Introduction

To improve pig production efficiency, it is necessary to decrease feeding expenses because they account for nearly 70 % of production costs (^{González-Razo et al. 2010}). This requires finding a low-cost feeding alternative. Therefore, fruit by-products and exogenous enzymes may be an effective means to meet this challenge.

It has been reported that exogenous enzymes (amilases, proteases, xylanases, phytases) improve digestibility of proteins, polysaccharides and oligosaccharides from soy and other grains, promote better digestibility of agro-industrial by-products, and may optimize productive efficiency, thereby improving pig growth (^{Oliveira-Teixeira 2005}, ^{Ristanović et al. 2009}).

Agro-industrial by-products from vegetables, fruit pomace, plant extracts, and distillery and extraction have been proposed as an effective low-cost alternative, as a source of nutrients, in order to meet the nutritional needs of animals (^{Kim et al. 2006}, ^{Xandé et al. 2007}, ^{Taasoli and Kafilzadeh 2008}, ^{Lee et al. 2009}, ^{Newman et al. 2011}, ^{Fang et al. 2016}). ^{Jeong et al. (2014)} have proposed the use of a fermented carrot by-product to feed pigs in the finishing phase, and a bamboo by-product was proposed to feed pigs by ^{Chu et al. (2013)} in order to improve pig productivity.

Apple pomace is a by-product of apple juice Processing and is produced in high amounts in Chihuahua State, Mexico, which produces 573.5 tons yearly (^{SIAP, 2016}). Due to this, pomace is not used after processing, and it is disposed of in the environment, which makes it a potential source of pollution (^{Rodríguez-Muela et al. 2010}). Apple pomace has good nutritional characteristics, palatability, and digestibility; moreover, it contains dietetic fiber and polyphenols which may improve intestinal function (Rodríguez-Muela et al. 2010). It has only been used as an ingredient in silage mixtures to feed pigs, with positive results in food intake, but with variant effects on carcass traits (^{Fang et al. 2016}, ^{Lee et al. 2009}, ^{Bowden and Berry, 1959}).

Apple pomace has been processed using the solid-state fermentation process, as a way to shift carbohydrates contained in it to a single cell protein, improving its crude protein contribution from 6 to 21 % and its true protein (TP) one to 15.5 % (^{Nikolić and Jovanović 1986}, ^{Bhalla and Joshi 1994}, ^{Joshi and Shandu 1996}, ^{Rodríguez-Muela et al. 2010}), transforming apple pomace into a highly nutritious ingredient for animal diets (^{Villas-Bôas et al. 2003}, ^{Teixeira-Macagnan et al. 2015}), which makes it feasible to be considered as an important ingredient in ruminant diet formulation (^{Pérez-Guerra et al. 2003}, ^{Rachana and Gupta 2010}).

However, it is important to determine if the nutritional improvement obtained from this agroindustrial by-product is suitable to feed pigs as an alternative way to increase their productivity (^{Joshi and Attri 2006}, ^{Descalzo and Sancho 2008}). However, although information about its use as feed for ruminants and other species is currently known (^{Gasa et al. 1992}, ^{Anrique and Dossow 2003}), there is a lack of information about its use in non-ruminants, especially pigs.

In this sense ^{Kim et al. (2006)} stated that diets with fermented by-products did not affect the productive performance of pigs when added at a level of 3 %. Nevertheless, other studies have mentioned that the addition of by-products in diets does affect the growth and development of pigs (^{Bowden and Berry 1959}), but, in the case of solid-state fermented apple pomace (FAP), there is very little information available about its effect on productive performance of pigs. In fact, there is no research focused specifically on feeding FAP and its effect on carcass traits and primary cut yield. Consequently, the aim of this work was to evaluate the productive performance and carcass traits of pigs fed with FAP and ENZ in a productive period.

