Digestibility and energy value of diets containing increasing proportions of olive soapstocks for Iberian crossbred pigs

Animal Feed Science and Technology 191 (2014) 83–90

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Digestibility and energy value of diets containing increasing proportions of olive soapstocks for Iberian crossbred pigs M.L. Rojas-Cano, V. Ruiz-Guerrero, L. Lara, R. Nieto, J.F. Aguilera ∗ Department of Physiology and Biochemistry of Animal Nutrition, Animal Nutrition Institute (INAN), Estación Experimental del Zaidín (CSIC), Camino del Jueves s/n, 18100 Armilla, Granada, Spain

a r t i c l e

i n f o

Article history: Received 10 July 2013 Received in revised form 30 January 2014 Accepted 30 January 2014

Keywords: Olive soapstocks Growth performance Digestibility N balance Energy value Iberian × Duroc crossbred pigs

a b s t r a c t The effects of increasing proportions of olive oil soap stocks (OSS) in diets for crossbred Iberian barrows on productive parameters, nutrient digestibility and N balance were studied. Twenty four Iberian × Duroc 50:50 growing barrows of 46.7 ± 0.60 kg initial BW were placed in individual pens and allotted to 4 dietary treatments in which olive oil soap stocks replaced 0 (diet A), 25 (diet B), 50 (diet C) and 75 (diet D) g/kg of a basal diet based on barley, corn, soyabean meal and wheat middlings. Crude fat content of diets increased from 20.8 to 86.6 g/kg with OSS inclusion. At the finishing stage of growth, 16 pigs were randomly allotted to two homogeneous groups of eight pigs each. Two experimental diets were prepared by mixing at equal proportions diets A and B, and diets C and D. In both experiments, daily feed allowance was fixed at 0.90× ad libitum intake, which was given in two equal meals (0900 and 1500 h). The inclusion of OSS in the diet of the growing pigs did not affect the average daily gain and gain:feed, being 765 ± 18.6 g and 0.296 ± 0.007, respectively. However, the ratio gain:metabolizable energy (ME) intake (g/MJ) decreased linearly on increasing the level of OSS dietary inclusion (P < 0.05). At the finishing stage, all performance parameters measured, except the gain-to-ME ratio, were improved (P < 0.05 to P < 0.001) or showed a tendency to be improved in pigs fed a diet containing the 62.5 g dietary proportion of OSS (P = 0.074 to P = 0.088). The apparent digestibility of DM, OM, CP, EE and GE was not altered by the level of OSS inclusion in the diet, remaining at 0.793 ± 0.008, 0.825 ± 0.007, 0.767 ± 0.013, 0.857 ± 0.013 and 0.799 ± 0.008, respectively. The energy density of the diets increased linearly (P < 0.001) from 14.6 to 15.8 MJ of digestible energy (DE)/kg DM and from 14.0 to 15.4 MJ of ME/kg DM with the increase in the dietary proportion of OSS. Olive oil soapstocks contain 32.2 MJ DE/kg and 31.2 MJ ME/kg. No differences in N retention were found between the experimental diets. The average value of 23.2 ± 1.04 g N retained daily corresponds to average efficiencies of utilization of total N (retained N/N intake) and N apparently absorbed (retained N/apparent digestible N) of 0.405 ± 0.018 and 0.530 ± 0.024, respectively. It is concluded that the inclusion of olive soapstocks in the diet of growing crossbred Iberian pigs does not affect the apparent digestibility of nutrients or body protein accretion but increases the energy cost of gain. Also, OSS increases substantially the energy value of the diet. In finishing pigs, OSS tend to improve the gain-to-feed ratio. © 2014 Elsevier B.V. All rights reserved.

