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Relationship between fecal crude protein concentration and diet organic matter digestibility in cattle¹

M. Lukas^*,2, K.-H. Südekum^*,3,4, G. Rave, K. Friedel and A. Susenbeth^*

^* Institute of Animal Nutrition and Physiology and and Variationsstatistik, Christian-Albrechts-University, 24098 Kiel, Germany; and and Institute of Farm Animal Sciences and Technology, University of Rostock, 18051 Rostock, Germany

	Abstract

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The positive relationship between fecal CP concentration and diet OM digestibility in cattle, which is based on increasing undigested microbial CP and decreasing fecal OM as OM digestibility increases, may be used as an indirect method for estimating diet OM digestibility from fecal CP concentration. Results of digestibility trials (445 individual observations) conducted at Hohenheim and Braunschweig, Germany, and at Gumpenstein, Austria, were used to study the relationship between CP concentration in feces (x, g/kg OM) and OM digestibility (y, %). The best fit was obtained with the curvilinear relationship y = a_i –107.7e^{(–0.01515 x x)}, with a₁ = 79.76 and a₂ = 72.86 (R² = 0.82; residual SD = 2.7; SE = 0.13), which takes into account the effects of location (i = 1 for Braunschweig and Hohenheim, and i = 2 for Gumpenstein). Dietary CP and crude fat concentration, and DMI had no effect on fecal CP content, whereas crude fiber content, proportion of concentrate in the diet, and forage type significantly affected CP concentration in feces; however, the magnitude of these effects was less than 2 percentage units, and the direction of the effect of proportion of concentrate in the diet was not uniform. The curvilinear relationship between fecal CP concentration (observed range, 100 to 300 g/kg of OM) and diet OM digestibility (observed range = 57 to 80%) may be used to estimate diet OM digestibility, particularly for field trials, as it requires no feed samples and does not physically restrict the animal.

Key Words: Cattle • Crude Protein • Digestibility Estimates • Fecal Composition • Feces

	Introduction

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The quality of a diet, particularly the energy value, is characterized by diet OM digestibility. In field trials or in grazing animals, OM digestibility cannot be determined by quantitative measurements of feed intake and excretion of feces. Therefore, indirect methods have to be used (e.g., in vitro techniques, internal markers, or the CP concentration in feces). The major difficulty associated with in vitro techniques and the use of internal markers may be the accurate collection of representative feed samples of the diet actually consumed by the animal, particularly when animals are selecting on heterogeneous pastures. Animals fitted with esophageal fistulas have been used to collect the selected material ingested on pastures, but this method requires the surgical preparation of animals (Le Du and Penning, 1982). Use of fecal CP concentration to estimate OM digestibility is suitable when complex diets and selective eating prevent representative sampling because this method does not require feed samples or impose any restriction to the animals.

Estimating OM digestibility from CP concentration in fecal samples requires generally applicable regression equations. Only a few general regression equations have been published based on conserved forage-based diets (Schmidt, 1993; Schmidt and Jentsch, 1994) or on grazing animals (Holloway et al., 1981; Leite and Stuth, 1990).

The aim of this study was to describe the relationship between fecal CP concentration and diet OM digestibility using a large data set from digestibility trials with a great variation in diet composition, and to investigate possible influences of diet composition (i.e., proportion of concentrate, type of forage, and feed intake). We also tested whether replacement of fecal CP with fecal acid detergent-soluble CP (ADSCP), which was assumed to represent nondietary CP (Van Soest, 1965; Mason, 1969), improved the relationship between fecal CP concentration and diet OM digestibility.

