1. A fecal P test for assessing P overfeeding on dairy farms
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Materials and Methods
Samples obtained from >90 dairy farms in NY, PA, MD, DE during
1 feed sample and 7-17 fecal samples each sampling time.
Feed samples analyzed for total P and nutritional parameters,
Fecal samples analyzed for total P and extractable Pi.
A total of 574 feed samples and >4,000 fecal samples used in this study.
Extraction: 2 g wet sample, 98 ml 0.1% HCl (v/v), 1 h shaking, filter (Whatman #42), filtrate tested for inorganic P (molybdate blue method; Ref. 2).
Z. Dou, C. Ramberg, J. Toth, C. Wang, J. Ferguson, R. Munson, Z. Wu, Univ Penn
R. Kohn, Univ Maryland; K. Knowlton, Virginia Tech; L. Chase, Cornell
Abstract
For various reasons many dairy farms still feed cows with excessive P in diets. We report the results of using a fecal P test to assess if P overfeeding is occurring on farms. Feed and fecal samples were obtained from >90 commercial dairies in the Northeast and Mid-Atlantic regions. Inorganic P (Pi) concentrations in 0.1% HCl extracts of feces closely reflect dietary P changes whereas extractable organic P (Po) and residual P was independent of dietary P changes. Apparently, excess P in feed intake is digested but not utilized by the animal and subsequently excreted in feces in a form that is extractable in 0.1% HCl. Therefore, fecal extract Pi provides a measure of the relative magnitude of excess dietary P. The potential usefulness of a fecal-based P test as a management tool is apparent. If a benchmark value is designated to represent adequate P status, we would be able to assess whether excess P intake is occurring in a herd by measuring 0.1% HCl extractable Pi in feces and comparing it with the benchmark. The fecal P test proposed here has the following features: Sample collection is easy and non-invasive, and the laboratory procedures are straightforward and inexpensive. From an environmental perspective, 0.1% HCl-extractable Pi is mostly labile and includes much of the P fraction that is most vulnerable to potential losses after the manure is land applied.
Results
Significant correlation between dietary P and fecal P ( total or extract Pi); higher R2 value for extract Pi (Fig. 2).
Results consistent with previous finding (comparing Fig. 2 with Fig. 3), that is, extract Pi reflects dietary P changes whereas the remaining (TP – extract Pi) does not.
Understanding P Flows in Dairy Cows
P in feed intake is partitioned into available pool and unavailable feed residue, the latter is excreted in feces. Some of the available P is absorbed, some is unabsorbed and excreted in feces. Microbial residues and sloughed cells excreted in feces also contain certain amounts of P.
Presumably, the greater the excess P in feed intake, the higher the unabsorbed P in feces.
In a previous study (Ref. 1): the higher the dietary P, the greater the extract Pi in 0.1% HCl, suggesting the possibility of using fecal extract Pi as an indicator of P status.
Feed P Intake
Available Pool
Unavailable Feed Residue
Absorbed
Microbial
Unabsorbed
Milk
Tissues
Feces
Saliva
Urine
Sloughed cells
Fig. 2. Both fecal total P (TP) and extract Pi (0.1% HCl) increase as dietary P increases. Extract Pi better reflects dietary P changes with a higher R2 value; TP-Pi not affected by dietary P.
Fig. 3. Results from a previous study (ref. 1). Fecal extract Pi responds to dietary P changes whereas extract Po as well as residual P not affected.
Discussion
Compared to results from research feeding trials (Fig. 4), the farm-based data show a wide range in fecal extract Pi at any given dietary P (Fig. 2). Possible explanations:
Variability in P bioavailability: low bioavailability resulting in a dilution effect and vice versa.
Variability in P utilization by the animal: low utilization leading to a high excretion and vice versa.
Feed samples not representative of what cows ate.
Fig. 1. A schematic illustration of P flow pathways in lactating dairy cows, linked with fecal P excretion (Ref. 1)
Objectives
To further test the relationship between dietary P levels and fecal extract Pi (0.1% HCl) with samples from >90 commercial dairies.
To discuss benchmark values of fecal extract P indicating adequate and excessive P status.
Fig. 4. Fecal extract Pi closely reflects dietary P changes; samples obtained from research feeding trials (ref. 3).

2. Discussion (Cont’d) Conclusions References Acknowledgement
Benchmark values indicating adequate or excess P status: Where to draw the line?
Adequate status: dietary P <4.0 g/kg, corresponding fecal extract Pi <≈ 5.0 g/kg
Sensitivity = 0.68, specificity = 0.80, false positive rate = 0.20
Excess status: dietary P >4.5 g/kg, fecal extract Pi >≈ 6.0 g/kg
Sensitivity = 0.33, specificity = 0.94, false positive rate = 0.06
Conclusions
Predicting P status of dairy cows will always be an inexact science. However, the fecal P test provides a pragmatically useful, “quick and dirty” assessment tool that produces answers that are good enough as a first approximation.
Fecal extract Pi ≈ 5.0 g/kg indicates adequacy, > 6.0 g/kg indicates excess.
References
1. Dou et al., Environ. Sci. Technol :4361.
2. Murphy and Riley. Anal. Chem. Acta :31.
3. Dou et al., J. Environ. Qual :2058
Acknowledgement
Funding provided by USDA CSREES IFAFS Program Grant