Public scrutiny of the impact of agricultural practices on the environment is growing. The livestock and poultry industries have been targeted for attention because of their visibility, and for real as well as perceived abuses. Large concentrations of animals in relatively small areas create difficult challenges in terms of odor and nutrient management, but problems of nutrient management can plague small as well as large animal operations. One of the fundamental challenges facing the livestock/feed industries is to recycle the flow of feed nutrients, particularly phosphorus and nitrogen, from animal operations back to cropland where they can again be used for crop production. Anything short of this is not sustainable, and will ultimately be unacceptable to the broader public.

To achieve effective nutrient recycling, and to minimize environmental damage, application of manure nutrients must be limited to an amount that crops can utilize. Herein lies the rub. Areas with high livestock densities will have to transport manure nutrients over larger distances to avoid overapplication of nutrients, or alternatively, relocate animals to where cropland is available for manure application. Full crediting of manure nutrients will be essential, and switching to a phosphorus standard is inevitable. Currently most states permit manure application in amounts that supply the crop need for nitrogen. Since the P:N ratio in manure is approximately twice that of the P:N ratio needed by crops, applying manure to meet nitrogen needs results in a build-up of soil phosphorus levels. Switching to a phosphorus standard would double the land area required for manure application compared to most current standards.

The objective of this paper is to review the current situation with regard to phosphorus supplementation of dairy cows. A reduction in dietary phosphorus would lead to less phosphorus in manure, thus easing the land requirement for manure application.

Large amounts of phosphorus are supplemented to dairy cows with lesser amounts of phosphorus supplemented to beef or feedlot diets. We oversupplement phosphorus, costing the U.S. dairy industry an unnecessary $100 million annually, as well as increasing risk of environmental damage through eutrophication of lakes and streams. What is the situation?

Based on a telephone survey of university extension specialists, nutrition consultants, and feed industry personnel, it appears that dairy producers feed phosphorus far in excess of National Research Council (NRC) (1989) recommendations. While the NRC recommends approximately .34 to .41% (dry basis) phosphorus in typical dairy diets, producers feed closer to an average of .48% dietary phosphorus, with a few herds receiving close to .60% dietary phosphorus. Is the NRC recommendation out of line, or are producers simply feeding more than they need?

Table 1 makes a comparison of phosphorus feeding recommendations, as of 1992, for several countries (Tamminga, 1992) (Note: Since the 1992 publication of Tamminga, Great Britain has increased and Germany has decreased the suggested feeding level for phosphorus). Compared to other standards shown in Table 1, the NRC recommendation in the United States is relatively low for the maintenance portion of the requirement, but very high for the milk portion of the requirement. The high dietary allowance suggested by NRC for milk production (1.98 g/kg fat-corrected milk) reflects the low value NRC uses for phosphorus availability in the gastrointestinal tract (50%).

Table 1. Phosphorus requirements for dairy cattle (Tamminga, 1992).

Maintenance                Milk Production              % Availability               Country

(g/kg of BW)                (g/kg of FCM)

.0286                                1.98                                 50               United States (NRC)

.042                                  1.50                                 60               Netherlands

.0207                                1.56                                 58               Great Britain

.062                                  1.25                                 70               France

.040                                  1.66                                 60               Germany

Compared to standards used in other countries, it is fair to say that NRC is on the high side of phosphorus recommendations. Caution is advised, however, in putting too much faith in any of the standards. All of the standards used in the various countries are based on a very old and meager data base. In the case of NRC (1989) dairy recommendations, 30 references on phosphorus averaging now 32 years of age are cited. Only 20% of these 30 studies used lactating dairy cows, with the balance utilizing laboratory or nonlactating animals. A combination of relatively new information, and a revisit of some of the older studies, allows us to put phosphorus feeding recommendations on firmer footing. What must we consider if we are to develop more reliable standards for feeding phosphorus?

One of the persistent questions through the years has regarded availability or true digestibility of phosphorus. This is of great importance for swine and poultry, because phytate phosphorus is largely unavailable to them, and the majority of phosphorus in feed grains is in phytate form. Phytate phosphorus is readily available to ruminants because the ruminal microorganisms produce phytase, the enzyme responsible for cleaving phosphorus from its phytate form. Nonetheless, questions about phosphorus availability persist. The availability of phosphorus was reduced from 55% in the 1978 NRC Nutrient Requirements of Dairy Cattle to 50% in the current NRC (1989) revision. The reason for this is not clear, because there appears to be no evidence supporting such a change. The available evidence would in fact suggest that the values used for availability or true digestibility of phosphorus should be increased. Most studies have underestimated true digestibility of phosphorus, because true digestibility can only be measured under conditions of phosphorus deficiency. Some studies and their measure of availability are as follows: Kleiber (1951)—50 to 64%; Lofgreen and Kleiber (1953 and 1954)—81 to 96%; Weiss et al (1986)—60 to 65%; Koddesbusch and Pfeffer (1988)--~90%; and Martz et al (1990)—64 to 75%. The working groups responsible for developing the dairy feeding standards in Germany have adopted a value of 70% for phosphorus availability (Kirchgeb ner, 1993). We believe this is a fair value to use.