	Materials and Methods

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Database
A data set of 445 individual observations was assembled from digestibility trials conducted at the Institutes of Animal Nutrition at Hohenheim University, Stuttgart (Hohenheim; n = 40) and at the Federal Research Center for Agriculture, Braunschweig (Braunschweig; n = 217) in Germany, and at the Federal Research Institute for Alpine Regions Gumpenstein, Irdning, Austria (Gumpenstein; n = 188). The trials were conducted with dairy cows (German Friesian, Simmental, and Brown Swiss) and adult steers (Hinterwälder) fed diets containing different types of forages and a variety of concentrate ingredients (see Appendix tables). The OM digestibility of the diets ranged from 56.6 to 80.0%, DMI from 4.2 to 21.2 kg/d, proportion of concentrate from 0 to 61% of DM, and CP from 6.5 to 19.2% of DM. The animals were kept in individual tie stalls. The adaptation period to the experimental diets lasted at least 10 d, followed by no less than 5 d of collection. Quantities of feeds offered and refused were recorded at each meal individually and analyzed for DM and nutrient contents. Total fecal output was collected quantitatively in Braunschweig and Gumpenstein, and in the steer trials at Hohenheim University. In the dairy cow trials at Hohenheim, digestibility was estimated from fecal samples that were collected by rectal grab sampling twice daily over 7 d according to a schedule wherein the 7-d sample represented 2-h intervals over a 24-h cycle. Titanium(IV)-oxide was used as digestibility marker. At the end of the collection period, the complete amount of feces from each animal was mixed and subsequently analyzed. Analyses of DM and proximate constituents of feeds and feces were conducted according to the official methods in Germany (Braunschweig and Hohenheim samples; Bassler, 1976) and Austria (Gumpenstein samples; ALVA, 1983). Briefly, feces DM was determined by drying at 65°C over 72 h. The feedstuffs DM was determined by drying at 105°C, and OM was estimated by ashing at 550°C overnight (OM = DM – ash). The CP (N x 6.25) was determined using the standard Kjeldahl procedure using Cu²⁺ as a catalyst. Fecal CP was determined on undried samples. Crude fat was analyzed as benzine extract by the Soxhlet method. Crude fiber was determined as loss on ignition of the dried residue remaining after digestion of sample with 0.13 M H₂SO₄ and 0.23 M KOH. Nitrogen-free extract (NfE) was calculated as NfE (%) = 100 – % ash – % crude fiber – % crude fat –% CP.

The ADSCP concentration of 404 fecal samples was calculated from the difference between total CP and CP in the ADF fraction, which was determined as the "C" fraction of the Cornell Net Carbohydrate and Protein System according to Licitra et al. (1996).

Calculations and Statistical Analyses
Application of Existing Equations.
Two regression equations were applied to the data set of the 445 individual observations described above. Equation [1] was derived from digestibility trials published before 1984 (K. Friedel and K. Nehring, personal communication):

[1]

where y is OM digestibility (%), and x is g of CP/g of fecal OM (n = 474; R² = 0.80; residual SD = 2.6). Trials were conducted with dairy cows, steers, and bulls fed diets based on artificially dried green crops, hay, or straw. The mean values for OM digestibility and fecal CP concentration that were based on 3 to 12 animals per diet were used for deriving Eq. [1].

Equation [2] was derived from mean values based on four to six observations per diet (Schmidt and Jentsch, 1994):

[2]

where y is OM digestibility (observed range, 50.8 to 81.8%), and x is g N/kg of fecal OM (n = 159; R² = 0.86; residual SD = 2.7). Trials were conducted with fattening bulls and steers fed silage-based diets, and with dairy cows fed diets based on artificially dried green crops. The OM digestibility ranged from 50.8 to 81.8%, DMI from 4.8 to 18.6 kg/d, and diet CP concentration from 7.8 to 26.7% of DM.

Development of New Regression Equations.
Regression equations were derived to describe the nonlinear relationship between CP or ADSCP concentration in fecal OM and OM digestibility using the data set of 445 individual observations described above. First, the following model was used:

[3]

where y is OM digestibility (%), and x is CP or ADSCP in fecal OM (g/kg), respectively. Data from Gumpenstein were significantly overestimated by this equation, whereas those from Braunschweig and Hohenheim were significantly underestimated. Therefore, the model was modified to take into account the effect of location (a_i):

[4]

where i = 1 for Braunschweig, i = 2 for Hohenheim, and i = 3 for Gumpenstein. The calculations were carried out with the NLIN procedure of SAS (SAS Inst., Inc., Cary, NC).

