Agro-industrial byproducts in rabbit food: Case of the complex of detoxified apricot kernel cake and dehydrated tomato pulp

Background and Aim: The use of agro-industrial byproducts as an unconventional source of raw materials for monogastric feed is one possible solution. This study aimed to determine the effects of incorporating detoxified apricot kernel meal (DAKM) as a substitute for soybean meal and dehydrated tomato pulp (DTP) as a substitute for alfalfa hay on the local rabbit fattening. Materials and Methods: A total of 120 white strain rabbits, weaned at 33 days, were randomly assigned to four groups of 30. The rabbits in each group were ringed, placed in cages at 6 rabbits/cage, and fed according to DAKM and DTP incorporation rates (0%, 30%, 40%, and 60%). Results: The weights at 77 days were improved (p<0.05) with unchanged mean daily intakes. The vast majority of slaughter parameters and carcass characteristics improved. The chemical composition of the meat constituents improved significantly, with a 60% increase in the protein content of the batch (26.55% vs. 28.53%), 38% reduction in the total feed cost, and 40 DA saved for each kilogram of feed consumed per rabbit. The relative economic efficiency improved in proportion to the substitution rates of soybean meal by DAKM and alfalfa hay by DTP. Conclusion: Substituting DAKM and DTP, as byproducts of agro-industrial processing, for up to 60% induced satisfactory results in rabbit fattening. Therefore, it would be more insightful to increase the incorporation rates to determine the optimal threshold.


Introduction
Algeria is one of the top producers of rabbit meat in Africa. The intensive production of rabbits (Oryctolagus cuniculus) has several advantages that solve a part of the worldwide shortage in animal protein [1,2]. Rabbit diet accounts for up to 70% of production costs [3], which are mainly due to the importation of most raw materials used in feed formulas. The short-term solution lies in the introduction of agricultural and agro-industrial byproducts into feed formulas [4][5][6][7][8], which would make it possible, in the short term, to make meat products available to the poorest populations at a lower cost. In rabbit farming, agro-industrial byproducts, such as apricot kernel cake and date scraps, have been successfully introduced at a rate of 30% [9,10] and apricot kernel cake alone at a rate of 60% [11]. In the framework of the continuity of this approach, this study aimed to introduce the byproducts, such as the apricot kernel cake versus soybean cake and tomato pulp versus dehydrated alfalfa, in rabbit feed.
The tomato pulp generated by processing the latter is important in the northeast region of the country. This weeded crop is grown on an area of 21,434 ha for an estimated production of 1.235 million tons per year, 85% of which is concentrated in the Wilayas of Skikda, Annaba, Guelma, and El-Tarf [12]. With 27 processing units, about 13% of these are byproducts [13], or 160,602 T/year, consisting mainly of seeds, husks, and stalks.
The tomato pulp in rabbit feed can be effectively used for up to 20% incorporation rate, as it was tested extensively by some authors [14,15]. However, this rate varies as the chemical composition and nutritional value of this byproduct depend on the environment, the soil in particular, and the level of fertilization. The tomato pulp from national processing units is relatively rich in cellulosic fiber, protein, and fat (35.3%, 19.90%, and 16.1%, respectively) [15].
In Algeria, the stone fruit arboriculture is dominated by apricot trees, with about 46,000 hectares for an average annual production of 293,486 T, which is mainly concentrated (40%) in the region of Hodna, inducing the establishment of several processing units [12]. These License (http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
units produce large quantities of byproducts each year, which are mostly cores. For El-Adawy et al. [16], the apricot kernel represents 15-16% of the apricot, while the almond represents 30-38% of the kernel. According to Ferradji et al. [17], the oil yield after crushing and pressing the apricot kernel is estimated to be 33% for 67% of the cake. Based on these data, the quantity of apricot kernel cake is estimated to be 106,970 T/year. There are only a few studies on the use of apricot kernel cake as a protein source in animal feed, and its incorporation has been tested in broiler chicken [4], sheep in fertilizer [5], and in association with other byproducts in rabbit farming [9,10]. This coproduct contains a significant protein content (42.3%) [15].

Ethical approval
The present study was conducted after approval of the Institutional Animal Ethics Committee laboratory of the Agriculture department of Ghardaia University, Algeria.

Study period and location
To guarantee the maintenance of environmental conditions, the test was carried out in a 200-m 2 building with a pad cooling system and fans in a professional rabbit breeding center (Wilaya of Sétif, Algeria), during the month of April 2019.

