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Effect of feed intake on endogenous losses and amino acid and energy digestibility by growing pigs^1,2

Department of Animal and Range Sciences, South Dakota State University, Brookings 57006

	Abstract

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An experiment was conducted to determine the effect of feed intake (FI) on endogenous losses and the digestibility of CP and AA by growing pigs. Six growing barrows (initial BW = 70.3 kg) had a T-cannula installed in the distal ileum and were used in a 6 x 6 Latin square design. A soybean meal cornstarch-based diet and a N-free diet were formulated. Chromic oxide (0.25%; as-fed basis) was included in both diets as an inert marker. Each diet was provided at three different levels of FI. Feed intake level 1 was equal to the estimated energy requirement for maintenance of the pigs, whereas levels 2 and 3 were two or three times this amount, respectively. Each experimental period lasted 7 d. The initial 4 d of each period comprised an adaptation period to the experimental diets. On d 5, fecal samples were collected, while ileal digesta were collected during two 10-h periods on d 6 and 7. Between each experimental period, a corn-soybean meal-based diet (16% CP) was fed to all pigs for 7 d. The basal ileal endogenous losses (IAA_end) of CP and AA were measured for each level of FI from pigs fed the N-free diet. Likewise, the apparent (AID) and standardized (SID) ileal digestibility coefficients for CP and AA in soybean meal were calculated for each level of FI. The total-tract digestibility coefficient of energy in the soybean meal-based diet also was calculated. The AID for CP and all indispensable AA except Lys, Met, and Thr increased (P 0.055) as FI increased. The IAA_end of CP and all AA except Pro decreased linearly (P < 0.05) as FI increased when expressed as g/kg of DMI; however, the total daily IAA_end increased as FI increased (linear, P 0.056) for all AA, except for Phe, Thr, Trp, Val, Cys, Gly, and Ser. The AA composition (% of CP) of endogenous protein was not affected by the level of FI, except for Arg, Thr, Pro, and Ser. As FI increased, the SID decreased linearly (P < 0.04) for CP and all AA, except Arg, Trp, Asp, Pro, and Tyr. The total-tract digestibility of energy was not influenced by the FI level. These results demonstrate that the FI level significantly influenced AID, SID, and IAA_end for CP and AA. Therefore, pigs used to measure AA digestibility coefficients and IAA_end should be fed at a level that is close to what is used under commercial conditions.

Key Words: Amino Acid Digestibility • Endogenous Losses • Feed Intake • Pigs

	Introduction

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The amount of feed consumed voluntarily by pigs is variable and is affected by many factors (Hyun et al., 1997). However, the level of feed intake (FI) has been shown to have only minimal or no effect on apparent ileal digestibility coefficients (AID) of CP and AA by growing pigs (Sauer et al., 1982; Haydon et al., 1984; Albin et al., 2001).

Standardized ileal digestibility coefficients (SID) yield more precise estimates of the amount of digestible AA in a mixed diet than do AID (Mosenthin et al., 2000; Jansman et al., 2002). To calculate SID, AID must be corrected for the basal ileal endogenous losses (IAA_end) of CP and AA (Stein et al., 2001). Endogenous losses of CP and AA are influenced by the FI level (Butts et al., 1993; Hess and Seve, 1999; Stein et al., 1999b). Therefore, the level of FI is expected to influence SID, but this hypothesis has not been investigated.

Previous experiments investigating the effect of FI on energy digestibility have yielded conflicting results. The DE of barley and of mixed diets has been shown not to vary with the level of FI (Dammers, 1964; Peers et al., 1976). However, Tollet et al. (1961) reported an improvement in DE as FI increased, whereas Morgan et al. (1975) found a significant decrease in DE with increasing level of FI.

The objective of the current experiment was to determine the effect of different levels of FI on IAA_end and on AID and SID of CP and AA in soybean meal by growing pigs. A second objective was to determine the influence of the level of FI on the total tract digestibility of energy in a soybean meal cornstarch-based diet.

