A complicated relationship

Farmers have a love/hate relationship with phosphorus (P). It’s a dietary requirement for livestock but significant P is excreted in manure and urine. Depending on on-farm conservation practices, P accumulates in runoff and soil erosion and is carried to important watersheds, resulting in eutrophication, algae blooms and fish kills.

Dr. Gerald Shurson, Department of Animal Science, University of Minnesota, said another problem with P and certain other nutrients is manure applied to cropland in amounts greater than crop nutrient uptake.

“We have a big phosphorus problem, and this problem is associated with the high concentration of a large number of livestock and poultry farms with high animal density in many regions of the U.S.,” Shurson said. “This results in large amounts of manure, in which phosphorus is a major component.”

According to Shurson, P is a finite resource that will be depleted within the next 30 to 40 years unless there’s more effort to conserve and recycle it for productive purposes.

“Agriculture and food production are major consumers of mined rock phosphate,” Shurson said. “Only 15% of phosphorus extracted from nature ends up in the foods we consume – 85% of phosphorus is lost.”

There’s a connection between animal feed and manure as major drivers of sustainable livestock and poultry production systems. Phosphorus utilization efficiency in livestock is poor and highly variable. Shurson said 42% of the P lifecycle loss in U.S. food production comes from livestock manure and runoff. Cattle are the main contributors to manure P throughout the U.S. In certain regions, swine and poultry are the major contributors.

“The amount of manure phosphorus being produced and applied to soil near these farms has exceeded crop uptake and the environmental capacity to absorb and neutralize it without causing ecological damage in many areas,” Shurson said.

The role of animal nutrition is critical in addressing challenges and solutions for reducing P in manure.

“Animal nutrition is relatively simple,” Shurson said. “It’s the execution that sometimes gets us into trouble. In feeding livestock and poultry, the goal is to meet the daily requirements … of all essential nutrients to optimize animal performance while minimizing overfeeding and underfeeding of nutrients.”

The two most common feed ingredients in U.S. animal diets are corn and soybean oil meal (SOM), both of which contain high proportions of phytate. Corn distillers grains (DG) with solubles, a byproduct of the ethanol industry, is the other main ingredient in swine, poultry, dairy and beef diets. Shurson said the advantage of DG over corn and SOM is that it contains a lower proportion of phytate relative to total P content, making it a better source of digestible P than corn or SOM.

“We can overcome the problems in swine and poultry diets with corn and SOM with phytase,” Shurson said. “Phytase enzymes are cost-effective and commercially available. As we increase the dosage, we increase phosphorus digestibility.”

One strategy for reducing the amount of excess protein or nitrogen (N) in livestock diets involves reducing the amount of SOM and adding synthetic amino acids to meet animals’ digestible amino acid requirements. The addition of phytase enzyme improves P digestibility in corn and SOM-based diets.

An important concept in P management is the variability in digestibility and water solubility of supplemental inorganic P sources commonly added to swine, poultry and cattle diets.

Whole-farm P efficiency improves when mono-dicalcium phosphate replaces dicalcium phosphate as an inorganic P source. Mono-dicalcium phosphate contains about 18% more water-soluble P, resulting in greater P digestibility, less inorganic P required in the diet, less P in manure and greater P crop uptake.

“We know how to formulate diets to reduce dietary phosphorus content,” Shurson said. “It involves using highly digestible phosphorus ingredients, accurately determining phosphorus for specific farm conditions, formulating diets on a digestible phosphorus basis and using phytase in a multi-phase feeding program to minimize over- and underfeeding of phosphorus.”

However, diet formulation to optimize P use is only one aspect of the challenge.

“It also involves execution,” Shurson said. “Our goal is to get the right amount of energy and nutrients in the right feed fed to the right animals at the right time.”

This can be accomplished by designing feeding programs that use multiple diets formulated to meet the changing requirements of animals at different stages of production. Feeding programs that include feed budgets can provide information to farmworkers about when to switch from one diet to the next for a specific group of animals on feed. Such nutrition practices minimize costs and optimize animal performance while minimizing over- and underfeeding nutrients throughout production phases.

Feed processing methods such as reducing particle size and pelletizing diets improve P digestibility by up to 29%. Properly adjusted feeders that minimize waste can significantly reduce P in manure.

Shurson said one issue that’s sometimes ignored is high levels of P in manure due to feeding biofuel co-products such as DG and SOM to poultry and livestock. This eventually contributes to excess P and N in manure.

Beef finishing cattle are often fed high amounts of wet DG because it’s available, economical and has higher energy value than corn. At a rate of 30% DG, cattle are overfed protein (N) and P relative to dietary requirements. Excess P ends up in manure. Hauling manure longer distances to where P is needed or reducing DG in diets are not reasonable solutions.

Shurson noted an increased demand for SOM due to higher demand for fats and oils, resulting in expanded soybean production.

“The net result is increased production of SOM and excess supply relative to demand while exports are declining and prices are falling,” Shurson said. “These market-driven factors are making SOM an economical protein and energy source, resulting in overfeeding protein and nitrogen relative to dietary requirements, which increases excretion of these nutrients.”

He said innovation is needed to manage the P challenge.

“We know how to formulate diets and formulate a multi-phase feeding programs to minimize phosphorus excretion in manure,” said Shurson, “but we need to improve on execution and connecting feeding program management with manure management to improve overall phosphorus utilization efficiency.”

by Sally Colby