Water quality in the U.S. is threatened by contamination with nutrients, primarily nitrogen (N) and phosphorus (P). Animal manure can be a valuable resource for farmers, providing nutrients, improving soil structure and increasing vegetative cover to reduce erosion potential. At the same time, application of manure nutrients in excess of crop requirements can result in environmental contamination. Environmental concerns with P are primarily associated with pollution of surface water (streams, lakes, rivers). Excess P in water causes algae to grow rapidly, or to “bloom.” The decomposition of this algae consumes dissolved oxygen in the water and impairs the survival and productivity of fish, clams, crabs, oysters and other animal life. An algae bloom and subsequent decrease in dissolved oxygen may be caused by runoff of P when application to land is in excess of crop requirements.

Role of animal agriculture

Increased specialization and concentration of livestock and crop production has led to the net export of nutrients from major crop-producing areas of the country to areas with a high concentration of animal agriculture. Livestock utilize P inefficiently, excreting 60 to 80 percent of that consumed. The majority of P brought onto the farm in feed stays on the farm, rather than being exported in meat or milk.

Animal manure is typically land-applied to supply nutrients for crop growth, but N and P are in imbalance in manure relative to crop needs. Land-application of manure to meet the N needs of the crop results in the overapplication and accumulation of P in soils. Historically, P contamination of surface water has been associated primarily with erosion.

As application of P in excess of crop requirements continues, however, soil loading of P has increased and runoff of P occurs independent of erosion. Concentrated animal agriculture has been identified as a significant source of P contamination of surface water (7 to 48 percent of total P loads, depending on watershed).


Increasing regulatory pressure

Increasing public concern with water quality and increased awareness of the impact of concentrated livestock production has led to the development and implementation of increasingly stringent environmental regulations. Greater pressure on states from the federal government in the last decade to enforce federal clean water regulations has significantly increased the level of regulatory pressure felt by farmers.

One key change in water quality regulations in the past five years is the shift from a primary focus on N to an increasing focus on P contamination of surface water. Limiting manure application to the P needs of the crop is one way to avoid continued accumulation of P in soil and to minimize potential P runoff and contamination of surface water.

Phosphorus-based nutrient management regulations dramatically increase the amount of land required to dispose of manure and will have a severe, detrimental effect on the agricultural economy in areas of intensive animal agriculture. Practices that reduce P losses from farms without impairing profitability must be developed and implemented.

“Precision feeding” to meet P-based nutrient management limits

Improving our understanding of P digestion and metabolism in dairy cattle will lead to improved efficiency of P utilization, thereby reducing P excretion and minimizing the imbalance of N and P in manure. In dairy cows, several studies indicate a direct link between P intake and P excretion. When diets containing P at 0.3 percent, 0.41 percent or 0.56 percent of dietary DM were fed, excretion increased linearly with increasing intake. The majority of additional P fed was excreted in feces.

In the past, phosphorus was often fed to dairy cattle 20 to 40 percent in excess of published requirements. Overfeeding P was practiced due to the perception that high P diets improved reproductive performance. Severe P deficiency appears to impair reproductive performance in range cattle. But such effects were not observed until dietary P concentrations were below 0.3 percent of dietary DM. This dietary concentration is far below the concentration found in most feedstuffs. Additionally, in all of the aforementioned studies, P intake was confounded with intake of energy and other minerals.

Although severe P deficiency may impair reproductive performance, there are no research data to suggest a benefit from feeding P to dairy cows in excess of NRC requirements.

A review by researchers in Wisconsin outlines 13 studies with 785 lactating cows fed diets low in P (0.32 to 0.4 percent P) or high in P (0.39 to 0.61 percent P). Dietary P had no effect on days to first estrus, days open, services per conception, days to first A.I. or pregnancy rate. Thus, the past practice of including additional P in the diet as cheap reproductive insurance is not warranted.

Another factor that may have led to overfeeding of P is variation in the P content of feeds. Phosphorus content of forages analyzed by the Northeast DHI Forage Laboratory from May 1994 through April 1995 was found to be highly variable. The coefficient of variation in P content of forages was 20 to 25 percent for most forage types, and P content was more variable for grasses than for legumes. If the P content of ingredients is unknown at the time of formulation, one must assume contents at the lower end of the range when constructing diets in order to avoid feeding a deficient diet.