Confusion has also surrounded the maintenance requirement for phosphorus, or perhaps a more appropriate term, the inevitable loss of phosphorus. As shown in Table 1, the feeding standards vary by as much as three-fold for the maintenance allocation. Excellent research from the group of Dr. Ernst Pfeffer at the University of Bonn in Bonn, Germany, has done much to clarify this question. It is unfortunate that the studies of this group are not better known in the United States, because their work has provided important basis for the recent revision of the German standard for feeding phosphorus to dairy cows. The working committee in Germany changed the basis for calculating the maintenance requirement from "g phosphorus per kg body weight" to "g phosphorus per kg dry matter intake." This decision was based on work from Pfeffer’s group (Spiekers et al, 1993) which showed that inevitable losses of fecal phosphorus in lactating cows averaged 1.2 g per kg dry matter consumed. This obligatory loss of phosphorus, measured with a phosphorus limited diet, appears to be largely in the form of microbial phosphorus. Microbial fermentation in the gut results in undigested microbial fragments, and these contain phosphorus. Unavailable dietary phosphorus and sloughed intestinal lining also contribute to the inevitable loss of phosphorus from the body. Very little phosphorus is excreted via urine in ruminants (Spiekers et al, 1993). Relating the ruminant maintenance requirement for phosphorus to dry matter intake allows for a much more accurate allocation of total phosphorus needs than does use of body weight.

The phosphorus feeding standards are undergoing review in several countries, including the United States. The NRC committee has not yet announced what changes, if any, will be made in the U.S. standard. We would encourage adoption of standards similar to those in Germany.

The German system (Kirchgeb ner, 1993) utilizes a factorial approach to calculating the requirement, and the component parts of the requirement are as follows:

Table 2 illustrates a comparison of current phosphorus recommendations based on Table 6.4 of the 1989 NRC publication, and the new recommendations developed in Germany. This example is for a 1320 lb Holstein producing milk containing 3.75% butterfat. The dry matter intake values are those suggested in Table 6.4 of the NRC publication. The dry matter intake value used will of course have a direct effect on the phosphorus requirement when expressed as a percent of diet dry matter.

Table 2. Phosphorus feeding recommendations

                                                                                     Current                            Recent

                                                       Estimated                  NRC                            German

                                                      Dry Matter         Recommendations           Recommendations

                          lbs milk/                  Intake

                             day                      lbs/d                P, g/day  Dietary P, %¹   P, g/day  Dietary P, %

1320 lb cow

3.75% BF             22                       28.7                    36.0         .27                 33.0            .25

                             44                       37.3                    55.5         .33                 52.8            .31

                             66                       44.8                    74.5         .37                 72.0            .35

                             88                       51.5                    93.0         .40                 90.7            .39

                           110                       60.1                  112.5         .41               110.6            .40

¹Recommends .48% dietary P during first three weeks of lactation

It is interesting that the two systems project very similar phosphorus requirements. The U.S. recommendation is based on what appears to be an unrealistically low maintenance requirement, but this is compensated for by an inflated requirement for lactation. The final NRC recommendation comes only slightly higher than what appears to be the more logically calculated German recommendation.

One more thing needs to be said about the NRC (1989) phosphorus recommendation. Table 6.5 of the NRC publication suggests that the dairy diet contain .48% phosphorus during the first three weeks of lactation. This is intended to ensure adequate consumption of phosphorus during the early part of lactation when feed consumption lags behind milk production. It is important to recognize that significant bone phosphorus is mobilized during the first few weeks of lactation. Bone provides 500-600 g of phosphorus in early lactation, and we should credit this source of phosphorus. Feeding an extra high amount of dietary phosphorus in the first weeks of lactation is likely not necessary.

While the NRC and European phosphorus standards differ greatly in their component parts, i.e., maintenance, production and phosphorus availability, the final feeding recommendations do not differ greatly. It may be a case of the standards being right for the wrong reasons! The glaring discrepancies in the component parts of the feeding standards do little to build confidence that the final feeding recommendations may in fact be pretty reasonable. This may be one explanation of why our feeding practices call for much more phosphorus than our feeding standards do.