Abbreviations: ADFI, average daily feed intake; ADG, average daily gain; BW, body weight; CP, crude protein; DE, digestible energy; DM, dry matter; DU, Duroc; EE, ether extract; FA, fatty acids; FFA, free fatty acids; GE, gross energy; IB, Iberian; ME, metabolizable energy; N, nitrogen; OM, organic matter; TTAD, total tract apparent digestibility. ∗ Corresponding author. Tel.: +34 958 572 757; fax: +34 958 572 753. E-mail address: [email protected] (J.F. Aguilera). 0377-8401/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.anifeedsci.2014.01.018

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1. Introduction Olive oil soap stocks (OSS), a by-product of the olive oil extraction industry, is a valuable source of oleic acid highly appreciated as supplement in diets for fattening Iberian (IB) pigs reared on intensive feeding systems (González and Tejeda, 2007; Serrano et al., 2009; González et al., 2012). This by-product is obtained by means of physical refinement processes which include pressing followed by distillation of the fatty acids (FA) present in the oil (Christakis et al., 1982). Its use is limited by the content in unsaturated fatty acids, due to the negative effect that these FA may have on meat quality [Fundación ˜ Espanola para el Desarrollo de la Nutrición Animal (FEDNA), 2010]. As other soapstocks derived from oil sources, OSS are a good source of energy, although their digestibility is lower than that of the oils they come from, due to its high proportion of free fatty acids (FFA), as found for other fat resources (Powles et al., 1993; Mateos et al., 1995; Jorgensen and Fernández, 2000). To the best of our knowledge, no specific work has been performed to determine the energy value of OSS when added to pig diets. Instead, estimations are currently made based on chemical parameters (FFA content, degree of unsaturation) by equations derived from studies carried out with other fat resources (Powles et al., 1995). Furthermore, in spite of their extensive use, no systematic evaluation has been carried out in IB pigs on the effects of the dietary inclusion of OSS on feed efficiency, digestibility of nutrients, energy value of the feed mixture and on protein accretion. It was hypothesized that the inclusion of OSS in the diet of growing crossbred Iberian pigs would not affect feed efficiency, the apparent digestibility of dietary nutrients or body protein accretion and that the energy value of OSS would be constant at all levels of inclusion. Bruce et al. (2006) observed in growing pigs a quadratic decrease in the apparent ileal digestibility of dry matter (DM), crude protein (CP) and some amino acids (AA) when increasing concentrations of soyabean soapstock were included in semipurified diets containing soyabean meal. With this background, the purpose of the present work was to determine the effects of OSS dietary content on growth performance, nutrient digestibility and N utilization in crossbred growing Iberian pigs. 2. Materials and methods The experimental protocol was reviewed and approved by the Bioethical Committee of the Spanish National Research Council (CSIC), Spain, and pigs were cared for following the Spanish Ministry of Agriculture guidelines (Boletín Oficial del Estado (BOE), 2005, 2007). 2.1. Animals, experimental design and feeding A feeding experiment followed by a digestibility and balance trial was performed in Iberian (IB) × Duroc (DU) 50:50 barrows of 46.7 ± 0.60 kg initial body weight (BW). The pigs (27.3 ± 0.32 kg BW), supplied by a commercial breeding company (Granja El Arenal, Almodóvar del Río, Córdoba, Spain), were individually housed in 2 m2 pens located in an environmentally controlled room at 20 ± 2 ◦ C, exposed daily to 12 h lighting, and given ad libitum a commercial pelleted feed (188 g CP, 9.7 g lysine (Lys)/kg DM) until they attained the target BW, when they were randomly assigned to four homogeneous groups of six or seven pigs each to carry out the feeding trial followed by the digestibility and nutrient balance experiment. Each group was offered restrictedly one experimental diet. Four diets were prepared by replacing 0, 25, 50 and 75 g/kg of a basal diet (A) with OSS to provide four different dietary fat contents (A–D; Table 1). The AA pattern of the dietary protein followed the ideal protein concept [National Research Council (NRC), 1998; British Society of Animal Science (BSAS), 2003]. The nutrient composition of OSS appears in Table 2. The feed mixtures were prepared by NANTA S.A. (Tres Cantos, Madrid, Spain) and offered as pellets at a level of 0.90× ad libitum intake, in two daily equal meals (0900 and 1500 h). Water was freely available from nipples. Feed refusals were daily collected, dried and weighed to calculate actual feed intakes. A second feeding experiment was also performed at the finishing stage of growth. Sixteen IB pigs of approx. 90 kg BW were randomly allotted to two homogeneous groups of eight pigs each. Two experimental diets were prepared by mixing at equal proportions diets A and B (diet AB) and diets C and D (diet CD). The daily feed allowance was fixed at 0.90× ad libitum intake, which was given in two equal meals (0900 and 1500 h). An estimation of the ad libitum intake, defined as what the pig would consume when given sole access to its diet for two periods of 60 min/day, had been previously obtained with diets of the same nature offered to purebred IB barrows placed in pens with individual feeding spaces (Nieto et al., 2001). The daily feed allowance was calculated based on the pig’s BW, measured once per week, according to the following regression equation derived for pigs of 50 to 85 kg BW: Voluntary intake (g/day as-fed basis) = 1525 ± 164 + 31.3 ± 2.33 × BW, kg (P < 0.001; n = 20; s.e. = 230; r 2 = 55.2)

