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Site and extent of digestion and amino acid flow to the small intestine in beef cattle consuming limited amounts of forage¹

Department of Animal Science, University of Wyoming, Laramie, WY 82071-3684

Abstract

Eight Angus x Gelbvieh heifers (445 ± 74.5 kg) fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square double double-cross-over designed experiment to assess the effect of restricted forage intake on site and extent of digestion and flow of essential AA amino acids to the small intestine. Heifers were fed chopped (2.54 cm) bromegrass hay (9.2% CP, 64% NDF on an OM basis) at one of four percentages of maintenance (30, 60, 90, and 120%). Experimental periods were 21 d in length, with 17 d of adaptation followed by 4 d of intensive sample collection, after which maintenance requirements and subsequent level of intake were adjusted for BW change. True ruminal OM, NDF, and N digestion (g/d) decreased linearly (P < 0.001) with decreasing forage intake. When expressed as a percentage of OM intake, true ruminal OM and N digestibility were not affected (P = 0.23 to 0.87), whereas ruminal NDF digestibility tended to increase (P = 0.09) as forage intake decreased. Total and microbial essential amino acid flow to the duodenum decreased linearly (P = 0.001) from 496.1 to 132.1 g/d and 329.1 to 96.0 g/d, as intake decreased from 120 to 30% of maintenance intake, respectively. Although the profile of individual essential amino acids in duodenal digesta (P = 0.001 to 0.07) and isolated ruminal microbes differed (P = 0.001 to 0.09) across treatment, the greatest difference noted for total and microbial essential amino acid profile was only 0.3 percentage units. Because total and microbial flow of essential amino acids to the small intestine decreased as OM intake decreased, but true ruminal degradability of individual essential amino acids (P = 0.17 to 0.99) and digesta essential amino acid profile were comparable across treatments, total essential amino acid supply to the small intestine was predicted using OM intake as the independent variable. The resulting simple linear regression equation was: total essential amino acid flow = (0.055 x OM intake) + 1.546 (r² = 0.91). The model developed in this experiment accounted for more of the variation in the data set than the current beef cattle NRC model, which under-predicted total flow of essential amino acids to the duodenum. The prediction equation developed herein can be used to estimate the supply of essential amino acids reaching the small intestine when formulating supplements to compensate for potential amino acid deficiencies resulting from restricted forage intake.

Key Words: Amino Acids • Beef Cattle • Digestion • Feed Restriction • Forage • Prediction Models

Introduction

Studies evaluating nutrition-reproduction interactions often use restricted intake to limit dietary energy (Schillo, 1992). However, when restricted intake is used to limit dietary energy, protein intake is concomitantly reduced—both of which will influence protein or essential AA supplied to the small intestine of the animal. Supplying equal amounts of protein to the small intestine across all levels of intake would be an initial approach to partitioning responses to energy and protein. Ruminally undegradable protein (RUP) can be used to boost postruminal supply of protein. However, greater ruminal retention time associated with restricted intake may confound extent of digestion (Riewe and Lippke, 1969) and a feedstuff’s RUP value (Shadt et al., 1999; Meng et al., 1999). Merchen et al. (1986) demonstrated that AA flow decreased with a decrease in forage intake; however, urea and sodium sulfate were added to the low-intake treatments to prevent limitations in microbial growth. Varga and Prigge (1982) and Tatman et al. (1991) found no differences in digestibility with restricted feeding of a diet composed entirely of forage, but intestinal AA were not evaluated.

The NRC (1996) metabolizable protein supply calculations were developed from experiments in which cattle were fed adequate diets, which may not be appropriate for beef cattle consuming restricted amounts of forage only. We hypothesized that a linear regression model could be developed to predict intestinal essential AA supply from data obtained in a digestion experiment. Objectives of this research were to determine site and extent of digestion and to characterize the quantity and profile of essential AA reaching the small intestine in beef cattle consuming restricted amounts of bromegrass hay. Our subsequent objective was to develop a regression model to predict intestinal essential AA supply in beef cattle fed restricted amounts of forage.