Based on the telephone survey of dairy extension specialists, nutrition consultants and feed industry representatives, it appears that dairy producers in the United States are feeding an average of .48% dietary phosphorus, considerably in excess of what NRC recommends. Why? Mention has been made of significant uncertainties in the feeding standards, and this has perhaps encouraged feeding extra phosphorus to provide a margin of safety. Aggressive marketing of phosphorus supplements has contributed to excessive levels of dietary phosphorus. Perhaps the most important factor responsible for excessive phosphorus supplementation is the notion that phosphorus is crucial to maintaining acceptable reproductive performance in dairy cows. A field study involving a number of farms in Scotland almost 50 years ago may be the source of this view (Hignett and Hignett, 1951). While there is an occasional observation that suggests phosphorus fed at levels recommended by the feeding standards is too low for optimum reproductive performance, the overwhelming evidence is that phosphorus has no effect on reproductive performance until dietary phosphorus drops below concentrations needed to support maximum microbial growth in the rumen. Dietary phosphorus levels of less than approximately .25% can reduce rumen microbial growth (Durand and Kawashima, 1980) resulting in less microbial protein and possibly lowered ration digestibility. Phosphorus can have an indirect effect on reproductive performance through its effect on digestibility and energy supply when very low phosphorus diets are fed. Modern dairy diets, however, never approach the low phosphorus content that can result in impaired function of rumen microbes.

The evidence available from lactation studies supports the view that current NRC recommendations, or the new standards being used in Germany, are adequate in their recommendations for phosphorus. Table 3 contains a summary of lactation trials where "low" or "high" levels of dietary phosphorus were fed. The low phosphorus treatments ranged between .30 and .39% dietary phosphorus, and the high treatments ranged between .39 and .65% phosphorus. Cows in these studies were producing approximately 16,500 to 24,600 lb of milk per 305 day lactation. Mean daily milk production values for the "low" and "high" phosphorus groups averaged 66.5 and 67.0 lb per day. The dietary phosphorus content of the "low" treatment groups was, in all but one case, below the level recommended by the NRC and German standards for the level of milk production reported, suggesting that the standards recommend more than an adequate amount of phosphorus.

Table 3. Milk production response to dietary phosphorus level

                                                         Dietary P                               Milk Production

Study                                           (% of Diet DM)                               (lbs/day)

                                                     (a)          (b)          (c)                (a)          (b)           (c)

Kincaid et al, 1981                       .30         .54                             61.6        66.0

(20 cows/trt) (10 mo. trial)

Brintrup et al, 1993                       .33         .39                            55.9         53.9

(26 cows/trt) (two

complete lactations)

Satter & Dhiman, 1997¹                    .39         .65                            52.6          53.7

(23 cows/trt)

(12 wk mid lactation)

Wu et al, 1997¹                            .35         .45                           65.3           63.6

(24 cows/trt)

(complete lactation)

Wu et al, 1998¹                             .37         .48                          86.5           84.7

(26 cows/trt)

(first 27 wks of lactation)

Wu et al, 1999¹                              .32        .41         .51            77.1           80.2          79.5

(8-9 cows/trt)

(complete lactation)

                                                                                            Ave 66.5           67.0

¹Unpublished studies. U.S. Dairy Forage Research Center. USDA-ARS. Madison, WI.

Based on responses from the aforementioned telephone survey, it appears that U.S. dairy producers are feeding about 25% more phosphorus than the NRC recommends. If NRC tabular values ("book values") for phosphorus content of feedstuffs are relied upon, rather than actual laboratory analysis of the feedstuffs used, it is likely that we are feeding even more phosphorus than we realize. Berger (1995) compared a large number of actual laboratory analyses of some common feedstuffs with their corresponding NRC "book value" (Table 4). In every case, the mean analytical value was higher than the NRC value. With alfalfa, laboratory measurements averaged 38% more than the NRC tabular value.

Table 4. Phosphorus analyses of feed samples submitted to commercial laboratories and the relationship to values reported in the United States-Canadian Tables of Feed Composition (Berger, 1995)

                                   Number of          Analyzed P            Ratio of

Feedstuff                       Samples           % (of DM)         Analyzed:NRC        SD

Corn silage                     8197                    .23                       1.05                .06

Alfalfa¹                           4096                    .30                       1.38                .06

Corn grain                        912                    .32                       1.07                .07

Ear corn                           905                    .29                       1.07                .08

Soybean meal (50%)        148                    .72                       1.03                .28

Brewers grain                   139                    .59                       1.08                .08

Distillers dried grains         114                    .83                       1.17                .17

Barley                               115                   .38                       1.02                .07

Oats                                    38                   .43                       1.13                .09

Berger, Anim. Feed Science and Technology 53:99, 1995.