(1)

2.2. Measurements After a period of 3 days on the restricted feed allowance at the pens, the feeding trial was initiated when the pigs attained 46.7 ± 0.60 kg BW, followed by the digestibility and balance trial, which was carried out with pigs of an average 57.2 ± 0.40 kg initial BW. Due to limited availability of metabolic cages, the digestibility measurements took place in two trial replicates with three or four pigs per dietary treatment. Average daily feed intake (ADFI), BW gain and feed efficiency (gain:feed)

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Table 1 Ingredients and nutrient composition of diets for IB × DU 50:50 pigs growing from 50 to 60 kg BW containing increasing proportions of olive soapstocks. Experimental diets

Olive soapstocks (OSS) (g/kg) Basal diet: OSS (g/g) Main ingredients (g/kg) Barley grain, 2-row Corn grain Soybean meal, 47% CP Wheat middlings Sodium bicarbonate Calcium carbonate Monocalcium phosphate Sepiolite Common salt l-Lysine HCl, 78.8% l-Threonine, 98% dl-Methionine, 99% l-Tryptophan, 100% Choline chlorhydrate Vitamin/mineral pre-mixa Analysed nutrient composition (g/kg) Dry matter Organic matter Crude protein Crude fat ADF Total ash Gross energy (MJ/kg)

A

B

C

D

0 1000:0

25 975:25

50 950:50

75 925:75

893 825 138 46.2 52.9 68.1 16.6

894 827 136 56.2 48.4 67.0 17.0

897 830 133 86.6 43.9 67.0 17.8

671 154 55.2 60.0 4.88 14.9 8.12 18.7 0.39 5.46 1.27 1.39 0.13 0.06 5.0 891 823 145 20.8 52.4 68.3 16.2

a Provided (per kg of complete diet): retinol, 10,000 IU as retinyl acetate; cholecalciferol, 1500 IU; dl-␣-tocopherol, 10 mg as dl-␣-tocopheryl acetate; menadione, 0.75 mg as menadione sodium bisulfite; thiamine, 2.0 mg; riboflavin, 3.0 mg; pyridoxine, 0.5 mg; cyanocobalamin, 20 ␮g; folic acid, 250 ␮g; nicotinic acid, 15 mg; d-pantothenic acid, 8.0 mg as calcium pantothenate; d-biotin, 50 ␮g; Mn, 30 mg as MnSO4 ·4H2 O; Fe, 80 mg as FeSO4 ·7H2 O; Zn, 110 mg as ZnO; I, 760 ␮g as KI; Cu, 150 ␮g as CuSO4 ·5H2 O; Co, 1.1 mg as CoSO4 ·7H2 O; Se, 200 ␮g as Na2 SeO3 ·5H2 O; phytase, 540 FTU.