Further dietary factors were tested as to their effect on the relationship between CP concentration in feces and OM digestibility; however, it was not possible to quantify possible effects within the model presented above (Eq. [4]) because DMI was not equally distributed over the range of fecal CP concentrations, and DMI was correlated with diet OM digestibility and with proportion of concentrate in the diets. Likewise, the proportion of concentrate was positively correlated with OM digestibility. Because quadratic and cubic models were sometimes superior to linear models in predicting forage consumption and digestibility (Holloway et al., 1981) and total daily DMI (Gruber et al., 2004), linear, quadratic, and cubic models were used to test possible effects of the factors (x): crude fat, CP, crude fiber, and NfE concentrations of the diet; type of forage; proportion of concentrate in the diet; and DMI, on the differences between observed and estimated OM digestibility values. The GLM procedure of SAS was used to determine the coefficients (b_i) of the following general type of equation:

[5]

where y is the difference between observed OM digestibility and that estimated from Eq. [6] given below, and x is the respective variable; only b-values with a significance level of P < 0.05 were considered further. The effects of type and quality of forage were tested with the Tukey-Kramer adjustment for multiple comparisons. In addition, the effect of dietary crude fat concentration also was tested within a selected number of data from experiments in which only dietary crude fat concentration was varied systematically.

	Results

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Mean values and SD of the differences between observed OM digestibility and those estimated from Eq. [1] and [2] (Table 1) were almost identical for both equations. For the data from Braunschweig and Hohenheim, the mean differences were small for both equations (–0.5 to 0.6 percentage units). However, Eq. [1] and [2] failed to estimate correct values for data from Gumpenstein (mean overestimations of 6.5 and 6.6 percentage units, respectively). The SD of the difference between observed and predicted OM digestibility values was the lowest for the data from Braunschweig (2.5 and 2.2 percentage units for Eq. [1] and [2], respectively), and greatest for data from Gumpenstein (3.0 and 3.2 percentage units for Eq. [1] and [2], respectively).

[6]

with a₁ = 79.76 and a₂ = 72.86 (R² = 0.82; residual SD = 2.7; SE = 0.13). The data plot is depicted in Figure 1. When ADSCP content in fecal OM was used as independent variable (Eq. [7]), the difference of –3.5 percentage units between the a-values of Braunschweig and Hohenheim and of –6.6 percentage units between those of Braunschweig and Gumpenstein did not permit pooling of data across locations. It should be noted that, in contrast to Eq. [1] and [2], where mean values of groups of animals were used, the coefficients of determination and SD of developed Eq. [4], [6], and [7] were based on individual observations.

View larger version (24K): [in this window] [in a new window]   Figure 1. The relationship between fecal CP (x) concentration and diet OM digestibility (y) derived from Equation [6]: y = ai – 107.7e(–0.01515 x x) with a1 = 79.76 (——), using data from Braunschweig () and Hohenheim (x), and a2 = 72.86 (— —), using data from Gumpenstein (); and estimated from the theoretical Eq. [8]: y = ([28.08 x + 13.7 diet OM (% of DM)])/(100 – x) (- - - - -). See text for details.

	Discussion

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Ruminal microbial CP is digested in the small intestine, whereas microbial CP synthesized in the hindgut is completely excreted, resulting in increased CP concentration in feces. Increased DMI resulted in increased ruminal passage rate, decreasing the extent of carbohydrate fermentation in the rumen, which may enhance hindgut fermentation (Owens and Goetsch, 1986; Owens et al., 1986). Undigested dietary CP will adversely affect the relationship between OM digestibility and CP concentration in feces because of increased CP concentration in feces without a proportional change in OM digestibility. Overheating of feeds or overprotection from ruminal degradation of protein concentrates will lead to a distinct rise of undigestible CP, which is bound in artifact lignin (Van Soest, 1965; Van Soest and Mason, 1991).

