater P utilization efficiency, self-sufficiency, security and supply resiliency in the U.S.

Potential for a P trading market

Creating nutrient (P) trading markets could be another part of the solution to reduce the amount of P in manure of animal feeding operations. This approach would allow regulated point sources (ethanol plants producing DG co-products) to trade P reductions with other point sources, and potentially with nonpoint sources (e.g., livestock operations) to minimize the cost of P reduction in a watershed. This is entirely feasible because ethanol biorefineries are located near a high proportion of intensive livestock and poultry production systems.

By removing a portion of P from DG to reduce excess P in animal feeding programs, an opportunity can be created to develop a centralized source of P reduction credits in a potential P trading market involving corn ethanol biorefineries in the Midwest region^12,13. If the uncertainties of this approach can be overcome and adherence to nutrient management commitments can be enforced, this seems like a viable long-term responsible P management strategies that would solve many of the current problems that exist today.

Conclusions

Phosphorus is an essential and expensive nutrient required in agriculture and food production, but its mismanagement and inefficient use has led to a dangerous tipping point where the Earth’s finite P reserves are being depleted, while at the same time, P from fertilizer and manure is being applied in excess to croplands resulting in losses due to erosion, and ultimately causing pollution of valuable freshwater resources in the U.S. and around the world. These problems can be solved if we choose to do so by adopting more circular P recovery, recycling and redistribution practices as well as better soil P management based on applying P from the “right source at the right rate at the right place at the right time”.

The interconnectedness and proximity between intensive crop production (corn and soybeans), the majority of corn ethanol biorefineries, and a high proportion of intensive livestock and poultry production facilities in the Midwest represents a perfect opportunity to achieve greater P utilization efficiency, self-sufficiency, security and supply resiliency in the U.S. This redesign would greatly enhance long-term sustainability of P use in agriculture by reducing the burden of manure P management in animal production systems, creating a new source of renewable P fertilizer for crop production, becoming a new revenue source for ethanol plants, reducing environmental burden of surplus P in soil and water pollution, enable the creation of a P trading market, and reduce dependency on imported phosphates from other countries.

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