Materials and Methods

General
Eight ruminally and duodenally cannulated Angus x Gelbvieh heifers (445 ± 74.5 kg) were used in a 4 x 4 Latin square double-crossover experiment (Neter et al., 1990) in accordance with an approved University of Wyoming Animal Care and Use Committee protocol. Heifers were fed chopped (2.54 cm) bromegrass hay (8.5% ash and 9.2% CP, and 64% NDF on an OM basis) at 30, 60, 90, and 120% of maintenance intake (equal proportions at 0600 and 1800 daily) according to the NRC (1996). Forage intake required for maintenance was determined using Level I of the NRC (1996) computer software program. Actual data were used as inputs for animal descriptions, environmental conditions, as well as forage chemical and nutrient composition. Microbial efficiency was set at 11% of TDN intake (Paterson et al., 1996). Each experimental period was 21 d in length with 17 d of adaptation, to ensure adequate adaptation of the digestive system to forage intake level, after which samples were collected for 4 d. In order to account for changes in BW, heifers were weighed once in the morning and in the evening on the final day of each period, and level of feed intake for each subsequent period was adjusted for changes in BW. Feed refusals were not detected from d 10 through 21 of the experimental periods. As a marker of digesta flow, boluses of 5.0 g of Cr₂O₃ were dosed intraruminally at each feeding (total = 10 g Cr₂O₃/d) from d 8 to 19 of each sampling period. Heifers were given ad libitum access to water and trace-mineralized salt (Champions Choice, AKZO Salt, Inc., Georgetown, SC; guaranteed analysis [percentage of DM]: NaCl, 95 to 99; Co, Cu, I, Mn, Zn, and Fe, less than 1) until d 14 of each sampling period. On d 14 of each sampling period, cattle were denied access to trace-mineralized salt to avoid any confounding effects with fluid passage rates due to increased water consumption associated with salt consumption.

Sampling
Beginning at 0400 on d 18 of each sampling period, duodenal (200 mL) and fecal (50 mL) samples were collected every 4 h. On d 19, collection times were advanced 2 h so that samples were collected to represent every 2 h in a theoretical 24-h clock. Fecal samples were dried in a 55°C forced-air oven, ground (Wiley mill; 1-mm screen, Thomas Hill and Sons, Philadelphia, PA), and composited (equal dry-weight basis) within heifer for each period. Duodenal digesta samples were composited (equal volume) within heifer for each period and immediately frozen. Duodenal digesta samples were lyophilized (Genesis SQ 25 Super ES Freeze Dryer, VirTis Co., Gardiner, NY) and ground (1-mm screen).

Immediately before the 0600 feeding on d 20 of each period (0 h), approximately 200 mL of whole ruminal contents were collected. Heifers were then dosed intraruminally with 200 mL of Co-EDTA for determination of fluid passage rate (Uden et al., 1980). Whole ruminal contents were then collected at 3, 6, 9, 12, 15, 18, 21, and 24 h post after dosing (only samples for Co determinations were collected at 24 h). Ruminal pH was immediately taken on whole ruminal contents using a combination electrode (Orion Research Inc., Boston, MA), and contents were strained through eight layers of cheesecloth. Ten mL milliliters of the resulting ruminal fluid was acidified with 0.1 mL of 7.2 N H₂SO₄ and immediately frozen. The unstrained sample of whole ruminal contents was placed in a blender (Hamilton Beach/Proctor Silex, Washington, NC) with an equal volume of 0.9% NaCl (wt/vol) solution and homogenized for 1 min to dislodge particulate-associated bacteria. The homogenized sample was then strained through eight layers of cheesecloth and immediately frozen for subsequent bacterial isolation by differential centrifugation (Merchen et al., 1986). The resulting bacterial isolate was lyophilized (Genesis SQ 25 Super ES Freeze Dryer, VirTis Co.) and ground with a mortar and pestle for subsequent laboratory analysis.

Laboratory Analyses
Feed, fecal, duodenal, and microbial samples were analyzed for DM, ash, and Kjeldahl N (AOAC, 1990). Neutral detergent fiber content of feed, feces, and duodenal digesta was determined using an ANKOM 200 fiber analyzer (ANKOM Technology, Fairport, NY). Chromium concentration in duodenal digesta and feces was determined by atomic absorption spectrophotometry (model 210 VGP AASpectr., Buck Scientific, E. Norwalk, CT) with an air-plus-acetylene flame (Hill and Anderson, 1958). Duodenal and isolated bacteria samples were analyzed for purine concentration (Zinn and Owens, 1986). Duodenal NH₃-N concentrations were determined by steam distillation over MgO (AOAC, 1990).