The CP concentration of fecal OM was considered to be inaccurate as a generally valid marker of OM digestibility (Le Du and Penning, 1982; Armstrong et al., 1989) due to effects of animals and feed (e.g., forage species, location, season of the year, and fertilization), which are not reflected by OM digestibility. Therefore, those authors (Le Du and Penning, 1982; Armstrong et al., 1989) recommended the use of only special equations to estimate digestibility in grazing animals, derived from indoor trials with the same feed. However, it can be assumed that the small databases in the previous studies and the inaccurate fit of linear regression models might influence the conclusions. Others, however, who evaluated the potential of CP concentration in fecal OM for estimating OM digestibility on pasture (Bartiaux-Thill and Oger, 1986; Boval et al., 1996) or for a variety of diets based on conserved forages fed indoors (Schmidt and Jentsch, 1994; Eq. [1], this paper), have confirmed that precise, generally applicable equations can be derived.

In this study, the effects of dietary factors other than digestibility on CP concentration in fecal OM were evaluated. The CP and crude fat concentrations of the diet had no effect, whereas, although small in magnitude, effects were observed for crude fiber and NfE concentrations of the diet, the proportion of concentrate, and for forage type. However, the small effect of the proportion of concentrate was not uniform because it was correlated with factors such as DMI, forage type, and concentrate ingredients, which also might affect the relationship between OM digestibility and fecal CP concentration. The effects of type of concentrate ingredient, ruminal degradability, and source of carbohydrates could not be assessed in this study. The effects of forage type were small within the data sets from Braunschweig and Hohenheim; however, the distinct difference compared with the data from Gumpenstein could be caused by forage type. Hay from Alpine grassland, which was used in the trials carried out in Gumpenstein, is characterized by exceptional species diversity, a short vegetation period, and specific management of the grassland swards (Buchgraber et al., 1997). These characteristics are not normally detected by routine chemical feed analyses such as CP, fiber fractions, and ash, or in vitro digestibility estimates. Likewise, there are no obvious reasons for the significantly higher fecal CP concentration in the Gumpenstein trials compared with those at Braunschweig and Hohenheim at similar OM digestibilities. Therefore, in Eq. [6], this difference is taken into account as a location constant. Equations [1] and [2] agreed well with Eq. [6] when using the location constant for Braunschweig and Hohenheim (a_BH). This confirms the broad validity of Eq. [6] with a_BH and the particularity of the Gumpenstein (i.e., Alpine) data. However, care should be taken when applying Eq. [6] to estimate the digestibility of diets of different origin (e.g., tropical forages or feeds causing an increased proportion of fermentation in the large intestine). Therefore, the equation should be validated against an independent data set to test its general validity.

In addition to the empirical regression equations (Table 2), an equation based on theoretical considerations was derived to describe the relationship between OM digestibility (x; %) and fecal CP concentration (y; g/kg). Based on published evidence, we assumed the following: 1) microbial CP synthesis of 156 g/kg of apparently digested OM intake (GfE, 2001; n = 335; 61 diets; RNA or ¹⁵N as microbial marker); 2) digestion in and absorption from the small intestine of microbial CP of 82% (Hagemeister et al., 1981); 3) endogenous N excretion of 2.19 g/kg of DMI (GfE, 2001). Microbial CP from hindgut synthesis and factors other than OM digestibility that might exert an effect on ruminal microbial and endogenous CP were not considered. Based on the assumptions given above, the following equation was derived:

[8]

where x is OM digestibility (%), and y is fecal CP concentration (g/kg OM). Figure 1 indicates that theoretical and empirical equations yielded similar patterns of the relationship. The shift to lower fecal CP concentrations at identical OM digestibility values for the theoretical equation is assumed to be caused mainly by undigested dietary CP, which was not accounted for in the theoretical equation.

