The present study demonstrated the potential of CM as an alternative feedstuff to replace SM in tilapia diets, which in recent years has increase the consumption for animal feeding and its price in the international market. The DM, CP and Ca in CM in the present study (921.5, 363.3 and 8.1 g kg−1) were similar to CM evaluated by Ledoux et al. (1999) (DM 910.0, CP 366.0 and Ca 10.0 g kg−1), and lower than those analyzed by Liu et al. (1995) (CP 442.0 and Ca 73.0 g kg−1). However, crude fiber (CF) content of CM of the present study was about three times higher than those showed by Liu et al. (1995), and lower than that reported by Carlson and Tookey (1983), whose values ranged between 220 and 260 g kg−1. According to these same authors, whole seed with shell presents 221 g kg−1 of CF, while the dehulled seed shows 36 g kg−1 of CF. All essential and non-essential amino acid concentrations of CM analyzed in the present study were lower compared to those reported by Liu et al. (1995). Therefore, the nutritional composition of CM differs according to the type of cultivar, and amounts of shell present with the seeds during processing, which influence the fiber and protein content.
Regarding the anti-nutritional factors, CM of this study showed 10.8 g kg−1 of erucic acid content. On the other hand, CM showed 41 µmol g−1 of glucosinolate, and this value was lower than that reported by Yong-Gang et al. (1993) (45-70 µmol g−1). Due to lack of information about anti-nutrients and ADC of CM for tilapia, SM was used as reference value. Furthermore, other meals prepared from other cruciferous species, such as canola meal (CaM) Brassica sp., and cultivated radish meal Raphanus sativus L. were used to compare the results of the present study by their similarity in nutritional profile and anti-nutritional compounds (erucic acid and glucosinolates). CM of this study contained 20.84 g kg−1 of phytate, and this value was higher that SM (10-15 g kg−1), and lower that rapeseed meal (RM) (50-75 g kg−1) (Francis et al. 2001).
The ADC of CM of this study for DM, CP and GE were similar to the ADC of SM for Nile tilapia (DM 65.49%, CP 89.28% and GE 71.38%) reported by Boscolo et al. (2002) and to CaM for tilapia (DM 66.38% and CP 87.00) found by Pezzato et al. (2002). The phosphorus availability in CM was higher than the values reported by Furuya et al. (2001) for CaM (59.68%). Since plant protein sources contain up to 80% of phosphorus in the form of phytate, which is unavailable to fish (NRC, 1993), phosphorus from CM can be considered more available for Nile tilapia than other cruciferous species. The ADC of methionine of CM in the present study was higher (98.56%) than the values determined by Guimarães et al. (2008) for SM (93.4%), while the ADC of cystine of CM (73.82%) was lower in comparison with SM (89.3%) (Guimarães et al. 2008). In general, ADC of nutrients and amino acids of CM and SM showed similarity.
The presence of some anti-nutritional compounds, such as glucosinolate, phytate and erucic acid in CM negatively influenced the ADC of some nutrients and amino acids, and consequently, the growth performance and feed efficiency. Therefore, some technologies for removing anti-nutritional compounds have been studied and considered in plant-based feedstuffs. The use of heating with or without chemical additives and aqueous extraction can remove glucosinolates present in CM (Yong-Gang et al. 1994), resulting in a product with good properties to be used in animal diets. The CM extracted by the isoelectric pH method showed higher protein content, better amino acid profile and lower concentrations of phenolic compounds (Lovatto et al. 2017).
In the present study, a reduction in the WG of tilapia with increasing inclusion of CM in the diets was verified. Similarly, Ledoux et al. (1999) observed reduction in WG of chicken fed with 150.0 g kg−1 of CM in diets, and Yong-Gang et al. (1994) in pigs fed with 30.0 g kg−1 of CM. On the other hand, Burel et al. (2001) also reported a decrease in the growth of rainbow trout fed 30.0 g kg−1 of RM (cruciferous) in diets. However, Pretto et al. (2014) observed no differences on growth parameters of jundia fed diets containing 208.4 g kg−1 of CM, in comparison with a control diet (0 g kg−1 CM) and chemically treated CM.
The increased replacement of SM by CM reduced the nutrient utilization efficiency by Nile tilapia juveniles. Similar results were described by Santos et al. (2009), who evaluated diets for Nile tilapia that replaced SM protein by cultivated radish meal protein at 12.5, 25.0, 50.0 and 75%, obtaining FCRs of 1.27, 1.17, 1.53 and 1.59, respectively. Furthermore, Pretto et al. (2014) also observed the worst FCR results in jundia fed diets with high levels of CM (208.4 g kg−1). The partial replacement of animal protein by CM protein concentrate (25 and 50%) in diets for Rhamdia quelen, worsened feed conversion and reduced the hepatic glycogen content of fish (Lovatto et al. 2018). Nagel et al. (2012), evaluated different levels of canola protein isolate in partial or total replacement (0, 33, 66 and 100%) of protein of fish meal (FM) for turbot, Psetta maxima, diets and determined values for PER of 2.31, 2.17, 1.55 and 1.45, respectively, similar to those in the present study.
Linear decreases in WG and PER, reinforcing those anti-nutritional factors of CM, reduced nutrient utilization, growth and protein efficiency. In addition, it is feasible that deleterious effects have been boosted by the complementation of the various anti-nutritional in CM. Furthermore, metabolites of glucosinolate hydrolysis can be consider the major toxic compound, which limit the use of non-detoxified CM in Nile tilapia diets.