Acidified ruminal fluid samples were centrifuged at 10,000 x g for 10 min, and a 2.5-mL aliquot of the resulting supernatant was added to 0.5 mL of 25% metaphosphoric acid containing 2 g/L of 2-ethyl-butyric acid (Goetsch and Galyean, 1983). These samples were then centrifuged for 10 min at 10,000 x g and the supernatant was analyzed for concentrations of VFA using a Hewlett-Packard 5890 GLC (Hewlett-Packard, Avondale, PA) equipped with a 15 x 0.533 mm (I.D.) column (Nukol: Supelco, Bellefonte, PA) with a ramp temperature of 110 to 150°C at 8°C per min. Helium was used as the carrier gas with a column flow rate of 20 mL/ per min. Injector and detector temperatures were 250°C. Ruminal NH₃ concentration was determined by the phenol-hypochlorite procedure (Broderick and Kang, 1980). Cobalt concentration was analyzed using atomic absorption spectrophotometry (model 210 VGP AASpectr, Buck Scientific) with an air-plus-acetylene flame (Hart and Polan, 1984).

Amino acid analyses were performed on feed, duodenal digesta, and isolated ruminal microbes by the University of Missouri Agric. Exp. Stn. Chemical Laboratories according to methods of the AOAC (2002; Method 982.30 E, subsections a, b, and c) via a Beckman 6300 interfaced with Beckman System Gold (model 6300, Beckman Instruments, Palo Alto, CA). The cationic HPLC system utilized postderivatization with ninhydrin followed by UV detection. Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis.

Calculations and Statistical Analysis
Organic matter flow was calculated by dividing the amount of Cr dosed by the concentration of Cr in the sample (duodenal and fecal). Duodenal flow of N, NDF, and EAA was calculated by multiplying nutrient concentration in duodenal OM by duodenal OM flow. The microbial purine:N ratio was calculated by dividing microbial purine content by N in bacteria (Zinn and Owens, 1986). The proportion of N flowing at the small intestine of microbial origin was calculated by dividing the purine:N ratio of duodenal digesta by the purine:N ratio of microbial isolates. Microbial OM flowing to the duodenum was calculated by dividing duodenal microbial N flow by microbial N as a percentage of OM. True ruminal digestibility was calculated based on the amount of nutrient ingested subtracted from the amount present at the small intestine without microbial nutrient contributions. Ruminal fluid passage rate was calculated by regressing the natural logarithm of Co concentration on time after dosing (Uden et al., 1980).

Data were analyzed using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC). Single-degree-of-freedom orthogonal polynomial contrasts were used to determine linear, quadratic, and cubic effects of level of feed intake (Steel and Torrie, 1980). Based on the most significant polynomial effect, regression equations were developed using the REG (STEPWISE) procedures of SAS (SAS Inst. Inc.) to predict total essential AA flow to the small intestine. Time course data were analyzed using the REPEATED statement within the MIXED procedure of SAS. Model included the effects of animal, period, treatment, time, and treatment x time. Animal x period x treatment was used to specify variation between animals using the RANDOM statement. Autoregression order one was determined to be the most desirable covariance structure according to the Akaike’s information criterion. No treatment x time interactions were noted P = 0.10 to 0.71); therefore, only the main effects of treatment will be reported.

Results

Intake and Digestion
Organic matter intake (linear, P < 0.001), OM truly fermented in the rumen (quadratic, P = 0.02), and OM digested in the total tract (g/d) (linear P < 0.001) increased as intake increased from 30 to 120% of maintenance (Table 1). Likewise, total, microbial, and nonmicrobial OM flows (g/d) increased linearly (P < 0.001) as forage intake increased. Due to the proportional increase in duodenal flow and fecal output of OM with increased intake, neither ruminal nor total tract OM digestibility (% of intake) differed (P = 0.23 to 0.97) among treatments.