One final reason P may be overfed is the inclusion of feeds in the diet that are naturally high in P. Many byproduct feeds are high in P, most notably the byproducts of corn processing and ethanol production. These are increasingly popular feed supplements for beef and dairy cattle because of the protein and energy they supply.

However, inclusion of these feeds in higher amounts often increases the dietary P content beyond the animal’s requirement. Koelsch and Lesoing constructed nutrient balances for Nebraska livestock farms and found producers who used these byproducts had greater imbalances between P inputs and outputs than producers who did not. In their study, the seven cattle operations who fed these products imported twice as much P onto their farms as they exported in meat, crops and manure (input to output ratio of 2-to-1). In contrast, the nine farms who did not feed these products exported nearly as much P as they imported (input to output ratio of 1.1-to-1).

Economic impact

Given the huge increase in ethanol production over the past 10 years and expected fuel prices in the future, feeding of these high P byproducts will likely continue. Producers using these feeds should at least remove unneeded supplemental inorganic P from diets. Removal of calcium phosphate or other forms of inorganic P can improve ration costs, although one must consider whole farm costs when making these decisions.

Sink et al. found a 100-cow herd increases their feed bill by $800 to $1,050 per year by feeding P at 0.5 percent of dietary DM compared to feeding at 0.35 percent P, and with P at 0.55 percent of dietary DM, the feed bill would be increased by $1,460 to $1,570 per year.

However, achieving dietary P contents below 0.35 percent can have negative economic impacts if economical ingredients are forced out of the ration due to P content. In the long run, however, the true cost of the use of these high P feeds should be carefully considered. If the inclusion of these byproducts will cause significant nutrient imbalance in the livestock operation and lead to difficulty meeting environmental regulations, then these feeds may not be as inexpensive as they appear.

For the farmer under mandatory P-based nutrient management, additional costs include the cost of exporting manure if it cannot be applied to land and the cost of purchased N fertilizer to meet the remaining N needs of crops (limiting manure application to P removal results in underapplication of N relative to crop needs).

A study at Virginia Tech University examined dairy and dairy/poultry operations of different sizes, estimated potential P losses and simulated net farm income under different policy scenarios. One of the policy scenarios was a restriction on P applications to that taken up by the crop harvested. In this study, the policy limiting P application was the only policy with any measurable impact on P losses from dairy and dairy/poultry operations. Phosphorus losses were reduced by 28 to 43 percent by this policy, but net farm income was dramatically affected, falling by 11 to 23 percent.

The reduced net farm income under P-based nutrient management in these simulations was due primarily to the increased cost of purchased nitrogen fertilizer to meet the N requirements of crops. The impacts on net farm income are likely underestimated in this study, as it was assumed farmers could dispose of excess manure off the farm at no charge.

Reductions in net farm income will result in increased farm failure. As national cow numbers reflect national consumption, the cows from displaced farms will be replaced by cows in less environmentally sensitive areas.

Will precision feeding help you meet your nutrient management plan?

Phosphorus-based nutrient management plans will be difficult for many livestock farms to implement, but fine-tuning the nutrition programs can help. The impact of reducing P intake on P losses from the farm system can be estimated several ways. In dairy cattle, one can predict P excretion simply as the difference between P intake, and P in milk, retained in bodyweight gain and fetal development.

As bodyweight gain and fetal development are minor contributors to the calculation, one can arrive at a fairly good estimate by simply accounting for the difference in P intake and milk P. Given allocation of manure to crops and estimated nutrient uptake by those crops, one can calculate acreage required to land-apply manure with changes in P intake.

On a dairy farm milking 100 cows with different cropping strategies, P intake has a significant impact on acreage required for disposal of manure on a P basis. Acreage required to dispose of manure generated by the herd increases by about 80 percent as dietary P concentration increases from 0.35 to 0.55 percent.

Alternatively, given a fixed land base and different cropping strategies, we calculated the maximum number of milking cows supported by that land base. As dietary P concentration increases from 0.35 to 0.55 percent, herd size that can be accommodated with P-based manure application limits decreases by 45 percent.


Phosphorus-based nutrient management regulations may dramatically increase the amount of land required to dispose of manure and may have a severe, detrimental effect on the agricultural economy in areas of intensive animal agriculture. Reducing the P content of manure through nutrition is a powerful, cost-effective approach to reduce P losses from livestock farms and will help farmers meet increasingly stringent environmental regulations. PD

References omitted but are available upon request at editor@progressivedairy.com

—From 2006 Virginia State Feed Association and Nutritional Management Conference Proceedings