Chemical composition of the broccoli by-products
Chemical analysis of BBP is presented in Table 2. Results showed that dried BBPs had an acceptable value of CP, CF, EE, NFE and ash contents. Data showed that BBPs had higher contents of the CP (24.9%) and ash (16.2%) on a DM basis. These findings are similar to those obtained by Alpuche-Solis and Paredes-Lo´pez (1992) who found a relatively high protein content in floret flours (36.6% of DM) and stalk flours (18.6% of DM) and reported that the protein level measured in the whole plant flour of four cultivars of broccoli ranged from 23.2 to 32.0% of DM. Results obtained by Ibrahim et al. (2011) revealed that the chemical composition of BBPs for CP, CF, EE, NFE and ash, was 14.3, 23.9, 2.35, 45.9 and 13.6%, respectively, being fairly similar in respect of EE and NFE with those recorded in the present work. Hu et al. (2012) reported higher CP content (27.1%) and lower CF content (8.85%) than those obtained in the present study (Table 2) for broccoli stem and leaf meal on a DM basis. In short, our findings suggest the possible incorporation of BBPs (heads, leaves and false stems) with potential added values to animal feed.
Growth performance
Effects of BBPs supplementation on growth performance parameters of NZW rabbits are presented in Table 3. Rabbits fed Broc2 diet showed the highest (P < 0.05) total body weight gain (WG) and feed intake (FI) at 10 and 14 weeks of age, followed by those fed the Broc1 diet versus the control diet which showed the lowest WG and FI values. Previously, rabbits fed diets containing 6% broccoli residues as a substitute for alfalfa hay showed an improvement in the final body weight and daily gain compared to those fed a control diet free from broccoli residues, however, it had no significant effect on feed intake and feeding value (Ibrahim et al. 2011). In lactating dairy cows, replacing concentrate mixture with 20% pelletized BBPs had no adverse effects on rumen fermentation but increased milk production numerically and increased milk fat content significantly (Yi et al. 2015). In sheep, Mahmoud (2016) pointed out that both live body weight and daily gain were higher for lambs fed diets containing BBPs at a rate of 20 or 40% level in replacing to berseem hay. In contrast to our findings, Hu et al. (2011) indicated that increasing dietary inclusion (0, 3, 6, or 9%) of dried broccoli leaves and stem meal for the diets of laying hens had no significant effect on productive performance. It has been shown that BBPs contain high levels of vitamins E and C, calcium and phosphorus, as well as the most valuable amino acids (Hu et al., 2012) in addition to glucosinolates, the bioactive compounds of Brassica vegetables (Gerendas et al., 2008). Thus, the growth-promoting effect of BBPs might be due to their good nutrient profile and their high content of bioactive compounds that promote productive performance (Gerendas et al. 2008; Hu et al. 2012; Vallejo et al. 2004). Madhu and Kochhar (2014) revealed that broccoli leaves are available at no cost and are rich in most micronutrients and macronutrients for the biological processes in animal feed.
Regarding the feed conversion (FC) and performance index, results showed no significant effect due to the addition of different levels of BBP into the diet of rabbits (Table 3). In contrast to our findings, Mahmoud (2016) concluded that the best FC (P<0.05) was found with 40% BBP in replacing berseem hay for lambs, while the poorest one was associated with the control one. Also, Mustafa and Baurhoo (2016) reported that incorporation of dried broccoli floret residues at 3 and 6% levels increased the FC ratio in broiler chicken diets. In piglets (four weeks old male weaned piglets), broccoli extracts positively influenced WG and FC during the 1st week of the experiment only and there were no significant differences in productive performance between the tested and control piglets over the whole trial period (Mueller et al. (2012). Regardless of the controversies among results that might be due to various factors, these findings emphasize that the utilization of vegetable residue is a sustainable move in animal production and environmental protection.
Digestibility
The effect of BBPs supplementation in the diets of rabbits on digestibility coefficients is summarized in Table 4. Data obtained indicated that DM, OM, CP, CF, EE and NFE digestibility for Broc1 and Broc2 diets was greater (P < 0.05) compared to those of the Broc0 diet. Accordingly, the feeding value in terms of total digestible nutrient (TDN) and digestible crude protein (DCP) was greater in both tested diets versus the control one. Similar results were reported by Mahmoud (2016) who indicated that digestion coefficients of CP and NFE and subsequently feeding values in terms of TDN and DCP were higher for lambs received diets containing BBPs at a rate of 20 or 40% level in replacing to berseem hay in their diets. Ibrahim et al. (2011) found that rabbits fed diets contained 3 or 6% BBPs replacing to alfalfa hay increased digestion coefficients of DM, CP and CF and feeding values (TDN and DCP) compared to the control diet, but no significant effect was recorded regarding digestibility coefficients of OM, EE and NFE. The improved digestion coefficients in rabbits fed BBPs might be due to the improved caecal ecosystem and caecal morphology exerted by broccoli fiber as recorded by Paturi et al. (2010). In contrast, Mustafa and Baurhoo (2016) reported that dried broccoli floret residues incorporated at a rate of 3 or 6% did not affect nutrient digestibility. However, the ileal and total tract nutrient digestibility were compromised in broilers fed broccoli floret at a high level of 9% compared to in those fed the control diet (Mustafa and Baurhoo, 2016). Similarly, Mustafa and Baurhoo (2018) showed that the digestibility of DM, OM and CP was decreased with increasing the level of dried broccoli floret from 0 to 12% in the diets of laying hens.