Table 2 The nutrient composition (g/kg) of the olive soapstocks used in diets for IB × DU 50:50 fattening pigs. Organic matter Crude fat Total N Total ash Moisture and volatile matter Unsaponifiable matter Free FA Gross energy (MJ/kg) Iodine number FA relative profile in the crude fat fraction (%) C12:0 C14:0 C16:0 C16:1 n7 C17:0 C18:0 C18:1 trans 9 C18:1 cis 9 C18:1 cis 11 C18:2 n6 (cis 9, cis 12) C18:3 n3 ␣ C20:0 C20:1 n9 C20:2 n6 C20:3 n6 C20:4 n6 C20:5 n3 C21:0 C22:0 C24:0 Unsaturated FA/saturated FA

970 966 0.63 1.34 28.9 92.0 598 38.8 145 0.010 0.072 13.3 0.688 0.109 3.39 0.360 67.4 2.04 9.99 0.911 0.479 0.286 0.031 0.009 0.163 0.032 0.069 0.353 0.369 4.55

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of pigs remaining at the pens were determined. The pigs were individually moved to the metabolic cages 2 days prior to starting excreta sampling, in an environment of 20 ± 2 ◦ C, to allow individual collection of faeces and urine separately. Total collection of faeces and urine was performed daily for 5 days. Aliquots were kept at −20 ◦ C until they were analysed. Urine was collected into 50 ml 4 M-H2 SO4 . At the end of the digestibility and N balance experiment, a composite sample of faeces and urine was obtained for the whole collecting period. Total tract apparent digestibility (TTAD) of DM, organic matter (OM), CP, crude fat (ether extract, EE) and gross energy (GE), and N balance were measured. At the finishing period, the feeding experiment began when the pigs weighed 90.2 ± 0.93 kg BW. The pigs were individually housed in 5 m2 pens located in an environmentally controlled room at 19.0 ± 1.5 ◦ C. Performance parameters were determined.

2.3. Chemical analysis All analyses were performed in duplicate. The following procedures were carried out according to the Association of Official Analytical Chemists (AOAC) (2003): DM content of diets and freeze-dried faeces (no. 934.01); ether extract and total ash of diets, OSS and freeze-dried faeces (nos. 920.39 and 942.05), and total N in diets, OSS and urine, by the Kjeldahl procedure (no. 984.13). Acid–detergent fibre (ADF) was determined in diets using an ANKOM220 Fibre Analyser Unit (ANKOM Technology Corporation, Macedon, NY, USA) and expressed inclusive of residual ash. In freeze-dried faeces total N was determined according to the Dumas’ method, by total combustion in a TruSpec CN equipment (Leco Corporation, USA). Crude protein was determined as total N × 6.25. Whenever an analysis was made on freeze-dried material, DM was determined on an aliquot sample to establish the residual water content after freeze–drying, and the corresponding analytical result was expressed on a DM basis. Gross energy of diets, OSS and freeze-dried samples of faeces and urine was measured in an isoperibolic bomb calorimeter (Parr Instrument Co., Moline, IL). Samples of approximately 10 ml of urine were weighed, freeze-dried in a polyethylene sheet of known energy value, and their GE value obtained by difference. The FA content of OSS was extracted as described by Folch et al. (1957), with small modifications. Methylation of FA was performed according to Kramer and Zhou (2001), slightly modified. Tricosanoic acid (C23:0; Larodan Fine Chemicals, Malmö, Sweden) was used as internal standard. Fatty acids methyl esters were identified by gas chromatography using a gas cromatograph equipped with a flame ionization detector (Perkin Elmer Autosystem Gas Cromatograph, MA, USA) and a capillary column (Teknokroma, phase: TR-CN100, 60 m × 0.25 mm i.d. and 0.20 ␮m film thickness). The temperature programme used was: 70 ◦ C 4 min, ramp of 8 ◦ C/min to 110 ◦ C, ramp of 5 ◦ C/min to 170 ◦ C, 10 min at 170 ◦ C and ramp of 4 ◦ C/min to 240 ◦ C, final time 14.5 min (total time 63 min). The injector and detector were maintained at 255 ◦ C. The carrier gas (helium) flow rate was 1.2 ml/min with a split ratio of 50, and 1 ␮l was injected. Free FA content in OSS was determined by titration with 0.25 M NaOH (Association of Official Analytical Chemists (AOAC), 2003; no. 940.28) and results were finally reported as g oleic acid/kg OSS. The moisture and volatile matter fraction in OSS was determined by drying at 105 ± 2 ◦ C in air oven. The unsaponifiable fraction in OSS was obtained by saponification of the sample with 2 N KOH in ethanolic solution, extraction of the unsaponifiable fraction with petroleum ether (b.p. 40–60 ◦ C), evaporation of the solvent and drying at 103 ± 2 ◦ C to constant weight. Iodine number, to evaluate the degree of unsaturation of OSS, was determined following the Wijs method (Association of Official Analytical Chemists (AOAC), 2003; no. 920.159).