View larger version (8K): [in this window] [in a new window]   Figure 1. Influence of forage OM intake on total essential AA flow (g/d) to the small intestine in beef cattle consuming restricted amounts of bromegrass hay. The regression equation was y = [0.055 (±0.003) x OM intake (g)] + 1.546 (±19.4); with an r2 = 0.91.

Including other independent variables in the prediction equation only increased the r² from 0.91 to 0.93. Microbial N flow to the duodenum was the only variable that significantly improved (P = 0.002) the regression r² (r² = 0.93); however, incorporating duodenal flow of microbial N into our prediction equation would be of limited value to future users because it would necessitate measurement or prediction of this variable preceding model operation. Thus, we concluded that additional inputs were not required in the above equation estimating duodenum flow of essential AA.

Discussion

Intake and Digestion
The decrease in duodenal OM flow as forage intake decreased in this experiment agrees with results of Merchen et al. (1986), who demonstrated that OM flow to the small intestine decreased in sheep fed forage at 60% vs. 100% of ad libitum intake. Varga and Prigge (1982) also noted that ruminal OM digestibility was unaffected in sheep fed 100% forage at ad libitum and 60% of ad libitum intakes. In the five experiments conducted by Andersen et al. (1959), OM digestibility was not affected by intake restriction with 100% forage diets, but restricting intake increased OM digestibility when forage was mixed with concentrate feeds. The response observed by Andersen et al. (1959) can be attributed to increased digestibility of concentrates, wherein microbial numbers increase with an increase in available energy (Dehority and Orpin, 1997) and subsequent propensity for ruminal digestion. Clark et al. (1992) showed that microbial N flow to the small intestine increased as OM intake and ruminally digested OM increased. If forages are fed at levels below maintenance, the ruminal environment may be limited in its capacity to improve digestion. This seems to occur despite decreased fluid passage rate, which would allow for increased time for microbial digestion (Owens and Goestch, 1986). Thus, diets containing 100% moderate to low-quality forage may not contain sufficient fermentation substrates to support more extensive ruminal degradation. Therefore, the improvement in ruminal digestibility often seen with restricted intake (Merchen et al., 1986; Murphy et al., 1994) was not noted in the present study. Ruminally degradable protein was most likely not limiting in these diets as evidenced by no differences for ruminal OM, N, and NDF digestibility (percentage of intake) across treatment. However, the potential exists for the diets to be equally limiting across all levels of forage intake. Reevaluation of the diets with the NRC (1996, Level I) using the data obtained from our experiment indicated ruminally degradable protein deficits averaged -35.7, -73.8, -63.4, and -205.5 g/d for the 30, 60, 90, and 120% of maintenance intake treatments, respectively.

Digestion in the rumen can be described as a kinetic model, with rate of passage as an important factor influencing diet digestibility (Galyean and Owens, 1991). Increasing intake inherently increases rate of passage from the rumen, which reduces the time available for microbial digestion (Riewe and Lippke, 1969). Although passage rates of solids in response to level of feed intake are usually not as dramatic as those observed for liquid passage rates (Owens and Goetsch, 1986), Poppi et al. (1981) suggested that feed particles passing from the rumen per unit time are influenced by fluid passage rate because the feed particles are suspended in the fluid passing through the reticulo-omasal orifice. In this experiment, fluid passage rate increased from 6.0 to 10.1%/h as intake increased from 30 to 120% of maintenance. Intuitively, a decrease in digesta passage from the rumen would allow more time for the ruminal bacteria to ferment the available OM. However, Firkins et al. (1986) indicated that intakes greater than 1.75% of BW would be required to elicit a significant response in digestibility due to changes in digesta passage rate when steers were fed a 75% forage diet. In the present study, OM intakes were 0.5, 1.0, 1.5, and 2.0% of BW for 30, 60, 90, and 120% of maintenance intake, respectively. Because there were no differences in diet OM digestibility as intake increased, intake of a 100% forage diet may need to be greater than what was fed in this experiment in order to elicit changes in ruminal OM digestibility.

