Feed Management: Phosphorus Levels in Component-Fed Herds


Source: Pennsylvania State University


Feed management is a key practice in livestock nutrient management. It includes ration formulation for optimal supplementation of nitrogen and phosphorus, consistency and accuracy of feed delivery, regular feed testing, and diagnostic tools to monitor results. In 2007, Mid-Atlantic Water Program (MAWP) scientists applied the national feed management program to meet the needs of dairy consultants in the Chesapeake Basin. This program certifies consultants in precision feed management, a practice that reduces nutrient loads in animal wastes by minimizing the phosphorus and nitrogen content in the feed. Over the years, the work of this project team has established precision feed management as both an economically and environmentally viable best management practice. As such, state watershed implementation plans include precision feed management as a method to meet load allocations.

In 2011, fifty-one Pennsylvania operations received Environmental Quality Incentive Program or Chesapeake Bay Watershed Initiative funding from the Natural Resources Conservation Service (NRCS) for feed management. Pennsylvania currently has twenty-three NRCS qualified nutritionists to write feed management plans. Many of the contracted feed management plans on dairy and beef operations have completed the first year of implementation.

Observations From Contracted Dairy Operations

Data from the first year’s plans submitted to NRCS have revealed several on-going challenges. Focusing on phosphorus, herds feeding a total mixed ration (TMR) have shown a poor agreement between the formulated versus the actual diet fed. A majority of the actual TMRs analyzed show phosphorus levels outside an acceptable margin of ± 0.03% from the formulated, which is an above average margin. There are opportunities to improve precision feeding.

The other areas creating greater challenges are component-fed lactating, heifer, and dry cow groups. In several herds, some rations were reformulated to remove all inorganic phosphorus and fecal phosphorus remained unchanged or even increased. Fecal phosphorous levels in young calf groups are proving to be high as well. The Mid-Atlantic region is unique in that component-fed herds are commonplace compared to other areas of the country. Phosphorus research reported in peer-reviewed journals focuses on TMR-fed diets. Component-fed herds present a unique set of challenges compared to TMR-fed herds as it relates to feed management practices and their influence on phosphorus excretion. These include feeding sequence of forages and grains, feeding times of energy feeds versus protein feeds, and the number of feedings throughout the day.

Current data is lacking on expected fecal phosphorus levels for young-stock. Fecal phosphorus levels in young calf groups on some farms are testing well above the recommended range of 0.65 to 0.85% on a dry matter basis. Young-stock typically receive poor oversight on consumption rates and are generally fed a standard pellet that may or may not match the animals’ requirement for phosphorus. Deviation between consumption rates of phosphorus and expected excretion amounts are questioned as well. These are key areas that need to be better defined.

Another area that has not been addressed is sampling procedures. Observations of high fecal phosphorus levels have prompted questions about the potential of contamination in the fecal samples collected from cow or heifer groups. Plausible results could be associated with the addition of organic matter, i.e. bedding, during fecal sample collection. It was hypothesized that this may be contributing to higher than expected fecal phosphorus levels. Also, some grain mixtures were analyzed using near-infrared spectroscopy (NIR), which may be providing erroneous information especially when evaluating phosphorus content. Wet chemistry for phosphorus analysis typically provides more reliable results. The Penn State Extension Dairy Team submitted a proposal to the Mid-Atlantic Water Program to evaluate if the high phosphorus levels being observed on contracted dairy operations is a reflection of the ration fed, sampling error, or other issues.

Component Fed Herds – Sampling Procedures

In January 2013, six component-fed dairy operations were selected for intense sampling of ration ingredients and manure. The operations were located in the Northern, Western, Central, and Eastern parts of Pennsylvania. The goal was to select farms that had a diversity of forage programs that consisted of grass and alfalfa haylage as well as corn silage. The farms were also diverse in how they fed. Two farms fed conventionally in a free-stall set-up and four were in tie-stall barns for the lactating cows. All heifer groups were housed in pens.

Feed and fecal samples were collected on average producing cows, peak producing cows (20 pounds over average), and three heifer groups (3 to 7 months, 7 to 12 months, and 13 to 22 months). All feeds and fecal samples were submitted to Cumberland Valley Analytical Lab and were analyzed by wet chemistry. Fecal samples were analyzed for solids and P2O5.

Each farm was visited at the same time of day: 9:30 a.m. to 12:30 p.m. This time period was generally when all animal groups were being fed either additional forage or one of their grain feedings. The same people collected feed and fecal samples on each farm. Buckets were used to collect individual lactating cow manure samples (5 cows representing average milk production and 5 cows representing peak milk). Samples were taken from the cows as they were defecating or from the ground right after defecating, or from the rectum. Individual cow samples were composited for the average and peak producers. Both individual cows and composites were analyzed for fecal phosphorus. Only the composites were analyzed for fecal starch. For the heifers, fecal samples were taken directly from the animal or if they were observed defecating, a sample was taken from the pile without contamination from bedding. Only composited fecal samples for the heifer groups were analyzed.

