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J. Anim Sci. 2007. 85:1727-1730. doi:10.2527/jas.2006-556
© 2007 American Society of Animal Science
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ANIMAL NUTRITION

Starch digestion by feedlot cattle: Predictions from analysis of feed and fecal starch and nitrogen

R. A. Zinn1, A. Barreras2, L. Corona3, F. N. Owens4 and R. A. Ware5

Department of Animal Science, University of California, Davis 95616


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
To evaluate the utility of N as a digestion marker to predict total tract starch digestion, data from 32 metabolism trials involving 147 steers and 637 individual starch digestibility measurements were compiled. All trials were conducted at the University of California Desert Research and Extension Center. Total tract starch digestibility was determined from concentrations of starch and chromic oxide in feed and feces. In all trials, the steers were adapted to diets for 10 d followed by 4 d for collection of samples of feces. During collection, fecal samples (approximately 200 g, wet basis) were obtained twice daily. Samples from each steer within each collection period were composited for analysis. Diets contained 46.5 ± 7.4% starch and 1.85 ± 0.20% N. Apparently digestible N as a percentage of diet DM was closely associated (r2 = 0.73; P < 0.001) with dietary N concentration. Fecal N concentration (FN, % of DM) explained 35% of the variation in fecal DM excretion (Sy.x = 4.3; P < 0.001). Incorporating FN into the model, starch digestion was estimated as follows: starch digestion, % of intake = 100 {1 – [(0.938 –0.497FN + 0.0853FN2) FS/DS]}, where FS is fecal starch concentration (% of DM) and DS is dietary starch concentration (% of DM; r2 = 0.94; Sy.x = 0.68; P < 0.001). Fecal starch concentration alone explained 96% of the variation (Sy.x = 0.45; P < 0.001) in total tract starch digestion: starch digestion, % = 99.9 – 0.413FS –0.0104FS2. Omitting cases in our data set in which observed total tract starch digestion was less than 95%, the r2 between FS and starch digestibility decreased to 0.82 (Sy.x = 0.26; n = 529). However, estimated starch digestion using the equation incorporating FN remained closely associated with the observed starch digestion (r2 = 0.90; Sy.x = 0.22; P < 0.001; n = 529). Equations also were developed to predict NEm and NEg concentrations of common feed grains based on starch digestibility and FS. Starch digestion can be accurately predicted based on FS. However, incorporation of FN into the model markedly enhanced the estimates of grain quality and the efficacy of processing when total tract starch digestion exceeded 95%.

Key Words: cattle • digestion • nitrogen • starch


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
The feeding value of cereal grains can be altered by grain source, grain processing, animal, and management conditions (Owens and Zinn, 2005Go). When grain is the primary or sole source of starch in the diet, the concentration of starch in feces (FS, % of DM) of feedlot steers can serve as an indicator of total tract starch digestion (TSD, % of intake = 100.5 – 0.6489FS; n = 64, r2 = 0.91; Zinn et al., 2002Go). This close relationship between FS and TSD was confirmed by Corona et al. (2005Go; TSD = 102.4 – 0.72FS (n = 16; r2 = 0.97). Decades ago, fecal N excretion was observed to be readily predicted from the concentration of dietary N in forage-based diets (Holter and Reid, 1959Go). More recently, in a summary of 445 digestion trials, Lukas et al. (2005)Go observed that fecal N concentration (FN, % of DM) could explain 82% of the variation in OM digestion of forage-based diets.

The objective of this study was to evaluate the utility of combining FN with FS into equations for estimating TSD of high-grain finishing diets in feedlot cattle.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
Database

A data set containing 637 individual starch digestibility measurements from 32 metabolism trials (147 cross-bred beef and Holstein steers) was compiled to evaluate the utility of FN as a digestion marker to enhance prediction of TSD when combined with measurement of FS. Dry matter intake ranged from 2.8 to 13.9 kg/d (1.99 ± 0.34% of BW); diets contained (DM basis) 16.0 ± 10.0% forage, 69.8 ± 11.5% grain, 25.2 to 61.9% starch (46.5 ± 7.4%), and 1.5 to 3.0% N (1.85 ± 0.20%). All diets contained supplemental urea (1.05 ± 0.18%) and an adequate amount of protein to meet requirements for maintenance and growth. In most cases diets were supplemented with an ionophore. Starch digestibility was determined using chromic oxide as an indigestible marker. In most of these experiments, grain processing methods, dietary grain levels, or feed supplements or additives were tested.