Increased microbial OM and N flow to the small intestine with increased forage intake in the present study may be attributed to the increased quantity of OM truly fermented in the rumen associated with the greater levels of forage OM intake. In a summary of over 100 experiments with cattle consuming a variety of diets, Clark et al. (1992) demonstrated that microbial flow to the duodenum is related to OM intake and total OM digested. The trend for ruminal NDF digestibility to increase from 54.4 to 59.8% when intake decreased from 120 to 30% of maintenance intake is in agreement with Merchen et al. (1986), who noted a numerical increase in ruminal NDF digestibility when sheep were fed restricted levels of forage-based diet.

Ruminal Fermentation Patterns
Ruminal NH₃ levels (2.1 to 1.9 mM) were above the minimum level (1 mM) determined by Satter and Slyter (1974) indicating that ruminal N levels were adequate for microbial growth. Likewise, pre-experimental analysis using Level II of the NRC (1996) ration-evaluation software also suggested that bacterial N balance was adequate for all intake levels. Post hoc analysis using Level II of the NRC (1996) indicated that bacterial N balance was as low as -2 g/d for the 60% of maintenance intake treatment, whereas peptide balance was -2, -4, -7, and -9 g/d for the 30, 60, 90, and 120% of maintenance intake treatments, respectively. Lastly, ruminal pH across treatments was around optimum for N degradation (pH 6.5; Blackburn, 1965).

Varga and Prigge (1982) noted that increased ruminal acetate coincided with a numerical increase in NDF digestibility. Because the quantity of NDF digested in the rumen in the current study increased as level of forage intake increased, greater ruminal acetate would have been expected because fiber fermentation results in the liberation of acetate (Hungate, 1966). Molar proportions of acetate decreased whereas butyrate increased as NDF digested in the rumen increased in the present study. Increased butyrate production is indicative of an increased proportion of acetyl-CoA being converted to butyrate (Wolin et al., 1997). There may have been an increase protozoa and/or Butyrivibrio fibrosolvens population in cattle fed forage at 90 and 120% of maintenance because the organisms are the predominant butyrate-producing organisms in the rumen (Russell and Wallace, 1997; Williams and Coleman, 1997).

Essential Amino Acids
Total essential AA flow to the small intestine increased from 132.1 to 496.1 g/d as forage intake increased from 30 to 120% of maintenance. Greater essential AA supply to cattle consuming increasing amounts of forage was due to a combination of increased dietary and microbial essential AA flows. These results are in agreement with Clark et al. (1992), who demonstrated increased microbial N flow with increased OM intake. Other researchers (Ludden and Kerley, 1997; Volden, 1999) have determined that intestinal flow of AA increased in cattle fed increasing levels of a common diet; however, these researchers fed various levels above maintenance rather than restricting intake to level of that reported herein.

Although duodenal essential AA (% of essential AA) differed for 8 out of the 10 essential AA, the individual AA profile only differed by as much as 0.3 percentage unit. Titgemeyer et al. (1989) demonstrated that methionine, lysine, and arginine were more ruminally degradable than other essential AA. Chalupa (1976) showed that ruminal degradation of methionine was low compared to other essential AA. True ruminal degradability of individual essential AA did not differ across forage intake in this study, suggesting that the differences noted for duodenal profile are due to a disproportionately small SEM.

Model Comparisons
Because the focus of this research was to evaluate the influence of forage OM intake on total essential AA flow to the duodenum, we regressed total essential AA flow against OM intake. Clark et al. (1992) developed a regression equation to predict microbial N flow to the small intestine in cattle based on OM intake. Unfortunately, the model of Clark et al. (1992) may not be appropriate to use in beef cattle consuming restricted amounts of an all forage diet due to differences in diet type and intake level by the cattle in the data sets used to develop the equations. These authors indicated that increasing feed intake increased the quantity of OM truly fermented in the rumen. Similarly, OM truly fermented in the rumen was highly correlated with OM intake (r = 0.88; P < 0.001) in our experiment. Increasing OM truly fermented in the rumen would increase supply of nutrients for microbial growth, which should, in turn, increase intestinal essential AA supply from microbial sources. This contention was supported by the improvement made in the regression model with microbial N flow to the duodenum as well as a positive correlation between OM truly fermented in the rumen and both microbial N flow to the duodenum (r = 0.73; P < 0.001) and total essential AA flow to the duodenum (r = 0.76; P < 0.001). For the sake of comparing the prediction equation presented in Figure 1, we regressed total essential AA against OM truly fermented. The resulting equation was total essential AA = (0.096 ± 0.005 x grams of OM truly fermented) + 50.753 ± 16.67 (r² = 0.91). Nonetheless, results of our stepwise regression analysis indicated that forage OM intake was the most significant independent variable and the only other independent variable that improved the model was microbial N flow to the duodenum. Forage OM intake was more highly correlated (r = 0.96; P < 0.001) with total essential AA than OM truly fermented in the rumen. Furthermore, percentage of OM truly fermented in the rumen was not affected by level of intake. Thus, our results demonstrated that the most appropriate and practical model to estimate total essential AA flow to the duodenum included forage OM intake as the only independent variable.