Materials and methods

For this experiment, twenty-four, 105-day-old (Landrace x York) pigs (12 males, 12 females) with 38.9 ± 3.6 kg live weight were placed in individual pens (0.9 X 1.5 m), and received an adaptation diet (Table 1) for two weeks prior to the start of the experiment. Four pigs per treatment were randomly assigned with the same number of males and females to each treatment. After the adaptation period, experimental feeding was applied using diets with different levels of FAP and ENZ (Allzyme Vegpro^®; Alltech, IL. USA) inclusion as treatments, where: T0-0 (0 g kg⁻¹ FAP - 0 g kg⁻¹ ENZ), T0-1 (0 g kg⁻¹ FAP + 1 g kg⁻¹ ENZ), T50-0 (50 g kg⁻¹ FAP + 0 g kg⁻¹ ENZ), T50-1 (50 g kg⁻¹ FAP + 1 g kg⁻¹ ENZ), T100-0 (100 g kg⁻¹ FAP + 0 g kg⁻¹ ENZ) and T100-1 (100 g kg⁻¹ FAP + 1 g kg⁻¹ ENZ). Iso-energetic and iso-proteinic diets were formulated according to National Research Council (NRC) requirements for pigs (^{NRC 2012}), at growing and finishing periods (Table 2). For FAP elaboration, a solid-state fermentation process using apple pomace as a basic substrate was carried out. Apple pomace was placed on a flat surface to create a bed 1.2 x 10 m and 30 cm in height, and 15 g kg⁻¹ of urea [CO (NH₂)₂; Univex, Gto. Méx], 4 g kg⁻¹ of ammonia sulphate Std. [(NH4) 2S04; Univex, Gto., Mex] and 5 g kg⁻¹ of mineral mix (BASE ELITE LE, Lechero 12 %, MNA, NL. Méx), on a wet basis, were added and mixed every four hours during a 72-hour period to keep pH between 5.8 - 6.2 and temperature in the range of 28 - 32 °C, to allow fermentation and to achieve the appropriate conditions for yeast growth. All processes were developed according to ^{Rodríguez-Ramírez et al. (2007)}. After fermentation, FAP was dried, ground, and stored until use. To incorporate FAP and ENZ into rations, they were premixed with minerals and synthetic methionine (MetAMINO^®; Evonik, USA), then added to corn and soy. Prepared meal was stored until the start of the experiment.

Table 1. Growth and finishing diet compositions for pigs fed with solid-state fermented apple pomace and an enzymatic complex (g kg⁻¹ as fed basis). 

FAP= Solid-state fermented apple pomace; ENZ = Allzyme Vegpro^® enzymatic complex; Alltech.

Table 2. Nutrient content (percentage on dry basis) of growing and finishing diets, and from solid-state fermented apple pomace. 

FAP = Solid-state fermented apple pomace, ENZ = Allzyme Vegpro^® enzymatic complex; Alltech.

∗∗ Estimated according to NRC (1998).

This experiment lasted nine weeks: growth phase (week one to week four) and the finishing phase (week five to week nine). For the evaluation of productivity growing characteristics, initial weight (IW), weight-gain (WG), average daily gain (ADG) and final weight (FW) were measured in both phases. In the same way, feeding characteristics, namely feed intake (FI), average feed intake (AFI) and feed/gain ratio (F:G), were measured. Digestible energy intake (kcal/day) was estimated according to NRC (2012) models. An economic analysis through profitability Index estimation (PI) was used to obtain the return on investment by monetary unit (^{Oliveira-Teixeira et al. 2005}) spent on feed cost, which was calculated using the formula:

Where Yi = animal weight in each treatment, P = Price per kg live weight, Cri = Diet intake in each treatment, PRi = Diet price for each treatment. It is important to consider that in PI calculation only the costs for meal formulation were considered, and the price per live-weight-kilogram came from the Mexican Pig Producers Confederation reports (^{CPM 2016}).

At the end of the feeding period the pigs were slaughtered, following Mexican harvesting regulations (^{SAGARPA 1995}). Carcass traits: slaughter weight (SW), hot dressing percent (HDP), commercial dressing percent (CDP), and back fat (BF) were measured and recorded. Primary cuts: Ham, Loin, Brisket, Ribs, Filet and Lard were cut, and their yield was calculated, according to ^{García-Macías et al. (1996)}.

The results from productive performance, carcass traits, and cut yields were analyzed according to the randomized complete block design in a 3 x 2 factorial arrangement, with gender (male, female) as the block, and FAP and ENZ as the fixed effects. The data were processed using the GLM procedure in SAS^® (1990) software; means were expressed as mean and variability data were expressed with standard error of the mean. To identify differences among treatment means, a principal mean-differences analysis was made with PDIFF statement from SAS^® (1990). Means were considered different when p < 0.05.