Animals and Sampling

Animal care and handling techniques were approved by the University of California Animal Care and Use Committee. All trials were conducted at the University of California Desert Research and Extension Center. All steers received growth implants. Steers were individually maintained in concrete slatted-floor pens (3.9 m2) with free access to water at all times. Equal proportions of the daily diet were fed at 0800 and 2000. Steers were adapted to the housing, feeding management, and their respective concentrate diets for at least 21 d before initiation of the individual experiments. Experimental periods were 2 wk, with 10 d for diet adjustment followed by 4 d for collection of feces. During the collection period, fecal samples were collected from the steers in the following sequence: d 1, 0750 and 1350; d 2, 0900 and 1500; d 3, 1050 and 1650; and d 4, 1200 and 1800. Individual fecal samples consisted of approximately 200 g (wet basis) of fecal material.

Sample Analysis and Calculations

Samples from each steer within each collection period were composited before being analyzed. Samples were subjected to all or part of the following analyses: DM (oven drying at 105°C until no further weight was lost), Kjeldahl N (AOAC, 1984Go), chromic oxide (Hill and Anderson, 1958Go), and starch (Zinn, 1990Go). Fecal starch excretion was calculated based on FS relative to chromic oxide, an indigestible marker included in the feed, whereas TSD was calculated from the relative ratios of starch and chromium in feed and feces. In addition, relationships of NEm were regressed against TSD and against FS for the individual sources of grain across processing methods. The NEm estimates were derived from growth performance trials that were concurrent with the metabolism trials.

Statistical Analysis

Data were analyzed by means of linear mixed model using PROC MIXED (SAS Inst. Inc., Cary, NC). In evaluation of statistical relationships between observed TSD (oTSD) and FS, the following model was employed: oTSDij = B0 +B1FSij + B2FSij2 + expi + bi1FSij + bi2FSij2 + eij, where expi = trial. The fixed effect component of the model is represented by B0 + B1FSij + B2FSij, and the random effect component of the model is represented by expi + bi1FSij2 + b2FSij2 + eij, where [expi bi1 bi2]' ~ iid N [(0 0 0)', their variance-covariance matrix]. In PROC MIXED, the TYPE = UN option of the RANDOM statement was used to specify an unstructured variance-covariance matrix. The variance-covariance components were subjected to a test of hypothesis using the option COVTEST. The resulting covariances between random parameters were not different from zero (P > 0.10). Therefore, a reduced model was fit without covariance components, changing TYPE = VC in the RANDOM statement of the PROC MIXED. In the analysis of oTSD vs. TSD predicted using FN correction (pTSD), the following linear mixed model was employed: pTSDij = B0 +B1oTSDij + expi+ bi1oTSDij + eij. The fixed effect component of the model is represented by B0 + B1oTSDij, and the random effect component of the model is represent by expi + bi1oTSDij + eij, where [expi bi1]' ~ iid N [(0 0)', their variance-covariance matrix]. The structure of the variance-covariance component employed was unstructured. In this case, the resulting covariances between random parameters were different from zero (P < 0.01). Correlation coefficients using PROC MIXED were estimated by regression of Yadjusted dependent variables (predicted Y values + residuals) against the respective observed independent variables (St-Pierre, 2001Go).