Growth and nutrient utilization were negative affected by the anti-nutritional factors of CM. According to Mawson et al. (1994), the hydrolysis of glucosinolates by myronsinase generates toxic compounds such as isothiocyanates, thiocyanate anions, oxazolidinethiones and nitriles that may contribute to glucosinolate-induced hyperthyroidism. High levels of glucosinolates lead to depressed growth in fish, since thyroid hormones (T3 and T4) affect the metabolic utilization of energy, amino acids and possibly carbohydrates (Burel et al. 2000). Furthermore, isolated isothiocyanates promoted negative effects on the digestive utilization of nutrients in common carp, Cyprinus carpio (Hossain and Jauncey 1989). Thus, the adverse effects of glucosinolate and their breakdown compounds on metabolism were the major reason to decrease growth and feed efficiency of tilapia fed with highest levels of CM.
On the other hand, tannin has the ability to inhibit the action of proteases, and to complex with proteins, as well as phytate, impairing the absorption of amino acids (Richardson et al. 1985). Several in vitro studies have demonstrated that phytate-protein complexes are more resistant to proteolytic enzymes (Selle and Ravindran 2007). A decrease in protein digestibility was found by Sajjadi and Carter (2004), when 8 g kg−1 of phytate was included in a diet for Atlantic salmon, Salmo salar. In addition, phytate may negatively influence nutrient uptake, due to its ability to chelate divalent ions and to form complexes with proteins. This may limit or reduce its availability and damage the ceca-pyloric region by interfering with the absorption of nutrients (Francis et al. 2001). Thus, the relation of phytate to protein uptake may be the most reasonable explanation for nutrient ADC interference in CM, which may have also interfered on the growth. Moreover, high erucic acid levels impair the growth of Coho salmon and promoted histopathological alterations in important organs (Hendricks 2002).
The average fillet yield was 33.21% among the different treatments, within the range (25.4 to 42.0%) previously observed for Nile tilapia (Clements and Lovell 1994). The inclusion of the CM in the diet did not influence the fillets composition, but with a decreasing trend in fillet ether extract levels. Hossain and Jauncey (1989) reported similar results for common carp fed diets containing graded levels of isothiocyanate (isolated) and mustard oilcake, showing a decreasing trend in carcass crude lipid content with increasing allyl isothiocyanate (isolated) or mustard oilcake. This observation suggest that decrease in the ADC of lipids is provided by breakdown compounds of glucosinolates and tannin.
Variations in hemoglobin concentration can be related to the interaction of phytic acid with proteins that can modify the biological action of hemoglobin and thus the oxygen dissociation curve, reducing the affinity of hemoglobin for oxygen (Rivera-CH et al. 1995). In this study, the MCHC was raised as a function of CM increase in the diet. This change possibly occurred due to an increase in hemoglobin production by erythrocytes to compensate for the low levels of oxygen available to tissues. Feldman et al. (2006) describe reference values for healthy Nile tilapia in the range of 1.91 to 2.83 for RBC, 7.0 to 9.8 g dL−1 for Hb and 27.0 to 37.0% for Htc. Despite significant variations in hemoglobin and MCHC, the values obtained in the present study for all hematological parameters were within the range considered normal for the species. Thus, the different levels of CM evaluated in this study did not interfere on the health status of Nile tilapia.
The replacement of 6% of SM by CM showed lower ALT activity compared to the other treatments, however, it did not differ from the control. A trend of increasing ALT and AST was observed towards the highest level of CM (24%). Similarly, Pretto et al. (2014) also observed an increase in the numerical values of ALT and AST in fish fed diets containing untreated and treated CM, but without significant differences. High ALT and AST activity is indicative of injury to some specific organs. Because of the high concentrations of these enzymes in hepatocytes, increased membrane permeability of these cells by necrosis or inflammation can be identified by the release of these enzymes into the plasma (Grizzle and Lovshin 1996). Thus, increased AST and ALT activity in plasma may indicate liver damage (Asztalos et al. 1988).
In the present study, no significant difference was observed in the hepatosomatic index for tilapia fed diets with graded levels of replacement of SM by CM. According to Quinsac et al. (1994), deleterious effects of glucosinolates in broilers fed with treated or untreated RM caused hypertrophy in the liver. Although AST and ALT showed an increasing trend with glucosinolate levels, the percentages of CM used in this study were not high enough to provoke severe damage such as liver hypertrophy.
The increase in glucose levels observed with increased replacement of SM by CM could be indicative of the metabolic reflex of the animal due to the physical effort needed for the degradation of toxic substances in the liver, which demanded a greater energy supply, and not necessarily due to stress (Landman et al. 2006).
The interest in crambe for biodiesel production has prompted researchers to evaluate its by-products (meal and cake) for use in animal feed. According to Barros et al. (2006), the use of cakes and meals derived from oilseed processing as feedstuffs is essential to the biodiesel production chain. However, studies on the use of CM in fish diets are still scarce. Even considering the presence of anti-nutritional factors in CM, the present study selected the use of untreated meal because it can be directly used and is less costly. On the other hand, it is also important to evaluate different processing techniques that detoxify or reduce the anti-nutrient content of CM, in order to increase its potential as an alternative protein source.
Currently, there is no CM price reference in the Brazilian and international market. However, some projections indicate that the estimated value of CM is about one-third of SM price (Salsgiver 1997), and its replacement by SM would reduce the formulation cost. Based on the results of the present study, replacement of up to 18.0% of the SM protein by CM protein (133.3 g kg−1 in the diet) could be used in Nile tilapia diets, but a rigorous cost-benefit evaluation is necessary, because the reduction on growth and feed efficiency caused by increasing CM in diets needs to be offset by the lower dietary cost expected from the CM inclusion.