Animals, food, and experimental protocol
One hundred and twenty white strain bunnies, weaned at 33 days, were randomly divided into four groups of 30. The rabbits in each group were ringed and placed in cages at a rate of six rabbits per cage, that is, five repetitive batches per group. The sex of the rabbits was not given particular attention because until 15 [18] and 20 weeks of age [19], the males and females followed a similar growth curve and body composition.
The apricot kernel cake was supplied by an oil extraction unit located in Beni Ourtilane, Wilaya of Sétif. It was detoxified according to the method of Gabrial et al. [20]. The tomato pulp was supplied by the Nouvelle Ere canning factory in the industrial zone of the Wilaya of Sétif. It was dried in the sun for 3 days. The chemical compositions of detoxified apricot kernel meal (DAKM) and dehydrated tomato pulp (DTP) are shown in Table-1 [21,22].
Four compound feeds were formulated using WUFFDA [23]; one control and three experimental feeds, in which we substituted 30, 40, and 60% of the soybean meal with DAKM and alfalfa hay with DTP (Table-2).
The animals were individually weighed at 33, 44, 58, and 77 days of age, and the feed was distributed at will; The consumption of each cage was checked every week of the experiment at a specific time. The average daily gains (ADG g/day), average daily intakes (ADI g/day), and conversion indices (CI) were calculated. The water was available ad libitum through the automatic pipette-type drinkers.
The slaughter parameters, carcass characteristics, and meat chemical composition were determined on the ten rabbits in each group according to the methods proposed by Dalle Zotte et al. [24].

Meat quality
In the longissimus lumborum muscle, the pH was measured directly 24 h postmortem using a pH meter, and the chemical composition of meat was determined according to the AOAC method [21] with three replicates. The analyses included the water content, protein content, fat content, and mineral content. After 24 h in the cold room (6ºC), the weight of the cold carcass was determined.

Statistical analysis
The data were prepared using the Microsoft Excel sheet. Statistical analysis and comparison of the means between the different diets (control and experimental) were performed by unidirectional analysis of variance using the SPSS software version 21 (IBM Corp., NY, USA). Student-Newman-Keuls and Duncan's test were done if a significant difference at 5% standard error was found (p<0.05).

Economic efficiency
The economic efficiency was calculated from the equation of Asar et al. [25]: Economic efficiency (%) = [Net income/Total feed cost]×100.
Where: Net income = Price of weight gain − Total feed cost.
Weight gain price = Average weight gain (kg/subject)×price of 1 kg body weight.
Total feed cost = Average consumption (kg /subject)×price of 1 kg of feed.
The cost of each kg of feed for the control and experimental diets was calculated based on the local market price of ingredients at the time of the experiment (the year 2018). Ancillary costs were not included.
• For the experimental batch, the total feed cost also included the cost of purchasing and detoxifying the apricot kernel cake and the cost of purchasing and dehydrating the tomato pulp (Table-3). The cost of the production of DAKM and DTP was calculated using the direct cost method, which consists of allocating the direct fixed costs specific to the production of apricot kernel cake. The common costs were not negligible, but they were reported on all the company's products since they were difficult to evaluate in this study, which deals with only one byproduct.
• The basic wage in Algeria (SMIG) was 18,000 DZD/month if 35,000 DZD (gross wage+employer's contributions) was assumed, since the average wage per month was about 200 DZD/h. • The cost of energy was calculated by estimating the energy consumption of the material used for the production of oil cakes, knowing in the professional context, each kWh was charged at 4.472 DZD/kWh [26]. • To rinse one ton of oilcake with water, about 10 m 3 of water was needed, at a price of 1 DZD/m 3 , according to the Algerian Water Agency [27]. • The depreciation of the equipment was estimated according to its purchase price (Dryer, Industrial Tank, etc.) on the market and according to the average capacity of the companies carrying out the oil extraction.

Results
The substitution of soybean meal by DAKM and alfalfa hay by DTP had little effect on animal health status. Over the entire experimental period for all groups, a mortality rate of <6% was recorded.

Zootechnical performances
The substitution of soybean meal with DAKM and dehydrated alfalfa with DTP improved the rabbit weights at 77 days for the experimental lots (Table-4). The weights at 44 days were higher for 40% and 60% groups. On day 58, the 30% group achieved better weight performance compared to the other groups (+65 points).
The ADG (33-44 days) for the 40% and 60% groups was optimal and similar, with an increase of +8 points over the control group, with the 30% group having a minimum of −14 points over the experimental group, and −6 points over the control group. However, for the ADG (45-58 days), the 30% lot had a significantly higher value, with a difference of +16 points compared to the 40% and 60% lots, and −10 points compared to the control lot. For the experimental groups, the ADG (59-77 days) was significantly higher for the 40% and 60% lots, with the 30% lot having a lower value with −4 points. The control lot was the one with a lower value. Throughout the entire rearing period, the ADG (33-77 days) was significantly lower for the control lot (−2 points) compared to experimental lots with similar values. The ADI and CI were similar (p>0.05) for all groups during the entire rearing period (33-77 days) ( Table-5). The CI (33-44 days) for the 0% group was optimal (3.08) and remained similar for the other groups, with +0.23 points compared to the control group. The CI performance remained the same for all groups in the 44-58 days period. The CI (59-77 days) was depreciated and remained similar for the 0% and 30% groups, with a difference of +0.44 points compared with the 40% and 60% groups, which had similar values.
The ADI (33-44 days) and ADI (45-58 days) were the highest (p>0.05) for the 30% group compared with the other groups. The ADI (59-77 days) for the 60% group was lower compared with the other groups, which had similar values.