Results

Results showed that during the growth and finishing phases, productivity was not affected by FAP and ENZ addition (Table 3), at any level of inclusion. Constant feed intake was observed while the F:G ratio showed better values in growth than during the finishing phase; in addition, weight increase was constant and similar during the growth phase since all pigs were above 60 kg at the end of the phase. As noted in the growth phase all pigs showed similar weight at the end of the finishing phase (p > 0.05).

Table 3. Growing and feed-intake characteristics of pigs fed with solid-state fermented apple pomace and an enzymatic complex during growth and finishing periods. 

Data displayed as Mean ± SEM, Significance level = significant effect is showed when p < 0.05.

SEM = Standard Error of the mean, FAP = Solid-State Fermented Apple Pomace, ENZ = Allzyme Vegpro^®, DEI = digestible energy intake.GEN = Gender (male, female) considered as block. FAP and ENZ = Individual effect of FAP or ENZ. FAP x ENZ = Interaction between FAP and ENZ.

Carcass characteristics shown in Table 4 indicate that T0-0 showed the highest (p < 0.05) HDP, while ENZ addition decreased (p = 0.0497) it. Treatments with ENZ addition (T0-1, T50-1, and T100-1) diminished HDP in a 2.8 to 3.1 percentage point range, while treatments T50-0 and T100-0 showed similar performance to the control (p > 0.05). With respect to CDP, T0-0 showed the best (p < 0.05) level, while an effect of the interaction of FAP and ENZ (p < 0.05) was observed, decreasing the dressing in treatments T50-1 and T100-1 in a range from 2.6 to 3.2 percentage points. However, treatment T0-1 showed a lower (p < 0.05) dressing in a greater level (3.1), with respect to treatment T0-0. Treatments T50-0 and T100-0 showed similar results (p > 0.05) to the control. The obtained results indicated that the effect of ENZ and its interaction with FAP decreased carcass yield. Back fat thickness and primal cut yields were not affected (p > 0.05) by FAP and ENZ inclusion and were similar (p > 0.05) to the values of conventionally-fed pigs.

Table 4. Carcass traits and primal cut yield from pigs fed with solid-state fermented apple pomace and an enzymatic complex. 

Data displayed as mean ± SEM

Rows with different letters mean difference at p < 0.05 significance level.

SEM = Standard Error of the mean, FAP = Solid-State Fermented Apple Pomace, ENZ = Allzyme Vegpro^®; Alltech

GEN = Gender (male, female) considered as block. FAP and ENZ = The fixed effect (individual effect of FAP or ENZ). FAP x ENZ = The interaction between FAP and ENZ effect.

Results from PI (Table 5) showed that the return on investment by monetary unit in the growth phase was better for T50-0, while T0-0 showed the lowest profitability. However, in the finishing phase it was observed that T0-1 had the best profitability and T100-0 the lowest.

Table 5. Profitability index for diet cost for pigs fed with a solid-state fermented apple pomace and an enzymatic complex. 

FAP= Solid-state fermented apple pomace ENZ= Allzyme Vegpro^®.

Discussion

Results showed stability in the average feed intake and F:G ratio, allowing a constant weight gain, which is positive to maintain pig productivity. The reason why the use of FAP showed this stability is probably because it is an appropriate feed for the pig diet since this product was able to maintain the pig intestinal function, because when it is fermented the proportion of crude fiber decreases from 19.6 - 3.5 % (^{Joshi and Sandhu 1996}) or from 2.8 to 6.7 %, as mentioned by ^{Noblet and Le Goff (2001)} and ^{Heimendahl et al. (2010)}. Furthermore, fermented feeds improve digestive functions by controlling pathogenic bacteria (^{Jeong et al. 2014}).