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
Apparently digestible N (ADN, % of dietary DM) was closely associated (r2 = 0.73, Sy.x = 4.28, P < 0.001; PROC MIXED covariance structure TYPE = CS), where Sy.x = standard error of the fit, and with the concentration of N in the diet (DN, % of DM): ADN = 0.983DN – 0.554. This equation is in fairly good agreement with the equation developed across a wide range of dietary roughage levels by Holter and Reid (1959)Go of ADN = 0.929DN –0.557. As these equations indicate, the intercept (presumably metabolic fecal N) comprises the majority (94 to 95%) of fecal N excretion for typical feedlot diets (1.8 to 2.1% N), whereas the slope, representing the true digestibility of N, was 98% (Sy.x = 0.069). The NRC (1985)Go compiled 21 different estimates of true N digestibility and noted that values ranged from 85 to 95% and were greater with diets based on concentrate than on roughage. Because dietary N contributes very little to fecal N excretion with these levels of dietary protein, Lukas et al. (2005)Go proposed that FN alone, due to its relatively constant output and high correlation with digestibility (r2 = 0.82), could be used to calculate DM digestibility. However, for our data set, FN explained only 35% of the variation in fecal DM excretion: fecal DM excretion, % of intake = 93.8 – 49.7FN + 8.53FN2 (R2 = 0.35; Sy.x = 4.1; P < 0.001). Accordingly, we estimated TSD from this estimate of fecal DM excretion and relative concentrations of starch in feed and feces: TSD, % of intake = 100 {1 – [(0.938 – 0.497FN + 0.0853FN2) FS/DS]}, where FS and DS are the percentages of DM represented by starch in feces and in the diet, respectively (r2 = 0.94; Sy.x = 0.68; P < 0.001; Figure 1Go).


Figure 1
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  		Figure 1. Relationship between predicted and observed starch digestion: predicted starch digestion, % of intake = 100 {1 – [(0.938 – 0.497FN + 0.0853FN2) FS/DS]}, where FN, FS, and DS are the percentages of DM represented by fecal N, fecal starch, and dietary starch, respectively [observed starch digestion, % = 1.003 (SE = 0.0009) x predicted starch digestion, r2 = 0.94].

 
Consistent with previous reports (Barajas and Zinn, 1998Go; Zinn et al., 2002Go; Corona et al., 2005Go; Owens and Zinn, 2005Go), FS alone could explain 96.6% of the variation (Sy.x = 0.453; P < 0.001) in TSD: TSD, % = 99.9 – 0.413FS – 0.0131FS2 (Figure 2Go). This simple method for estimation of TSD compares quite favorably with the more complex equation incorporating FN. However, the correlation coefficient in both equations is augmented statistically by the broad range in FS (0 to 44.2%) as compared with the mean value for FS (5.9%). Omitting 108 cases in our data set where observed TSD was < 95%, the r2 between FS and observed TSD decreased to 0.82 (TSD, % = 100.1 – 0.551FS; Sy.x = 0.256; n = 529), whereas estimates of TSD that utilized FN remained closely associated with observed TSD (r2 = 0.90; Sy.x = 0.216; P < 0.001; n = 529). Because field estimates of TSD often are used to appraise adequacy of processing method (quality control) of high moisture and steam-flaked grains where TSD usually exceeds 95%, the increased precision from including FN in equations to estimate TSD should prove relevant and useful.


Figure 2
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  		Figure 2. Relationship between fecal starch percentage (FS) and total tract starch digestion {starch digestion, % = 99.9 (SE = 0.0540) – [0.413 (SE = 0.0306) x FS] – [0.0131 (SE = 0.00192) x FS2], R2 = 0.96}.

 
Zinn et al. (2002)Go observed that TSD was closely associated (r2 = 0.88) with the NE value of corn: Corn NEm = –0.70 + 0.032 TSD. Thus, where corn is the principal source of dietary starch: corn NEm = 2.49 – 0.0127FS – 0.000292FS2, and NEg = 0.877 NEm – 0.41. The increase in NE value of corn with increased TSD cannot be a reflection of changes in TSD alone because the GE of starch is 4.18 Mcal/kg, corresponding to a NEm of 2.39 Mcal/kg [NEm = –0.568 + 0.708DE, r2 = 0.998; derived from NRC (2000)Go]. This suggests that changes in TSD alone can account for only about 75% of the increase in NEm. The remainder of the increase in NEm presumably is due to concomitant increases in digestion of the nonstarch OM constituents of the grain (Zinn et al., 1995Go, 2002Go). For practical assessment of grain processing effects, similar equations were developed by regression for other common feed grains based on previously measured NEm values calculated from growth performance of cattle fed the same diets, TSD, and FS:


Formula

Across cereal grains, the change in grain NEm with changes in TSD was remarkably constant, averaging a 0.0328 Mcal reduction in NEm for every 1 percentage unit decrease in TSD (r2 = 0.97; Figure 3Go), or a 0.162 Mcal reduction in NEm for every 1 percentage unit increase in FS (r2 = 0.85). This would imply that the deduction in feeding value for undigested starch is similar across grain sources.