Slaughter parameters, carcass characteristics, and chemical composition of the meat
The incorporation of DAKM and DTP complex (up to 60%) as a substitute for soybean meal and alfalfa hay, respectively, significantly improved the slaughter parameters (p<0.05) ( Table-6). The HCW and yields (HCW/LWS) from the 40% and 60% groups were similar and dominant over the 0% and 30% groups. The CCW of the experimental lots was higher (p<0.05) than that of the control group, and so was the yield (CCW/LWS), which was superior for the 40% and 60% groups; the muscle/bone ratio remained dominant for these groups.
The liver and PRFW of the experimental groups were higher than those of the control group (p>0.05), and so were the liver/Pva, Pgpr/Pva, and Pgpr/Pcc ratios. The SWs (pp) of the 40% and 60% lots were significantly higher (+14 points) than the 30% lot, which was +8 points higher than the control lot. The solid GI tract weights of all lots were similar (p<0.05). The weights of the front part of the control and 30% groups remained the same and lower than those of the 40% and 60% lots by about 50 points. The weight of the middle part remained unchanged for all groups. While similar, the weight of the rear part of the experimental groups was significantly higher than that of the control group.
The meat chemistry parameters of the experimental lots were significantly improved by substituting DAKM for soybean meal and DTP for alfalfa.

Economic efficiency
The substitution of soybean meal with DAKM and alfalfa hay with DTP in the diet of growing rabbits had a positive effect on economic efficiency (Table-7). Indeed, the 60% group achieved optimal economic efficiency of 25.82% compared to the control group, and a 25.95% reduction in the total feed cost, saving 9.1 DZD for each kg of feed produced. As a result, the net income per kg of meat produced changes proportionally to the substitution rates of soybean meal with DAKM and alfalfa hay with DTP.

Discussion
The overall mortality rate was low (<6%). It was due to the transfer after weaning and the adaptation of the subjects to their new rearing conditions, as stated by De Blas [28]; however, it was still within the norms and range of what was recorded in national rabbit farms [29].
From the substitution of soybean meal with DAKM and alfalfa hay with DTP, the improvement of the overall performance of the experimental batches was mainly due to the increased efficiency in the use    All slaughter parameters were improved in proportion to the incorporation rate of DAKM and DTP (from 30% to 60%) (p<0.05), except for the weight of the full digestive tract and saddle. However, for DAKM and DTP incorporation rates of 10-30%, Mennani et al. [9], Mennani et al. [10] reported no differences in slaughter parameters and carcass characteristics between the batches. This suggests that these agro-industrial byproducts, especially DAKMs, incorporated in up to 60% in rabbit fattening diets, did not affect the digestive tract and saddle weights.
The nutritional value of rabbit meat is highly variable [45,46], resulting in a highly variable chemical composition depending on the part of the carcass studied [47] and different production factors [48], especially the feed [49]. In this sense, the introduction of DAKMs and DTPs in rabbit fattening diets induced better meat composition performances in the experimental lots (30%, 40%, and 60%) compared to the control lot, while remaining similar between them. For lower levels of DAKM (10%, 20%, and 30%), Mennani et al. [9], Mennani et al. [10] observed an improving meat chemical composition; this remained different between the experimental lots, with a significant decrease in fat content which, on the other hand, significantly increased in our study. This was contrary to the increase in the fat content of experimental lots due to higher DAKM incorporation rates.
The price per kilogram of feed produced by the experimental 60% group created a financial gain of 40.21 DZD due to the low-cost price of agro-industrial byproducts and inflationary prices of soybean meal and alfalfa, which are entirely imported, depending on the fluctuations in world stock markets.

Conclusion
The DAKM/DTP complex can be considered a good source of protein (58.41%) and fiber (45.62%). This can be used as an alternative to the soybean cake/ alfalfa hay complex, with a substitution rate of up to 60%, without any negative effects on growth rates, food conversion, and food consumption. It improved the live weights, all slaughter parameters, carcass characteristics, and meat chemical composition while reducing the cost of feed consumed. These encouraging results allow us to put forward the idea of increasing the substitution rates to determine the optimal incorporation rates.