Short communication
A comparison of the predictions of digestible energy
content of compound feeds for pigs by
chemical or in vitro analysis
M. Spanghero
*, L.A. Volpelli
Department of Animal Production Science, via S. Mauro 2, 33010 Pagnacco, Udine, Italy
Received 9 October 1998; received in revised form 23 February 1999; accepted 1 June 1999
Abstract
The comparison of the predictions of the digestible energy (DE) content of commercial compound pig feeds from the organic matter in vitro digestibility (OMD) and five recent and accurate multiple regression equations based on chemical components was examined. The OMD was determined by submitting each sample (17 compound feeds for lactating or pregnant sows and 23 for piglets and growing±fattening pigs) to three sequential incubations with enzymatic solutions in a closed system to simulate the gastric and pancreatic digestion and the degradation in the hind gut (2, 4 and 18 h of incubation, respectively). OMD values were converted into energy digestibility coefficients (dE), which together with gross energy (GE) content were used to calculate DE (=GE*dE). Chemical components (ash, ether extract (EE), crude protein (CP), crude fibre (CF), NDF and ADF) and GE content were used to predict the DE content using five multiple equations, selected in terms of their accuracy (r2and/or RSD).
The DE content was closely correlated with NDF (r,ÿ0.91,p<0.01), while ADF and CF were correlated to a lesser degree (r,ÿ0.75, and ÿ0.79, respectively, p<0.01) and no significant relationship was found with ash. The average DE contents estimated with the multiple equations were slightly lower than those predicted in vitro (from 15.15±15.40 vs. 15.58 MJ/kg DM). The differences between the DE content estimated with the multiple equations and that predicted in vitro (DE) were highly, and negatively, correlated with the ash content of the feeds (rfromÿ0.68 to
ÿ0.84,p<0.01) and it was estimated that theDE varied by 4±5% (ca. 0.71 MJ/kg DM) when the ash content (6214 g/kg DM) ranged from low to high levels (fromÿ1 to +1 SD). All theDE values had negative correlations with both CP (r, fromÿ0.62 toÿ0.68,p<0.01) and EE (r, from
ÿ0.48 toÿ0.53,p<0.01). The fibre components, present in all equations either as NDF, ADF or NDF-ADF, were not significantly related with any of theDE considered. The prediction of DE content based on the in vitro technique considered or chemical parameters showed a general
81 (1999) 151±159
*Corresponding author. Tel.: +39-432-650110; fax:+39-432-660614
E-mail address: mauro.spanghero@dspa,uniud.it (M. Spanghero)
coherency, the only exception concerning the ash fraction.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Pig; Feedstuffs; Digestibility; Energy
1. Introduction
When the ingredient composition is unknown, as is the case for commercial compound pig feeds, the digestible energy (DE) content can be predicted by in vitro analysis or dietary chemical parameters, generally included in multiple regression equations.
Several in vitro techniques have been proposed over the last two decades (Vervaeke et al., 1989; Furuya et al., 1979; Boisen and Eggum, 1991; Van der Meer and Perez, 1992) and the one originally developed in Denmark (Boisen, 1991) has recently been updated and, after a verification of its accuracy, definitively adopted for the practical evaluation of mixed diets for pigs (Boisen and Fernandez, 1997). Prediction by means of multiple regression, based on dietary chemical parameters, has been widely studied (Morgan et al., 1987; Batterham, 1990; Longland and Low, 1995) and one of the most recent and extensive experimental investigations was that of Noblet and Perez (1993), with several equations presented in the review by Noblet and Henry (1993).
The predictions based on in vitro assays and chemical composition should be compared in terms of accuracy and precision by simultaneous comparison with in vivo data, but these evaluations are lacking in the literature.
The present note, which does not contain in vivo data, has the objective to compare the predictions of DE using an in vitro technique and some of the most recent and accurate multiple regression equations. Both types of prediction are applied to the same set of compound feeds, which were representative of commercially available products for different categories of pigs.
2. Material and methods
Forty samples of compound pig feeds (17 for lactating or pregnant sows and 23 for piglets and growing-fattening pigs) were collected in northeastern Italy (Friuli-Venezia Giulia) by the local Farmers' Association. Samples were ground (Ciclotec Tecator, 1 mm) and analysed for dry matter (DM), crude protein (CP), ether extract (EE), ash (AOAC, 1995), NDF and ADF (Goering and Van Soest, 1970). Gross energy (GE) was determined by adiabatic calorimeter (IKA C 400, Janke and KuÈnkel, D-79219 Staufen, Germany). The organic matter in vitro digestibility (OMD) of each sample was determined according to the method officially adopted in Denmark for energy evaluation (Boisen and Fernandez, 1997). Briefly, this method is a gravimetric technique, in which the sample (500 mg) is submitted to three sequential incubations with enzymatic solutions in a closed system to simulate the gastric and pancreatic digestion and the degradation in the hind gut (2, 4 and 18 h of incubation, respectively). The samples were analysed in duplicate
and were partitioned into 10 batches, with each batch having two blanks for the correction of enzymatic organic matter and a standard to uniform data between the different batches.