¹Only samples reported as pure alfalfa were included. The NRC description for alfalfa hay, sun-cured, early bloom was used as the standard because of similarity in protein values.

The overfeeding of phosphorus to dairy cows is costly and is contributing to environmental risk. If we reduce dietary phosphorus and bring it in line with the feeding standards, can this make a difference in terms of nutrient management? Table 5 illustrates an example where all of the manure from a 100 cow dairy operation (includes lactating and dry cows, plus 80 replacement heifers) goes back to the land required for producing all of the grain and forage for this herd. The import of nutrients into the system consists of protein supplement, mineral supplement, and starter fertilizer for corn grain and corn silage. In this example of a herd averaging 20,000 lb of milk, annual import of the protein supplement (soybean meal in this example) will result in 1,220 lb phosphorus entering the system. Assuming dicalcium phosphate is used to elevate the dietary phosphorus level from .36% (dry basis) of the unsupplemented feeds to the average .48% being used in dairy diets in the United States, an additional 1,500 lbs of phosphorus enter the system. Starter fertilizer is probably used for corn production, and if 100 lbs of 9-23-30 is applied per acre of corn used for grain and silage (~120 acres for this herd), 1,180 lb of elemental phosphorus is added to the system. On the export side, milk from 100 lactations of 20,000 lb each, or a total of two million lb of milk, will result in the export of 1,800 lb phosphorus from the system. Cull cows and bull calves from this herd will result in an additional 300 lb of phosphorus export from the system. In this example, a net buildup of 1,600 lb of phosphorus per year occurs on this farm. If the dietary level of phosphorus is brought into line with the feeding standard, say .38% of dietary DM, import of supplemental phosphorus could be reduced by 1,250 lb, a substantial portion of the annual accumulation of phosphorus in this example.

Table 5. Annual phosphorus balance for a 100 cow dairy averaging 20,000 lbs milk per cow¹

                       Import to farm (lbs of P)                         Export from farm (lbs of P)

100 tons of soybean meal = 1,220                             2,000,000 lb milk     = 1,800

Dicalcium phosphate²         = 1,500                             Cull cows and calves = 300

Grain³                              = 0                                    Surplus feed³             = 0

Forage³                                    = 0                                    Manure export           = 0

Fertilizer⁴                         = 1,180                             Runoff⁵                      = ~200

                                           3,900                                                                  2,300

¹Includes lactating and dry cows, and 80 heifer replacements. In this example, the farm produces all of the grain and forage, and purchases only protein and mineral/vitamin supplements.

²Assume dicalcium phosphate is used to raise dietary P from .36 to .48% of diet dry matter.

³If shelled corn is purchased or sold, figure 5.4 lbs P per ton. If alfalfa hay is purchased or sold, figure 6.0 lbs per ton.

⁴Assume 120 acres of corn is used to grow corn grain and corn silage for this herd. Fertilizer (9-23-30) is added at the rate of 100 lbs per acre. This results in 1,180 lbs of elemental P imported annually.

⁵Average P losses in runoff are likely to range between .2 to 3.0 lb per acre per year (Bundy, 1998).

With the crop yields we have in Wisconsin, 2-3 acres will be required for each mature cow (this includes her replacement heifer) averaging 20,000 lbs per lactation to utilize the phosphorus contained in the manure. Maintaining equilibrium in soil phosphorus levels will be easy for those dairy farms importing only protein supplement and necessary phosphorus supplement and necessary starter fertilizer. Dairy operations buying in all of their protein supplement and part of their grain needs will manage only if they eliminate excess phosphorus supplementation to the cows, and eliminate phosphorus fertilizer (which in all probability they will not need). Dairy operations importing all of their protein and grain needs will have to find additional land for manure application, for there is no way they will be able to achieve equilibrium in soil phosphorus levels without additional land for manure. We have increasing numbers of dairy producers in Wisconsin that currently produce sufficient forage to meet their needs, but purchase all of their concentrate feeds. This group of producers will need to make some adjustments.

The days are numbered for simply disposing of manure. It is likely that in the near future manure application to cropland will be restricted to the amount of nutrients that can be utilized by the crops. Phosphorus content of manure will likely be the determining factor for the application rate of manure, since the P:N ratio in manure is approximately twice the P:N ratio needed by crops. We can reduce dietary phosphorus in dairy diets from approximately .48% to .38%. This will reduce phosphorus excretion in the manure by 25-30%. This translates into 25-30% less land required for manure disposal. A strategy of reducing dietary phosphorus levels is a win-win situation for dairy producers, since both feed costs and environmental risk/cost of manure disposal are reduced.