The authors of the Nutrient Requirements for Dairy Cattle (NRC, 2001) suggest that models are needed to accurately predict the essential AA composition of duodenal protein, and more importantly, metabolizable protein. Currently, the Nutrient Requirements for Beef Cattle (NRC, 1996) ration software (Level II) is widely utilized used to determine essential AA composition of metabolizable protein. Unfortunately, this model was developed using data from cattle fed at or above maintenance intake levels. Therefore, it was necessary to validate the NRC model for cattle restricted below maintenance by comparing the NRC-predicted duodenal supply of total essential AA to those observed in this experiment.

For comparison purposes, the NRC (1996, Level II)-predicted metabolizable AA supplies were divided by 0.80 (intestinal digestibility; NRC, 1996) and expressed as supply of essential AA to the small intestine (duodenal flow). Input parameters for Level II of the NRC (1996) software model were based on actual forage composition and digestibility measurements, animal BW, and environmental conditions measured in the current experiment. Actual and NRC-predicted total and individual essential AA flows to the small intestine are presented in Table 9. There was an intake level x measurement of essential AA flow interaction (P = 0.001 to 0.008) for all 10 of the essential AA. These interactions were due to greater differences between actual duodenal flow of the essential AA and the NRC-predicted flows as forage intake increased.

Kohn et al. (1998) indicated that some of the shortcomings of the NRC model are that it does not adjust for unusual feeding management. Therefore, the NRC (Level I; 1996) Level II model may not be appropriate to use when balancing total essential AA flow to the duodenum in beef cattle consuming restricted amounts of bromegrass hay below maintenance. Although such severe feed restriction may rarely occur in most production settings, the overall goal of this experiment was to evaluate essential AA supply reaching the small intestine across all levels of intake in an effort to develop a model that can be used to estimate essential AA to the small intestine of beef cattle fed forage at various levels below maintenance. The resulting model may eventually be used to balance intestinal essential AA supply to beef cattle fed similar quantities of forage as that used in the present study.

Although there has been a great deal of research conducted concerning prediction of essential AA supply in beef cattle, there is considerable lack of information available concerning severely restricted intake of all-forage diets. The data presented in the current study indicate that digestive effects often observed in studies designed to evaluate intake levels at multiple levels above maintenance (ARC, 1980) are not apparent when beef cattle are fed all-forage from 120 to as low as 30% of maintenance. Moreover, we present a model that can be used to predict intestinal supply of essential AA when forage OM intake by beef cattle ranges from 2.3 to 8.5 kg/d.

Implications

Digestibility of a bromegrass hay diet was not affected by restricting forage intake to as low as 30% of maintenance. Consequently, flow of essential amino acids to the small intestine increased linearly when forage intake increased from 30 to 120% of maintenance, and a simple linear regression model was developed to predict essential amino acid flow to the small intestine when beef cattle consume restricted amounts of forage. The simple linear regression equation developed from the data presented herein may be useful to feed managers formulating supplements to provide adequate amounts of the essential amino acids to beef cattle consuming restricted amounts of forage.

Footnotes

¹ This research was supported by the University of Wyoming Faculty Grant-in-Aid Program and the USDA-NRI Competitive Grants Program (USDA-NRI #99-03628).

² Correspondence: P. O. Box 3684 (phone: 307/766-5173; fax: 307/766-2355; e-mail: brethess{at}uwyo.edu).

Received for publication July 15, 2003. Accepted for publication November 14, 2003.

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