Regressions of OM digestibility on the CP-to-OM ratio in feces have been performed with linear (Bartiaux-Thill and Oger, 1986; Armstrong, 1989; Boval et al., 1996), quadratic (Bartiaux-Thill and Oger, 1986; Boval et al., 1996), and hyperbolic (Schmidt, 1993; Schmidt and Jentsch, 1994; Boval et al., 1996) models. The relationship between fecal CP concentration and OM digestibility is not linear and, therefore, nonlinear models should be used. The disadvantage of the quadratic model is a decrease of estimated OM digestibility values for CP concentrations beyond the maximum of the curve (e.g., at 314 g/kg of OM in Eq. [1]), although remarkably higher CP concentrations in feces may occur. The hyperbolic model was hypothesized as being superior to others (Schmidt, 1993; Schmidt and Jentsch, 1994; Boval et al., 1996). Such functions describe the rapidly increasing OM digestibility per unit of CP in fecal OM and the relatively sharp curvature before reaching maximum OM digestibility, which is not related to CP concentration. This type of relationship also is well described by a first-order exponential function, which was used in this study, and which resulted in a better fit of data than a hyperbolic function.

The relationship between OM digestibility and the CP concentration in feces OM is based on changes in nondietary CP in feces, and inclusion of indigestible dietary CP in the model might impair the relationship. Therefore, we determined the ADSCP fraction, which was assumed to represent nondietary fecal CP (Van Soest, 1965; Mason, 1969). Nonetheless, ADSCP did not improve the estimate, as the coefficients of determination (R²) were lower and the residual SD were higher when OM digestibility was related to ADSCP concentration in fecal OM using the same type of equation as that used for fecal CP concentration. In contrast, the coefficient of determination of Eq. [7] was less and the residual SD greater compared with the equations in which CP concentration in fecal OM was used as an independent variable. Relating OM digestibility to the ADSCP fraction did not lead to a decrease in the difference between data from Gumpenstein and other locations. Therefore, the higher CP concentration in fecal OM at similar OM digestibility values of Gumpenstein cannot be explained by a higher proportion of undigested dietary CP in feces. The inability to improve the estimate by using ADSCP instead of CP may be caused partially by the fact that, in this study, there were no diets containing elevated levels of heat-damaged or overprotected undigested dietary CP. Similarly, Boval et al. (1996) were not able to obtain an improved estimate of OM digestibility of green crops, which also do not contain higher levels of indigestible CP, by including in their estimates fecal ADIN in addition to CP concentration. Moreover, the weaker relationship of OM digestibility with ADSCP than with CP may be due to the inevitably greater analytical error that is associated with ADF plus CP analyses on feces than with the robust CP determination alone.

In conclusion, the curvilinear relationship between fecal CP concentration and diet OM digestibility may be used to estimate diet OM digestibility, in particular for field trials, as it requires no feed samples and does not physically restrict the animal. Dietary factors other than digestibility affected fecal CP concentration only to a small extent; however, diets with hay from Alpine regions led to higher fecal CP concentrations than forage diets at other locations at a given OM digestibility. Replacing fecal CP in the regression equations, with ADSCP representing nondietary CP, did not improve the relationship between fecal CP concentration and diet OM digestibility. Further studies should validate the equation (OM digestibility, % = 79.76 –107.7e^{(–0.01515 x fecal CP [g/kg OM])}; R² = 0.82; residual SD = 2.7) by an independent data set and should provide information about the variation in fecal CP concentration of grab samples to determine the necessary number of random fecal samples in field trials.

	APPENDIX

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	Footnotes

 
¹ Appreciation is extended to the Stiftung Schleswig-Holsteinische Landschaft for financial support under the research program "Nitrogen Cycle on a Specialized Dairy Cattle Farm." We thank L. Gruber, Federal Res. Inst. for Alpine Regions Gumpenstein, Irdning, Austria; P. Lebzien, Federal Res. Center for Agric., Braunschweig, Germany; and H. Steingaß, Hohenheim Univ., Stuttgart, Germany, for kindly sharing their extensive databases and for their generous willingness to provide us with any information we needed.

² Current address: Vollkraft Mischfutterwerke GmbH, Friedrich-Voss-Strasse 11, 24768 Rendsburg, Germany.

³ Current address: Inst. Anim. Sci., Univ. of Bonn, Endenicher Allee 15, 53115 Bonn, Germany.

⁴ Correspondence—phone: +49-228-73-2287; fax: +49-228-73-2295; e-mail: ksue{at}itz.uni-bonn.de.

Received for publication August 17, 2004. Accepted for publication February 24, 2005.

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