	Materials and Methods

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Animals, Housing, and Experimental Design

Six growing barrows (average initial BW = 70.3 kg) were obtained from the South Dakota State University Swine Research Farm. A T-cannula was installed in the distal ileum of each pig using a procedure adapted from Stein et al. (1998). Following surgery, pigs were allowed to recuperate for 14 d. The pigs were housed individually in 1.2 x 1.8 m pens for the duration of the experiment in an environmentally controlled room. Room temperature was maintained at 20°C. A 6 x 6 Latin square design was used, with six periods and six animals representing the rows and the columns, respectively. The experimental protocol was reviewed and approved by the South Dakota State University Animal Care and Use Committee (No. 01-A023).

Diets, Feeding, and Sample Collection

Two diets were prepared. Diet 1 was a soybean meal-based diet, and Diet 2 was a N-free diet (Tables 1 and 2). Chromic oxide (0.25%; as-fed basis) was included in both diets as an inert marker. Vitamins and minerals were included at levels that met or exceeded the estimated requirements for growing pigs (NRC, 1998).

Chemical Analyses

At the conclusion of the experiment, digesta and fecal samples were thawed, mixed within animal and diet, and a subsample was taken for chemical analyses. All digesta samples were lyophilized and finely ground prior to chemical analysis. Fecal samples were oven dried at 60°C. Dry matter was determined in diets, digesta, and fecal samples (AOAC, 2000). The concentrations of Kjeldahl N and AA were determined in diets and digesta samples (AOAC, 2000). Amino acids were analyzed on a Chrom-tech HPLC AA analyzer (Thermo Quest, Inc., San Jose, CA), using ninhydrin for post derivitization and nor-leucine as the internal standard (AOAC, 2000). Samples were hydrolyzed with 6 N HCL for 24 h at 110°C. Methionine and Cys were determined as Met sulfone and cysteic acid after cold performic acid oxidation overnight prior to hydrolysis (AOAC, 2000). Tryptophan was determined after samples had been flushed with nitrogen and hydrolyzed with 6 N NaOH for 22 h at 110°C (AOAC, 2000). The Cr concentration of diets, digesta, and fecal samples were determined by spectrophotometry (Fenton and Fenton, 1979). The soybean meal-based diet and fecal samples were analyzed for GE according to AOAC (2000) using a bomb calorimeter (PARR 1563, Moline, IL).

Calculations and Statistical Analyses

Apparent ileal digestibility coefficients for AA were calculated as follows (Stein et al., 1999a):

[1]

where AID is the apparent ileal digestibility coefficient of an AA (%), AAd is the AA concentration in the ileal digesta DM (g/kg), AAf is the AA concentration in feed DM (g/kg), Crf is the chromium concentration in the feed DM (g/kg), and Crd is the chromium concentration in the ileal digesta DM (g/kg). The AID of CP and the total tract digestibility of energy were also calculated using Eq. [1].

The IAA_end of each AA was determined based on the flow obtained after feeding the N-free diet as follows (Stein et al., 1999b):

[2]

where IAA_end is the basal endogenous loss of an AA (g/kg DMI).

The daily flow (g/d) of endogenous CP and AA was calculated by multiplying the IAA_end per kilogram of DMI for CP and each AA by the daily DMI.

By correcting the AID for the endogenous losses of each AA, the SID were calculated for each level of FI as follows (Stein et al., 2001):

[3]

where SID is the standardized ileal digestibility coefficient (%). In this calculation, the IAA_end used to correct the AID were obtained at FI levels that were identical to those used to determine the AID.

The AA composition of endogenous protein was calculated for each level of FI by expressing each AA as a percentage of total endogenous protein.

The GLM procedure of SAS (SAS Inst., Inc., Cary, NC) was used to evaluate the effect of the level of FI on IAA_end, AID, SID, and DE. An analysis of variance was conducted with FI, pig, and period as the main effects. Linear and quadratic effects of FI on IAA_end, AID, SID, and DE were determined using GLM with contrast statements.