OMD was converted into energy digestibility (dE) with the following equation (Boisen and Fernandez, 1997):
dE ÿ0:140 0:021 1:106 0:026OMD r2 0:94;RSD0:034
Chemical components (g/kg DM) and GE content (MJ/kg DM) were used to predict the DE content (MJ/kg DM) with the following multiple regression equations:
DE18:50-0:0435Ash0:0056CP0:0153EE-0:0161NDF
The selection of the equations was based on their accuracy (in terms ofr2and/or RSD) from those which contained the chemical parameters analysed in the present work. Eqs. (1) and (5) were reported in the review by Noblet and Henry (1993) and in the work of Morgan et al. (1987), respectively. Eqs. (2)±(4) were obtained by Noblet and Perez (1993) and the original coefficients were transformed to express energy values in MJ. Eq. (2), which originally gave the energy digestibility (dE, %), was multiplied by GE to give a DE value, but this manipulation does not allow the conversion of the original RSD value (=1.6). The difference between the DE content estimated by the multiple regres-sion equations and that predicted in vitro was calculated for each equation (DE1, DE2, DE3, DE4 andDE5, respectively, for (Eqs. (1)±(5)), in % of the in vitro value).
3. Results and discussion
Chemical composition, in vitro digestibility and DE contents of the compound feeds are presented in Table 1. Average and variability data for the main chemical components of the dataset can be compared with the respective values for the larger number of diets (n= 114) from which three equations (Eqs. (2)±(4)) were obtained (Noblet and Perez, 1993). Mean, SD, minimum and maximum values for CP and NDF contents of the current dataset were very close to those used in vivo. The present dataset had a higher average EE concentration (61 vs. 42 g/kg DM), a slightly lower ash concentration (62 vs. 71 g/kg DM) and, consequently, the GE content was higher (19.07 vs. 18.15 MJ/kg DM). It is worth noting that the present average in vitro prediction of digestibility of energy was very close to that reported as the mean of 114 diets (average and SD of energy
digestibility: 0.8160.046 vs. 0.8120.054) and that the average DE contents estimated by the multiple equations were slightly lower than those predicted in vitro (from 15.15 to 15.40 vs. 15.58 MJ/kg DM).
For each of the selected five multiple equations, the differences between the DE content, estimated by the multiple equations, and that predicted in vitro, labelledDE1, DE2,DE3,DE4 andDE5, respectively, for Eqs. (1)±(5), were calculated. Table 2 presents the correlation matrix of chemical components, the DE content estimated by the in vitro technique and the differences in terms ofDE. In addition to the expected close relationship between fibre components (NDF, ADF, hemicellulose and CF), a high correlation was found between the CP content and ash (r, 0.74,p<0.01) and between CP and EE (r, 0.76,p<0.01). This reflects the fact that compound feeds for animals with high protein requirements (piglets or lactating sows) often contain supplementary fat and are generally formulated by using animal protein supplements, which contain high levels of ash.
The DE content estimated from in vitro digestibility was moderately correlated with CP and EE and, as expected, it was highly, and negatively, related with the fibre components, which are generally considered the main feed chemical factors affecting digestibility (Noblet and Perez, 1993; Noblet and Henry, 1993). In agreement with in vivo investigations, NDF was the chemical factor most closely correlated with DE content (r, ÿ0.91, p<0.01), while ADF and CF were correlated to a lesser degree (r, ÿ0.75 and ±0.79, respectively,p<0.01). No significant relationship was found with ash, and this was probably due to the fact that the in vitro procedure has recently been modified by the addition of a chelating agent (EDTA), which prevents mineral precipitation and avoids any interference from the ash in the feed.
Table 1
Chemical characteristics (g/kg DM), in vitro digestibility and energy contents (MJ/kg DM) of the compound feeds examined (n=40)
Mean SD Minimum Maximum Crude protein (CP) 186 30 118 270
Ether extract (EE) 61 20 31 119
Ash 62 14 25 87
Crude fibre (CF) 55 15 20 87
NDF 168 50 45 255
ADF 60 17 19 102
Gross energy (GE) 19.07 0.50 18.43 20.31 In vitro OMD digestibility 0.865 0.042 0.802 0.975 Energy digestibility (dE)a 0.816 0.046 0.746 0.939
Digestible energy
adE =ÿ0.140 + 1.106 OMD (Boisen and Fernandez, 1997).
Table 2
Correlation coefficients between chemical characteristics, DE predictedin vitroand differences between the DE content estimated by the multiple chemical regression equations and that predicted in vitro (DE1,DE2,DE3,DE4 andDE5, respectively, for Eqs. (1)±(5)
Item Ash CP EE CF NDF ADF Hemica DE DE chemical-in vitro prediction
DE1 DE2 DE3 DE4 ADE5
aHemicellulose = determined as the difference NDF-ADF. **p<0.01; ns = not significant.