Figure 3
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  		Figure 3. Relationship between the change in grain (corn, wheat, sorghum) NEm and the change in total tract starch digestion (CSD, %), where the change in grain NEm, Mcal/kg = 0.0328CSD, % (r2 = 0.97; Sy.x = 0.022; P <0.001).

 
In conclusion, starch digestion can be assessed quite accurately based on fecal starch alone. However, incorporating fecal N into the model enhanced precision for estimating grain quality and efficacy of processing when total tract starch digestion exceeded 95%.


   Footnotes
 
2 Present address: Instituto de Investigaciones en Ciencias Veterinarias, UABC. Mexicali, B.C. México. Back

3 Present address: Facultad de Medicina Veterinaria y Zootecnia, UNAM. D.F. México 04510. Back

4 Present address: Pioneer Hi-Bred International Inc., Johnston, IA 50131. Back

5 Present address: SarTec Corporation, Anoka, MN 55303. Back

1 Corresponding author: razinn{at}ucdavis.edu

Received for publication August 16, 2006. Accepted for publication March 23, 2007.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 


AOAC. 1984. Official Methods of Analysis. 14th ed. Assoc. Off. Anal. Chem., Arlington, VA.

Barajas, R., and R. A. Zinn. 1998. The feeding value of dry-rolled and steam-flaked corn in finishing diets for feedlot cattle: Influence of protein supplementation. J. Anim. Sci. 76:1744–1752.[Abstract/Free Full Text]

Corona, L., S. Rodriguez, R. A. Ware, and R. A. Zinn. 2005. Comparative effect of whole, ground, dry-rolled and steam-flaked corn on digestion and growth performance in feedlot cattle. Prof. Anim. Sci. 21:200–206.[Abstract/Free Full Text]

Hill, F. N., and D. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587–603.[Abstract/Free Full Text]

Holter, J. A., and J. T. Reid. 1959. Relationship between the concentrations of crude protein and apparently digestible protein in forages. J. Anim. Sci. 18:1339–1349.[Medline]

Lukas, M., K. H. Sudekum, G. Rave, K. Friedel, and A. Susenbeth. 2005. Relationship between fecal crude protein concentration and diet organic matter digestibility in cattle. J. Anim. Sci. 83:1332–1344.[Abstract/Free Full Text]

NRC. 1985. Ruminant Nitrogen Usage. Natl. Acad. Press, Washington, DC.

NRC. 2000. Nutrient Requirements of Beef Cattle. 7th rev. ed., 2000 Update. Natl. Acad. Press, Washington, DC.

Owens, F. N., and R. A. Zinn. 2005. Corn grain for cattle: Influence of processing on site and extent of digestion. Proc. Southwest Nutr. Conf. 86–112.

Salinas, J., E. G. Alvarez, and R. A. Zinn. 1999. Influence of tempering on the feeding value of steam-flaked sorghum for feedlot cattle. Proc. West. Sect. Am. Soc. Anim. Sci. 50:325–330.

St-Pierre, N. R. 2001. Invited Review: Integrating quantitative findings from multiple studies using mixed model methodology. J. Dairy Sci. 84:741–755.[Abstract]

Zinn, R. A. 1990. Influence of flake density on the comparative feeding value of steam-flaked corn for feedlot cattle. J. Anim. Sci. 68:767–775.[Abstract]

Zinn, R. A. 1993. Influence of processing on the comparative feeding value of barley for feedlot cattle. J. Anim. Sci. 71:3–10.[Abstract]

Zinn, R. A. 1994. Influence of flake thickness on the feeding value of steam-rolled wheat for feedlot cattle. J. Anim. Sci. 72:21–28.[Abstract]

Zinn, R. A., C. F. Adams, and M. S. Tamayo. 1995. Interaction of feed intake level on comparative ruminal and total tract digestion of dry-rolled and steam-flaked corn. J. Anim. Sci. 73:1239–1245.[Abstract]

Zinn, R. A., F. N. Owens, and R. A. Ware. 2002. Flaking corn: Processing mechanics, quality standards, and impacts on energy availability and performance of feedlot cattle. J. Anim. Sci. 80:1145–1156.[Abstract/Free Full Text]


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