^{Fang et al. (2016)} reported that pigs fed with 50 g kg⁻¹ of apple pomace-mixed silage decreased average feed intake and feed efficiency, but maintained average daily gain and the finished body weight was not affected. On the other hand, ^{Lee et al. (2009)} reported that using fermented Apple pomace (40 g kg⁻¹ and 60 g kg⁻¹) in Berkshire pigs increased food intake, while feed conversion decreased when the by-product was added at 20 g kg⁻¹. ^{Bowden and Berry (1959)} reported that pigs fed with dried apple pomace (inclusion ranged from 100 to 400 g kg⁻¹) did not show negative effects on feed intake, recording an average feed intake of 2.7 kg, similar to the results obtained in this experiment. However, they mentioned that the F:G ratio and average feed intake were negatively affected when apple pomace levels increased to 400 g kg⁻¹. In this study, when 50 and 100 g kg⁻¹ were used, average feed intake and F:G ratio did not decrease during the growth period, which indicates that pigs were able to intake FAP at this level, since the start of the experimental period. On the other hand, using other by-products in pig feed, ^{Jeong et al. (2014)} reported that the addition of 671 g kg⁻¹ of carrot by-product on the finishing pig diet improved average daily gain and final weight. ^{Kim et al. (2006)} reported that feeding finishing pigs with persimmon by-product (30, 50, and 70 g kg⁻¹ addition levels) improved feed efficiency when 50 g kg⁻¹ were added, but when the addition level was increased to 70 g kg⁻¹ the evaluated characteristics did not improve. Thus, the importance of this experiment can be found in the fact that when FAP addition was 100 g kg⁻¹, productive characteristics were similar to control feeding conditions, probably because FAP contains good quality protein (^{Cuoto and Sanromán 2006}, ^{Nasseri et al. 2011}). In this regard, it has been demonstrated that the crude protein provided by FAP maintains its biological value similar to soy, wich is used as a main protein source in the feeding of pigs (^{Ahmadi et al. 2010}). In addition, ^{Slagle and Zimmerman (1979)} mention that the protein obtained from a solid-state fermentation process combined with soy maintains its biological value and does not affect the productive characteristics of the pig.

ENZ addition did not show improvement for average feed intake, F:G ratio, and average daily gain. However, results obtained in this experiment using ENZ are contrary to those of ^{Oliveira-Texeira (2005)} and ^{Soria-Flores (2009)}, who reported that the addition of this enzymatic complex improved average feed intake and weight gain. However, our results agree with those of ^{Ruiz et al. (2008)}, ^{Ristanović et al. (2009)}, and ^{Zamora et al. (2011)}, who did not assure productivity improvement with ENZ addition; this means that its usage is controversial because it only increased the production costs, when added or combined with FAP. Additionally, results obtained in this experiment agree with those of experiments made using other agricultural by-products to feed pigs (^{Xandé et al. 2007}, ^{Oddoye et al. 2009}, ^{Newman et al. 2011}, ^{Chu et al. 2013}, ^{Jeong et al. 2014}), where growth and finishing characteristics were not negatively affected.

Regarding carcass traits, the addition of ENZ, alone and with 50 g kg⁻¹ and 100 g kg⁻¹ of FAP, negatively affected HDP, while CDP was affected by the addition of 50 g kg⁻¹ and 100 g kg⁻¹ of FAP in combination with ENZ, although an effect of ENZ (treatment T0-1) was observed. There are currently no reports on the effect of ENZ on the dressing percent or pig carcass, so the results of this experiment are considered relevant. However, for the use of apple pomace, ^{Fang et al. (2016)} reported that feeding 50 g kg⁻¹ of apple pomace-mixed silage did not affect the dressing ratio. ^{Lee et al. (2009)} reported an increase in dressing percentage with 20 g kg⁻¹ of apple pomace mix addition, but when the level increased to 40 and 60 g kg⁻¹ it resulted in a dressing percentage that was similar to the control group. ^{Bowden and Berry (1959)} reported that 100 to 300 g kg⁻¹ of dried apple pomace addition did not affect HDP, but when 400 g kg⁻¹ were included, HDP decreased significantly. ^{Jeong et al. (2014)} reported that the use of a fermented carrot by-product did not affect HDP or CDP and that the carcass traits were positively related to the addition of the by-product. Finally, the same performance in carcass was reported by ^{Xandé et al. (2007)} and ^{Newman et al. (2011)}, who used agricultural by-products. Results of this experiment may indícate that 50 g kg⁻¹ and 100 g kg⁻¹ of FAP without ENZ were suitable to feed pigs because productivity and dressing percentages were not negatively affected, and a beneficiaI effect on commercialization may be shown.