As can be noted from Table 2, all the differences in DE contents (DE1, DE2, DE3,DE4 andDE5) were highly, and negatively, correlated with the ash content of the feeds (rfromÿ0.68 toÿ0.84,p<0.01). This implies that for low ash contents, all the multiple regressions overestimate the DE with respect to the in vitro data, while the opposite situation occurs for high ash contents (see Figs. 1±3). This is due to the high values attributed to ash coefficients, which are higher than would be expected for a simple dilution effect. Eqs. (1), (3) and (5) had high negative coefficients (ÿ0.0435, ÿ0.0393 andÿ0.0326 MJ/g, respectively), but even in Eqs. (2) and (4) there was a high negative impact of ash content: in fact, in these latter equations, the effect of ash content on DE was not only due to the negative coefficient for ash, but also to the concomitant reduction of the GE content of the feed.
This relevant negative effect of ash on in vivo DE prediction was also mentioned by Noblet and Perez (1993), (p. 3396): these authors, in their cited work, found that elevated
Fig. 1. Regression of difference (DE,y) between DE estimated by multiple chemical regression Eq. (1) (&, - - ) and (2) ( ,_____) and by in vitro assay, on ash content (x).
Fig. 2. Regression of difference (DE,y) between DE estimated by multiple chemical regression Eq. (3) (&, - - ) and (4) ( ,_____) and by in vitro assay, on ash content (x).
ash contents increased the protein endogenous losses causing a reduction of the apparent in vivo digestibility. In addition to this explanation, the same authors postulated that elevated ash contents could be responsible for forming less digestible complexes with fatty acids. The mechanism of the phenomena, however, was not elucidated and requires more research effort. Whatever the true explanation of the process, the relevant result of the present paper is that the in vitro procedure is not sensitive to this effect and, therefore, increasing the ash content increases the difference between the DE contents, determined by the in vitro technique or derived from multiple chemical regression equations obtained from in vivo trials. An examination of the regression equations reported in Figs. 1±3 allowed the calculation of the mean value of the slopes ofÿ1.6% for each 10 g of ash in the DM of the feed. This, together with the SD for ash content in the current dataset (14 g/ kg DM, Table 1), allowed the calculation of an averageDE value of about 4.6% moving from low to high ash contents in the compound feeds (fromÿ1 to +1 SD, respectively). This is equivalent, given our average DE value of 15.48 MJ/kg DM, to a difference in the prediction by the chemical equations with respect to the in vitro method of about 0.71 MJ of DE/kg DM, which is about 2±3 times the RSD of the equations considered (range, 0.25±0.32 MJ/kg DM).
All the deviations had a moderately negative correlation with both CP content (rfrom ÿ0.62 toÿ0.68,p<0.01) and EE content (rfromÿ0.48 toÿ0.53,p<0.01): however, this was not attributable to the coefficients in the equations, which were only present in Eqs. (1), (3) and (5), but was due to the high correlation with the ash content, firstly with CP and, to a lower degree, also with the EE content. In fact, an attempt was made to relate the deviations (DE) with several chemical parameters by using a step-wise procedure, but only the ash content explained most of the variability, as the other chemical contents were excluded from the multiple regression.
Finally, the fibre components present in all equations, either as NDF (Eqs. (1), (4) and (5)), ADF or NDF-ADF (Eqs. (2) and (3)), were not significantly related with any of the deviations considered. Therefore, it was surprising to note that variations in the expected main factor influencing digestibility did not interact with the type of prediction.
Fig. 3. Regression of difference (DE,y) between DE estimated by multiple chemical regression Eq. (5) and by in vitro assay, on ash content (x).
4. Conclusions
The present work showed that the difference of the DE content of compound feeds obtained by several multiple chemical regression equations (from in vivo trials) and that from an in vitro technique did not vary by changing the main chemical parameters (e.g. NDF, CP, EE) of compound feeds. The only exception concerns the ash fraction, and it was estimated that the difference in the predictions between the two procedures varied by 4±5% (ca. 0.71 MJ/kg DM) when the ash content ranged from low to high levels (ÿ1 to +1 SD). Therefore, apart from the ash depression on in vivo digestibility, which requires additional research efforts (e.g. increasing endogenous protein losses or nutrient digestibility depression; Noblet and Perez, 1993), we conclude that the present results are an indirect positive validation of the accuracy of the in vitro technique examined. It is a candidate to become an alternative to multiple chemical regression equations when competitive in economic terms (e.g. analytical costs) and/or if more investigations could demonstrate the superiority of the technique in accounting for any digestive effect (e.g. associative effects, hydrothermal treatments, anti-nutritional factors, etc.) which are not appreciated by proximate analysis.
Acknowledgements
The authors wish to thank the Associazione Allevatori del Friuli (Friuli Venezia Giulia region, Italy) for providing the compound feed samples. This research was supported by the Departmental Research Funds (Dipartimento di Scienze della Produzione Animale, Udine, Italy) and represented part of the thesis work of Dr. Luca Brunissco.
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