2.4. Calculations Two procedures were followed to calculate the energy value of OSS: (I) From the linear regression that relates the increase in dietary crude fat content (EEOSS ; g/kg) due to the incorporation of OSS to the experimental diets in replacement to the basal diet (A), as independent variable, and DE or metabolizable energy (ME) content (kJ/g), as dependent variable; (II) From the product of the coefficient of apparent digestibility of crude fat (EE) fraction in the experimental diets attributed to OSS and the GE value of OSS, determined in the bomb calorimeter (38.8 MJ/kg), to obtain digestible energy (DE) content (MJ/kg OSS). To obtain the apparent digestibility of the OSS fraction, the apparent digestible fat in basal diet (A) was subtracted from total apparent digestible fat in diets B, C and D, respectively, and the resulting amount was related to the corresponding OSS dietary incorporation determined by ether extraction. 2.5. Statistical treatment Orthogonal polynomial contrasts were used to determine linear and quadratic effects of dietary increasing proportion of OSS on performance, digestibility and N balance variables of IB pigs derived from the digestibility experiment. Initial BW was used as co-variable in the analysis of performance traits of the growing pigs. The GLM procedure of SAS was used. For the finishing stage of growth a one-way ANOVA was performed using the GLM procedure of SAS. Statistical differences among mean values were assessed by the Tukey t-test. The level of significance was established at 5%. The pig was the experimental unit for all the variables studied.

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Table 3 The effects of increasing proportions of olive oil soap stocks in the diet of IB × DU 50:50 pigs on growth performancea . Experimental diet

Olive oil soap stocks (OSS), g/kg Basal diet: OSS, g/g Fattening stage n Initial BW (kg) Body-weight gain (g/day) Feed intake (g/day) DE intake, MJ/day ME intake, MJ/day Gain:feed Gain/ME intake (g/MJ)

Olive oil soap stocks (OSS) (g/kg) Basal diet: OSS (g/g) Finishing stage n Initial BW (kg) Body-weight gain (g/day) Feed intake (g/day) DE intake (MJ/day) ME intake (MJ/day) Gain:feed Gain/ME intake (g/MJ) a b c d e

A

B

C

D

0 1000:0

25 975:25

50 950:50

75 925:75

SEM

Linear

Quadratic

7 45.2 809 2593 33.7b 32.6b 0.312 24.6

6 47.5 734 2583 34.4c 33.4c 0.284 22.2

6 46.7 760 2599 34.9d 33.9d 0.293 22.5

6 46.3 759 2580 36.7e 35.7e 0.294 21.2

0.77 18.6 4.7 0.07 0.07 0.020 0.54

0.703 0.458 0.603 <0.001 <0.001 0.485 0.039

0.386 0.337 0.633 0.002 0.016 0.323 0.399

Experimental diet CD 50:50 AB 50:50 12.5 62.5 987.5:12.5 937.5:62.5

SEM

ANOVA P value

8 90.1 837 3415 44.9 43.4 0.246 19.3

0.96 17.5 12.3 0.16 0.16 0.006 0.43

0.939 0.028 0.088 <0.001 <0.001 0.074 0.521

8 90.3 924 3461 47.8 46.5 0.267 19.9

Polynomial contrast P value

Mean values adjusted with initial BW as covariate. Within a row values with different superscripts differ (P < 0.05). Within a row values with different superscripts differ (P < 0.05). Within a row values with different superscripts differ (P < 0.05). Within a row values with different superscripts differ (P < 0.05).