Lactating Cow Results

Table 1 presents results from the lactating cows. The ideal range for fecal phosphorus based on field work conducted by The University of Pennsylvania (personal communication) is 0.55% to 0.80% on a dry matter basis. Five farms fell within this range and one farm slightly above, however, there was variation in the phosphorus intake and it did not appear to correlate to levels tested in the manure. There did not appear to be any relationship to the amount of corn silage fed, amount of phosphorus coming from the grain mix, feed efficiency, or fecal starch. The fecal phosphorus levels within farm for the average and peak producers were very similar (Figure 1).

Table 1. Summary results on the average and peak producing cows from six component fed herds.

Farm 1 Farm 2 Farm 3 Farm 4 Farm 5 Farm 6
CS – corn silage; TRDM – total ration dry matter; FE – feed efficiency (calculated using energy corrected milk divided by dry matter intake); P – phosphorus; DM – dry matter.
Housing Tie-stall Free-stall Tie-stall Free-stall Tie-stall Tie-stall
Feeding sequence Grain first Grain first Forage first Forage first Forage first Forage first
Cows at average production
% CS of TRDM 39.0 47.0 30.0 37.0 44.0 28.0
% P from grain 38.9 54.5 59.0 53.9 44.0 55.7
P intake, lbs. 0.14 0.15 0.22 0.25 0.18 0.25
FE 1.95 1.44 1.37 1.49 1.62 1.16
Fecal P % DM 0.75 0.59 0.85 0.79 0.73 0.64
Fecal starch % DM 2.50 5.30 5.40 13.8 5.30 9.60
Milk lbs. 70.0 62.0 67.0 71.0 71.0 64.5
Cows at peak production (Average + 20 pounds)
% CS of TRDM 30.0 42.0 28.0 34.5 35.0 27.0
% P from grain 61 66.8 60.5 53.5 59.4 58.9
P intake, lbs. 0.22 0.21 0.23 0.27 0.24 0.27
FE 1.49 1.71 1.61 1.81 1.65 1.44
Fecal P % DM 0.73 0.67 0.88 0.67 0.71 0.61
Fecal starch % DM 2.70 6.50 6.50 15.60 11.70 7.70

FM Fig 1
Figure 1. Fecal phosphorus as a percent of dry matter for the average and peak producing cows from six component fed herds.

All farms were evaluated using the Penn State Comprehensive Feed Management Monitoring Spreadsheet for Dairy Cows. The section was used that evaluates the phosphorus requirements using the 2001 NRC. Requirements are based on milk production, dry matter intake and stage of lactation. The phosphorus availability can be adjusted.

For the average producers 64% phosphorus availability was used and represents forages. Three of the six farms were close to feeding 100% of requirement while the other three farms exceeded 130%. On those farms, if the phosphorus availability was reduced to 50-55%, then they were within 110% of requirement.

For the peak producers, four of the six farms were at 100% requirement using 64% availability. The other two farms were within requirements using 56% availability. In the 1989 edition of the NRC 50% was used. As other data became available the absorption coefficients were increased. However, when working at the farm scale it appears that phosphorus availability may not be as high as what the 2001 NRC suggests. On herds feeding excellent quality forages and high forage based diets, phosphorus availability may be higher than the 64% currently used in the 2001 NRC.

Heifer Results

Table 2 and Figure 2 represent the heifer data. Phosphorus intake level and the percent of phosphorus coming from the grain did not appear to affect fecal phosphorus level. In general phosphorus fell within the recommended ranges regardless of the levels of forages or grains fed and over a wide range of intakes. Based on the limited research data for heifers, the goal for fecal phosphorus is 0.65 to 0.85% on a dry matter basis. For the weaned heifer group, calves from Farm 3 received a commercial calf starter as the only feed. Their fecal phosphorus was within the recommended range and matched the other farms (except for Farm 6) where the heifers were receiving hay and grain combined. Farm 6 was feeding the weaned calves and breeding age heifers the milk cow grain mix and very low quality hay. This farm had the highest fecal phosphorus levels for these age groups.

Table 2. Intake results for the heifer groups from six component fed herds.
Farm 1 Farm 2 Farm 3 Farm 4 Farm 5 Farm 6
3 to 7 month old
% P from grain 20.6 43.3 100.0 55.0 66.9 79.6
P intake, lbs. 0.050 0.046 0.038 0.028 0.044 0.043
7 to 12 month old
% P from grain 15.4 37.8 38.9 43.7 44.6 60.9
P intake, lbs. 0.036 0.055 0.036 0.090 0.040 0.057
13 to 22 month old
% P from grain 10.0 31.7 17.7 15.3 40.3 44.7
P intake, lbs. 0.040 0.066 0.080 0.076 0.048 0.070

FM Fig 2
Figure 2. Fecal phosphorus as a percent of dry matter for weaned calves, breeding age heifers and bred heifers from six component fed herds.