In this work, only when ENZ was added (alone or in combination with FAP) was dressing percentage affected. Furthermore, an improvement in productive characteristics was not observed; therefore, it is possible that for farmers the use of ENZ is not economically feasible, when the pigs are sold and when the carcasses are retailed.

Back fat thickness (BFT) was similar for all treatments, although a gender effect was observed. Back fat for males was higher (p = 0.0291) than females (2.5 cm versus 2.02 cm; respectively); nevertheless, these effects were considered as a block in the statistical model, due to the differences that exist between genders for pig carcass traits, due to the effects of castration (^{Bérard et al. 2010} and ^{Monziols et al. 2005}). ^{Fang et al. (2016)} reported that a diet with 50 g kg⁻¹ of apple pomace did not affect BFT. Conversely, ^{Lee et al. (2009)} using an apple pomace mix including reported an increase in BFT and they mentioned that their results were related to the high fat content in the diet. For this experiment FAP provided only 32 g kg⁻¹ of fat, which could explain the non-difference in BFT. The fact that the carcass characteristics were not affected is an advantage for the use of FAP.

Primal cut yields were similar in all treatments. Reports on the effect of FAP on primal cuts have been poorly reported and only ^{Bowden and Berry (1959)} mentioned that feeding pigs with 100 to 300 g kg⁻¹ of dried apple pomace did not affect ham shoulder, loin and bacon yield. But when they included 400 g kg⁻¹ of by-product, loin and ham yield increased.

In general, pigs fed with 50 and 100 g kg⁻¹ of FAP grew consistently and after nine weeks of feeding the FW was similar. Thus, if a producer decides to use FAP, adding 100 g kg⁻¹ will maintain adequate growth and result in similar characteristics when compared with animals fed in a standard way. Also, carcass dressing percentage will not show a significant decrease that would result in financial losses. Furthermore, at retail sale, the fact that similar yields for ham, loin, shoulder, and bacon, which are considered to be the most important pig cuts (^{Correa et al. 2008}, ^{Kouba and Bonneau, 2009}), were not affected implies that economic profitability will be maintained, making FAP addition a good option in pig diet formulation.

^{Oliveira-Teixeira (2005)}, who used ENZ, mentioned that PI has increased when 1 g kg⁻¹ of ENZ was added but when ENZ addition was 4 g kg⁻¹ only a tendency to increase was shown. This agrees with the PI increase in this work, although a positive ENZ effect was observed when it was added alone in the finishing phase; when ENZ was combined with FAP, PI showed a tendency to decrease. Finally, the FAP and ENZ addition did not affect pig productivity, but affected HDP, while 100 g kg⁻¹ of FAP addition resulted in an HDP similar to that for pigs fed with a standard diet (corn and soy), and 50 g kg⁻¹ of FAP kept a good PI, which means that 50 g kg⁻¹ of FAP with or without ENZ could be added to obtain a similar final weight and good PI.

In reference to the above information, if producers want to sell pigs at farm point they may use ENZ in combination with 50 g kg⁻¹ or 50 g kg⁻¹ of FAP as there are no negative effects on growth. If producers want to sell pork carcasses, they may use 50 g kg⁻¹ or 100 g kg⁻¹ of FAP addition without ENZ, since the HDP will be similar to pigs conventionally fed and the economic income will not be drastically affected. Moreover, if producers want to sell primary cuts, an addition of 50 g kg⁻¹ or 100 g kg⁻¹ of FAP with ENZ will produce a similar cut yield, improving PI and stabilizing the price and value of the meat.

Conclusion

Pigs fed with FAP and ENZ displayed similar growth as conventionally-fed pigs. Addition of 1 g kg⁻¹ of ENZ with or without FAP decreased HDP by 4.6 % and CDP by 5.4 %. Inclusion of 100 g kg⁻¹ of FAP produced similar HDP, CDP, and primal cuts. Pigs fed with 100 g kg⁻¹ of FAP and 1 g kg⁻¹ of ENZ obtained a similar PI to conventionallyfed pigs. Under these experimental conditions, the use of FAP may be considered as a suitable option to feed pigs.