	Results

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Apparent Ileal Digestibility Coefficients

The AID for CP, the mean of the indispensable AA, and for all indispensable AA except Arg, Lys, Met, and Thr, increased (quadratic, P 0.055) as FI increased (Table 3). For CP, Ile, Leu, Trp, and Val and for the mean of the indispensable AA, a linear effect (P < 0.05) was observed in addition to the quadratic effect. For Arg, only a linear response was observed (P < 0.02), whereas there was no effect of FI on the AID for Lys, Met, and Thr.

Endogenous Losses of CP and AA

The IAA_end measured in grams per kilogram of DMI for CP and all AA except Pro decreased linearly (P < 0.02) as the FI increased (Table 4). For Pro, there was a tendency for a linear decrease (P = 0.064) in IAA_end as FI increased. For CP and all AA except Phe, Thr, Trp, Val, Cys, Gly and Ser, the daily flow to the distal ileum increased (linear, P < 0.008 to 0.056) as FI increased (Table 5). In contrast, except for minor changes in the concentration of Arg, Thr, Pro, and Ser, the AA composition of IAA_end was not affected by FI (Table 6).

Except for Arg and Trp, the SID (Table 7) for CP and all indispensable AA and for the mean of the indispensable AA decreased linearly with increasing level of FI (P = 0.003 to 0.035). The SID for the mean of the dispensable AA and for all dispensable AA except Asp, Pro, and Tyr also decreased linearly (P = 0.002 to 0.034) with increasing level of FI, as did the SID for the mean of all AA (P = 0.006).

The digestibility coefficients for energy in the soybean meal-based diet were 84.8, 87.2, and 86.9% for FI levels 1, 2, and 3, respectively. These values were not significantly different.

	Discussion

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Apparent Ileal Digestibility

The AID obtained in the current experiment for pigs fed the highest level of FI are close to values reported for soybean meal by Green and Keener (1989) and by Fan et al. (1995a). However, the numbers are lower than the AID reported for dehulled soybean meal by Traylor et al. (2001) and by Dilger et al. (2004).

Previous experiments have shown limited or no effects of FI on AID (Sauer et al., 1982; Haydon et al., 1984; Albin et al., 2001). The results of the current experiment disagree with those reports; however, the lowest level of FI used in the above studies was higher than the lowest level used in the present experiment. This may explain why a different response was obtained in the current experiment because the increases in AID are mainly found between low levels of FI.

The reason why AID increase for most AA as FI is increased from a low to a medium level is that IAA_end contribute more to the total output of AA at lower levels of FI than at higher levels. Because the ileal output consists of both IAA_end and undigested dietary AA, a higher proportion of IAA_end will lead to a lower calculated AID. However, the relative influence of IAA_end decreases as FI is increased, which partly explains why no significant differences in AID are observed at higher FI levels.

Endogenous Losses

The linear decreases in IAA_end measured in grams per kilogram of DMI for CP and AA that were found in the present experiment confirm previous findings (Butts et al., 1993; Hess and Seve, 1999; Stein et al., 1999b). In contrast, the daily flow of IAA_end increased as FI increased. This is in agreement with James et al. (2002), who observed an increase in the daily loss of all AA with increasing DMI in rats. Similar results were also reported by Butts et al. (1993) and Hess and Seve (1999). Thus, as FI is increased, the IAA_end decrease when expressed relative to the DMI of the animals, but they increase when expressed as the daily flow. The reason for this is that one fraction of the basal daily endogenous flow of CP and AA is secreted in response to the DMI of the animals, whereas another fraction is a daily loss secreted regardless of the DMI of the animal (Furuya and Kaji, 1991).