3. Results Throughout the trials, all pigs remained healthy, showed a normal behaviour and consumed readily the feed on offer. Very exceptionally, some pigs had small amounts of feed refusals that were quantitatively recovered and weighed. During the digestibility trial, one pig (diet C) showed persistent scouring problems and was removed from the experiment. In the growing pigs, the level of inclusion of OSS in the diet did not influence ADFI, being the average value 2.59 ± 0.005 kg (Table 3). However, positive linear (P < 0.001) and quadratic effects (P < 0.01–0.05) of OSS dietary proportion on DE and ME intake were found due to the increased energy content of the diet detected with increasing the level of OSS. Average daily gain (ADG) and gain:feed were not altered by dietary inclusion of OSS, being 765 ± 18.6 g and 0.296 ± 0.007, respectively. However, the gain:ME intake ratio (g/MJ) decreased linearly on increasing the level of OSS dietary inclusion (P < 0.05). As shown also in Table 3, all performance parameters measured in IB pigs of approx. 90 kg initial BW were improved (P < 0.05 to P < 0.001) or showed a tendency to be improved in pigs fed the diet containing the highest dietary proportion of OSS (P = 0.074 to P = 0.088), except for the gain:ME intake (g/MJ), although differences were small. The data obtained in the digestibility and N balance trial appear in Table 4. The coefficients of TTAD of DM, OM, CP, EE and GE were not altered by the level of OSS inclusion in the diet, remaining at 0.793 ± 0.008, 0.825 ± 0.007, 0.767 ± 0.013, 0.857 ± 0.013 and 0.799 ± 0.008, respectively. Also, the coefficient of metabolizability of GE and the ME:DE ratio remained unchanged (0.775 ± 0.008 and 0.972 ± 0.002, respectively). The energy density of the diets increased linearly (P < 0.001) from 13.0 to 14.2 MJ of DE/kg and from 12.5 to 13.8 MJ of ME/kg with the increase in the proportion of OSS in the diet. When energy losses in urine (GEu ) were related to urinary N excretion (Nu ), the following linear regression equation was obtained (P < 0.001), which allows calculating ME content of the experimental diets at N balance = 0 (MEn = 0 ; Table 3): GEu , kJ/day = 353 ± 73 + 29.8 ± 3.3 × Nu, g/day; n = 23; r 2 = 79.0; s.e. = 85.4

(2)

The slope of this equation indicates an energy loss of 29.8 kJ/g N excreted in urine. The MEn = 0 increased linearly from 12.2 to 13.6 MJ/kg with increasing proportions of OSS in the diet (P < 0.001; Table 4). According to approach (I), the following, highly significant (P < 0.001), equations were obtained: DE = 12.87 ± 0.10 + 0.0186 ± 0.0026 × [EEOSS ]; n = 23; r 2 = 69.2; s.e. = 0.32

(3)

EM = 12.49 ± 0.10 + 0.0187 ± 0.0027 × [EEOSS ]; n = 23; r 2 = 69.0; s.e. = 0.32

(4)

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Table 4 The effects of increasing proportions of olive soapstocks in the diet on nutrient digestibility and N balance of IB × DU 50:50 growing pigs. Experimental diet

Olive soapstocks (OSS) (g/kg) Basal diet: OSS (g/g) n Initial BW (kg) Final BW (kg) DM intake (g/kg BW per day) Coefficients of TTAD Dry matter Organic matter Crude protein Ether extract Gross energy (GE) ME/GE ME/DE Energy value DE (MJ/kg) ME (MJ/kg) MEn = 0 (MJ/kg) N balance Retained N (g/day) Retained N (g/kg BW per day) Retained N (g/kg BW0.75 per day) Retained N/N intake Retained N/N apparently absorbed a b c