A recent paper by Bjelland et al. (2011) evaluated heifer diets with and without supplemental phosphorus. On heifers consuming 19 to 21 pounds of dry matter and consuming 0.07 to 0.08 pounds of phosphorus, fecal phosphorus on a dry matter basis was 0.54% without supplementation compared to 0.70% with supplementation. Similar aged heifers were sampled at the Pennsylvania State University dairy herd. They were consuming 0.07 pounds of phosphorus (no supplemental phosphorus). The fecal phosphorus tested 0.70% on a dry matter basis. These values fall within the recommended guidelines.

Prediction Equations For Phosphorus Excretion

There are several prediction equations developed for lactating cows and heifers that estimate the amount of phosphorus excreted based on different inputs. Since all the pertinent input information was collected on these six component-fed farms, their data were entered and evaluated. Figure 3 lists the equations evaluated. Of the three equations used for the lactating cow phosphorus excreted, the ASAE 2005 equation required the most inputs. This equation estimates the total volume of manure excreted and coupled with the actual manure analyses the total phosphorus output was calculated. As an example, using the average of all six farms, the total amount of manure excreted was 147 pounds, the manure solids percent was 14.57 and the percent fecal phosphorus was 0.72. This equated to an average of 0.154 pounds of phosphorus excreted on a dry matter basis. For this paper, the ASAE 2005 equation was used as the standard to compare against the other equations.

FM fig 3
Figure 3. Prediction equations used for phosphorus excretion.

There was extreme variation in the estimated phosphorus excretion numbers using the various equations, especially between the average and peak producing cows. The Nennich 2005 equation overestimated phosphorus excretion by 21% on two out of the six farms for the average producers and by 18% on five out of the six farms for peak producers. The ASAE 2001 equation consistently underpredicted phosphorus excretion by 11% on all farms regardless of milk production.

One striking observation was the amount of phosphorus excreted compared to what was consumed. It has been generally stated that 60% of phosphorus consumed is excreted in the feces. Regardless of the equation used to calculate phosphorus output, the percent excreted ranged from 51 to 92% of what was consumed. One potential influencer is the digestibility of the diet and the volume of manure dry matter excreted. Many of the peer reviewed papers examining phosphorus excretion fed high concentrate or highly digestible diets (Knowlton et al., 2002). The trend in the dairy industry the last several years has been to feed high forage based diets, which many of the research papers do not represent.


The data collected on six component fed farms and comparing the results to some farms contracted through USDA-NRCS for feed management does show some multifaceted issues. Dou et al. (2010) examined the phosphorus status on farms from several states in the northeast. Based on their data, Dou et al. recommended using a composite of at least five cow fecal samples and obtaining fecal samples from healthy, lactating cows in the same feeding group by rectal grab or mid-air catch to avoid “contamination” from urine or other non-fecal materials. Fecal sampling error is definitely a possibility, especially for heifers and dry cows. The fact that heifers sampled on this project were fed rations ranging from heavy concentrate to heavy forage and the fecal phosphorus percent tested within the recommended ranges suggests sampling errors may be occurring on contracted farms. Using the percent of requirement for phosphorus intake from the 2001 NRC does not show a consistent correlation. The different prediction equations available to estimate phosphorus output can vary greatly and do not match very well with actual fecal analyses.

For the lactating groups, herds that have implemented precision feeding and removed all inorganic phosphorus can maintain fecal phosphorus levels from 0.55 to 0.80% on a dry matter basis. Herds exceeding this level have opportunity for improvement. Some contracted farms are observing fecal phosphorus levels over 1.0% on a dry matter basis. Based on the herds in this project, fecal phosphorus below 0.80% is achievable. The mismatch of fecal phosphorus to the ration being fed can still be an indication that precision feeding is not being achieved or sampling techniques can be improved.


  • Bjelland, D. W., K. A. Weigel, P. C. Hoffman, N. M. Esser, and W. K. Coblentz. 2011. The effect of feeding dairy heifers diets with and without supplemental phosphorus on growth, reproductive efficiency, health, and lactation performance. J. Dairy Sci. 94:6233-6242.
  • Dou Z., C. F. Ramberg Jr., L. Chapuis-Lardy, J. D. Toth, Z. Wu, L. E. Chase, R. A. Kohn, K. F. Knowlton, and J. D. Ferguson. 2010. A fecal test for assessing phosphorus overfeeding on dairy farms: Evaluation using extensive farm data. J. Dairy Sci. 93:830-839.
  • Knowlton K. F., and J. H. Herbein. 2002. Phosphorus partitioning during early lactation in dairy cows fed diets varying in phosphorus content. J. Dairy Sci. 85:1227-1236
  • National Research Council. 1989. Nutrient Requirements of Dairy Cattle. 6th rev. ed. Natl. Acad. Sci., Washington D.C.
  • National Research Council. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Sci., Washington D.C.

Written by Virginia Ishler, Rebecca White, Department of Animal Science, Penn State and Dan Ludwig, Pennsylvania USDA-NRCS

Reviewed by Dr. Gabriella Varga and Coleen Jones, Penn State, and Dr. Jamie Jarrett, Purina Animal Nutrition LLC