Standardized Ileal Digestibility

The SID were calculated in the current experiment using the principles previously described for correcting AID for the basal IAA_end of CP and AA (Stein et al., 2001; Jansman et al., 2002). The limitation to the use of AID is the assumed lack of additivity of digestibility coefficients for individual feed ingredients in mixed diets (Jansman et al., 2002). The most likely reason for this lack of additivity is that the digesta collected at the distal ileum contains IAA_end along with undigested dietary protein which may lead to an underestimation of the AID in feed ingredients with relatively low concentrations of AA (i.e., cereal grains). As the dietary AA levels increase, the contribution of IAA_end as a percentage of total ileal output will decrease, which leads to an increase in AID (Fan et al., 1995b). Therefore, calculated AID will change with the dietary concentration of CP and AA. In contrast, SID are independent of the AA concentration in the assay diet because SID are corrected for IAA_end. Thus, by using SID, it has been suggested that diets can be formulated with greater accuracy (Jansman et al., 2002). Nonetheless, the results from the current experiment demonstrate that SID are influenced by the FI level of the animals. To our knowledge, such a finding has not been previously reported.

Because of the design of this experiment, the intake of CP and AA in the soybean meal-based diets increased as the DMI increased. The increased DMI leads to a decrease in basal endogenous losses as illustrated in Table 4. Thus, the DMI per se will affect AA digestibility. This effect is responsible for the observed increase in AID at low DMI levels (Table 3) because the endogenous losses influence AID. If the entire effect of FI on AA digestibility could be explained by the increase in DMI and the resulting decrease in endogenous losses, then the SID should have been constant regardless of the FI of the animals because IAA_end are excluded from the calculations when SID are estimated. However, the SID for CP and all AA except Arg, Trp, Asp, and Pro decreased linearly as FI was increased, as illustrated in Table 7. This observation demonstrates that the digestibility of dietary AA decreases as FI increases. This effect is caused by the increased AA intake of the animals and is unrelated to the increased DMI. It follows from the above that the increase in DMI and the increase in CP and AA intake independently affect the AA digestibility in a feed ingredient. This combined effect is included in the calculation of AID, whereas the effects on SID are caused only by the increase in CP and AA intake. The reason why AID tends to plateau at higher FI levels (i.e., greater than two times the energy required for maintenance) is that the decreased IAA_end seem to be offset by the reduced digestibility of dietary AA. A consequence of this observation is that SID are only accurately predicting the digestibility of a diet if the animals that consume this diet have a FI similar to the FI of the animals used to measure the SID. Because many of the SID in the literature were obtained from animals that were restricted in their FI, these values may not be representative of animals that are allowed ad libitum access to feed.

Energy Digestibility

In the current experiment, the total-tract digestibility of energy was not affected by FI. This result agrees with previous reports (Zivkovic and Bowland, 1963; Peers et al., 1976; Haydon et al., 1984). These results indicate that growing pigs are capable of digesting the energy of a diet with the same efficiency at high levels of FI as at low levels. Thus, substrate availability seems not to influence energy digestibility. Another implication of this finding is that there seems to be no net loss of energy caused by the endogenous loss of CP and AA.

	Implications

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The level of feed intake influenced basal ileal endogenous losses of crude protein and amino acids and apparent and standardized ileal digestibility coefficients. Although apparent ileal digestibility coefficients were influenced mainly if feed intake were low, there was a linear effect of feed intake on standardized ileal digestibility coefficients. As a consequence, feeding the animals close to the voluntary feed intake during digestibility trials will lead to decreased basal ileal endogenous losses per kilogram of dry matter intake and increases the accuracy of the results. Therefore, growing pigs and lactating sows used in studies aimed at determining apparent and standardized ileal digestibility coefficients should be fed close to their voluntary feed intake; however, in gestating sows, restricted feeding is recommended to reflect commercial conditions.

	Footnotes

 
¹ Publication No. 3407 from the South Dakota Agric. Exp. Stn. Journal Series.

² Presented in part at the 2003 ASAS Midwest Meeting (J. Anim. Sci. 81(Suppl. 1):182 (Abstr.).

³ Correspondence: Box 2170, Brookings (phone: 605 688 5434; fax: 605 688 6170; e-mail: hans.stein{at}sdstate.edu).

Received for publication February 26, 2004. Accepted for publication August 30, 2004.

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