A

B

C

D

0 1000:0 7 57.1 60.5 39.2

25 975:25 6 58.4 60.4 39.1

50 950:50 6 56.8 59.8 40.0

75 925:75 6 56.4 59.3 40.4

0.791 0.825 0.773 0.865 0.803 0.772 0.971

0.799 0.831 0.774 0.859 0.803 0.780 0.971

0.785 0.817 0.760 0.857 0.791 0.768 0.971

13.0a 12.5a 12.2a

13.3b 13.0b 12.7b

24.8 0.422 1.17 0.412 0.540

23.6 0.398 1.10 0.412 0.535

Polynomial contrast P value

SEM

Linear

Quadratic

0.40 0.38 0.23

0.367 0.249 0.038

0.906 0.824 0.498

0.797 0.826 0.760 0.846 0.800 0.779 0.975

0.004 0.004 0.007 0.007 0.004 0.004 0.001

0.772 0.741 0.371 0.306 0.545 0.799 0.302

0.826 0.793 0.996 0.852 0.609 0.857 0.548

13.4b 13.1b 12.8b

14.2c 13.8c 13.6c

0.08 0.08 0.07

<0.001 <0.001 <0.001

0.192 0.329 NS

22.7 0.391 1.08 0.402 0.527

21.8 0.379 1.05 0.394 0.518

1.04 0.018 0.050 0.018 0.024

0.321 0.434 0.403 0.717 0.742

0.931 0.862 0.879 0.925 0.969

Within a row values with different superscripts differ (P < 0.05). Within a row values with different superscripts differ (P < 0.05). Within a row values with different superscripts differ (P < 0.05).

The intercept of these equations estimates the DE and EM content (kJ/g) of the basal diet (A) and the coefficient of the independent term, the increase in energy value per g of increase in dietary inclusion of OSS (assessed by the analyzed dietary fat content) in replacement of the basal diet. Consequently, the DE and ME contents of the OSS used in the present trial are calculated to be 31.5 (12.87 + 18.58) and 31.2 (12.49 + 18.75) MJ/kg, respectively. Calculation of DE value of OSS following approach (II) resulted in an average value of 32.9 MJ/kg. An average apparent digestibility of fat of 0.849 ± 0.008 was calculated for OSS. No differences in N retention were found between the experimental diets, although a numerical decline was observed as the dietary OSS content increased, which seems to be due to a slight and steady loss in the efficiency of utilization of dietary N with increasing the proportion of OSS in the feed. The average level of protein accretion observed was 145 [(23.2 ± 1.04) × 6.25] g/d, corresponding to average efficiencies of utilization of total N (retained N/N intake) and N apparently absorbed (retained N/apparent digestible N) of 0.405 ± 0.018 and 0.530 ± 0.024, respectively. 4. Discussion In the production of the crossbred IB pig, the incorporation of OSS to the feed mixture as provider of oleic acid is usually practiced to enhance the presence of this FA in animal tissues, thereby upgrading meat quality (González et al., 2012). Levels of incorporation are highly variable depending among other factors on the stage of growth. For fattening pigs these levels may stay in the range of 15 to 65 g/kg (González and Tejeda, 2007; Serrano et al., 2009; González et al., 2012). The present experiment was designed to cover the range of usual levels of incorporation of OSS in the diet for crossbred IB pigs at the fattening/finishing stage of growth to validate the practical application of the experimental observations. The digestibility trial was performed in pigs heavier than 50 kg BW. In their experiments in pigs of BW ranging from 27 to 80 kg to determine the energy value of several fats and fat by-products, Powles et al. (1993) did not observe increases in the apparent digestibility of fat with increases in BW from 50 kg BW onwards. In the present trial it was found that the inclusion of up to 75 g OSS in replacement of the basal diet of the growing pigs did not alter ADG or gain:feed, but increased the energy cost of growth, as shown by the decrease in body gain from 24.6 to 21.2 g/MJ of ME intake. However, growth performance improved in the heavier pigs fed the diet containing 62.5 g OSS/kg compared to those fed 12.5 g OSS/kg. Atteh and Leeson (1985) studied the effects of supplementing the diets of weaner pigs with 0, 50 or 100 g/kg acidulated soapstock for a period of 6 wk. No significant effect of the dietary soapstock level on feed intake and on the apparent digestibility of protein and fat was found, although there was a decrease (P < 0.05) in ADG when dietary soapstock level increased to 100 g/kg. Feed:gain ratio was not influenced by diet. In the present experiment, the apparent digestibility of DM, OM and GE of the experimental diets was close to 0.80 and not influenced by OSS dietary content. The apparent digestibility of the EE fraction was not altered by the level of OSS inclusion and attained an average value of 0.857 ± 0.001. In fact, a slight reduction in the coefficient of apparent

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digestibility of the EE fraction (%) occurs with increasing the level of inclusion of OSS in the diet. Free FA content and degree of saturation are known to influence the efficiency of digestion and absorption of fats and fats blends in an opposite way. In growing/finishing pigs, Powles et al. (1993) observed that the degree of saturation was the major determinant of the apparent digestibility of fats (and therefore, of their DE value), the higher coefficients being obtained for unsaturated fats (rape oil) than for saturated fats (tallow and palm oil). Free FA content appeared to be another major determinant of DE value of fats and acid oils, as a decrease in their apparent digestibility was noticed with increasing FFA content, probably as a result of insufficient solubilization of FFA in the intestinal lumen. Procedures (I) and (II) lead to DE values which differ in 4.61%. Wiseman (1990) and Powles et al. (1993) rely more on approach (II), arguing that “it merely assesses fat digestibility and does not have to account for possible interactions between added fat and the non-fat component of the basal diet”. However, it can be contended that fat digestibility is not free of being interacted by other components of the basal diet. So, we believe that both approaches are equally valid and, then, an average DE value has been adopted, namely 32.2 MJ/kg OSS. This value is somewhat lower than that of 33.5 ˜ para el Desarrollo de la Nutrición Animal (FEDNA) (2010) (8000 Mcal × 4.184) MJ DE/kg tabulated by Fundación Espanola for OSS and also lower than the value of 33.8 MJ DE/kg fat which can be calculated from the prediction equation derived by Powles et al. (1995) with ratio of unsaturated to saturated FA and FFA content as independent additive terms. This predicting equation was obtained for pigs growing from 30 to 90 kg BW. Consequently, these tabulated/predicted values overestimate the energy content of OSS. An important factor influencing the energy value of soapstocks, irrespective of their origin, is the level of their digestive utilization. It is well known that their digestibility is comparatively lower than that of the oils they come from, mainly due to the high proportion of FFA they contain (Powles et al., 1993; Mateos et al., 1995; Jorgensen and Fernández, 2000). As described above, we have not observed any significant negative effect of OSS incorporation in TTAD of nutrients and energy. The results of the current trial differ from those of Woerfel (1981), who observed decreased DM, CP, and certain AA digestibilities following the addition of soybean soapstock to the diet. Also, Bruce et al. (2006) found in growing pigs a quadratic decrease in the apparent ileal digestibility of DM, CP and 5 AA (Ile, Phe, Trp, Gly, and Ser) when increasing concentrations of soybean soapstocks were included in semi-purified diets containing soybean meal. However, in the current experiment there was a slight tendency for all parameters related to the N balance measurements to decline as the level of dietary inclusion of OSS increased. The effect, although not significant, should not be overlooked. The small reduction which takes place on the digestibility of CP and on the efficiency of utilization of the N absorbed causes a reduction of up to 18.8 g in protein accretion. The average value obtained for N retention (23.2 ± 1.0 g/d) indicates that 145 g protein are accreted daily, a figure much higher than the peak value of 71 g of protein retained in the body of purebred IB fattening pigs under the same feeding level (Barea et al., 2007). 5. Conclusions The inclusion of olive soapstocks in the diet of fattening crossbred Iberian pigs up to 75 g/kg does not affect growth performance or body protein accretion, measured by the N-balance technique, but increases the energy cost of gain. It does not alter the apparent digestibility of nutrients, but increases substantially the energy value of the diet, as a result of the high enthalpy and digestive utilization of olive soapstocks. Olive soapstocks contain 32.2 kJ DE/g and 31.2 kJ ME/g. In finishing pigs, when added to the diet, olive soapstocks tend to improve the gain-to-feed ratio. Acknowledgements This study was financed by NANTA S.A. (Tres Cantos, Madrid, Spain), who also sponsored M.L. Rojas-Cano and V. RuizGuerrero contract expenses. Thanks go to L. Lara for their skilful assistance. Also we are very grateful to Granja El Arenal (Almodóvar del Río, Córdoba, Spain) for the provision of piglets. References Association of Official Analytical Chemists (AOAC), 2003. 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