Carcass traits
Data of carcass traits of slaughtered NZW rabbits are presented in Table 5. Results revealed that insignificant differences were observed in pre-slaughter and non-edible parts weights due to the addition of different levels of BBPs in the diets of rabbits. The obtained results indicated that the weights of the empty carcass (with head), giblets and dressing (total edible parts) were higher (P < 0.05) for both Broc1 and Broc2 diets than those of the Broc0 diet. A similar trend among the dietary treatments was observed for head, liver, heart and kidney weights, with the highest values were found in the Broc2 diet and the low ones were associated with the Broc0 diet. This improvement induced by the Broc2 diet might be due to the ability of BBPs in preventing fat deposition, especially in inedible offal as has been previously reported by An et al. (2010). In contrast, Ibrahim et al. (2011) revealed that rabbits fed a diet that contained 3% or 6% BBPs as a substitute for lucerne hay in their diet had no change in weights of inedible offal and carcass, digestive tract and chemical analysis of the 9, 10 and 11th ribs but has a lower dressing percentage in comparison to control rabbits.
Blood biochemical parameters
The results of the blood plasma biochemical parameters of rabbits are presented in Table 6. Total protein, albumin and globulin concentrations were higher (P < 0.05) for rabbits that received BBPs than those of the control one with the highest value was observed with the Broc2 diet. Inversely, plasma glucose, total cholesterol, uric acid and creatinine concentrations were lower (P < 0.05) with both tested diets than those of control one with the lowest value was associated with the Broc2 diet. Similarly, Mahmoud (2016) showed that lambs received BBPs at a rate of 40% in replacing to berseem hay increased the concentrations of total protein, albumin and triglyceride, while those received BBPs at a rate of 20% recorded the highest values of globulin, creatinine and total cholesterol compared with lambs received diet without any replacement. Kummer et al. (1981) showed that total protein and its fractions can be used as an indicator to estimate the ruminant nutritional and physiological cases, so, the increase in globulin production by the liver could reflect the good hepatic functions of these animals and correlates very well with high immunity (Griminger, 1986). In the same context, Craig (1999) mentioned that albumin is one of the important protein fractions that keeps the osmotic pressure stable in the blood. Both albumin and globulin results reflect the ability of animals to store reserve proteins even after their bodies have reached the maximum capacity of depositing tissues (Stroev, 1989). The present results are in harmony with those reported by Cox-Ganser et al. (1994) who recorded that transfer of sheep from grazing grass to Brassica forage results in a reduction in plasma cholesterol. Similar results in human patients were observed by Suido et al. (2002) who found that daily consumption of broccoli may be useful in lowering the low-density lipoprotein cholesterol levels in hypercholesterolemic.
The AST and ALT enzyme activity was higher (P < 0.05) in rabbits that received Broc1 and Broc2 diets in comparison with the control one. Additionally, Hu et al. (2011) mentioned that hepatic hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase activity was decreased with increasing dried broccoli leaves and stems meal supplementation. In general, the values obtained respecting blood constituents in this work indicated that all rabbit groups were under normal physiological and healthy status.
Caecum activity
The results concerning the caecal activities in terms of pH value, concentrations of TVFAs and NH3-N are presented in Table 7. Results revealed no significant differences between treatments in cecal pH value. Rabbits fed Broc2 diet showed the highest (P < 0.05) concentrations of TVFAs while the lowest values occurred with rabbits that received either 1% BBP (Broc1) diet or those of control diet (Broc0). The concentration of ammonia followed the opposite trend of TVFAs among dietary treatments, but no significant difference was observed between Broc0 and Broc1 groups and both were significantly higher than that of the Broc2 diet. These results might be due to the glucosinolates content that is considered as an important class of organic compounds including nitrogen and sulfur in BBPs which are health-promoting compounds (Vallejo et al. 2004). Similarly, Mahmoud (2016) recorded that the values of rumen pH and TVFAs concentration were increased, and the concentration of NH3-N was decreased with the increase of BBPs meal supplementation compared with the control lambs. Also, Yi et al. (2008) found little effect on in vitro gas production and ruminal fermentation in ruminant diets after replacing soybean meal with pelletized BPPs. In addition, Yi et al. (2015) indicated that replacing concentrate mixture with 20% pelletized BBPs had no adverse effects on rumen fermentation in lactating dairy cows. Moreover, it has been reported in rats that fermentable oligosaccharides in BBPs may be beneficial to cecal microbiota, alter the cecal TVFAs and increase the crypt depth and goblet cell number in the colon (Paturi et al. 2010) and thus protecting the gut ecosystem. Also, it has been revealed that BBP improved the utilization of a low protein diet by conferring some protection in terms of the colon morphology, via its antimicrobial properties against several microorganisms of clinical importance (Brandi et al. 2006; Paturi et al. 2010).
To this end, broccoli residues may vary in percentages of stalks, stems, leaves, and florets which in turn reflect the variation in the content of nutrient and antinutritional factors in broccoli restudies preparations. Broccoli residues contain the antinutritional compounds tannin, phytate and oxalate, saponins, goitrogens, and trypsin inhibitor (Madhu and Kochhar (2014; Nagraj et al., 2020) which negatively affect the assimilation of nutrients, particularly protein and minerals. A higher intake of broccoli residues may impair gut function and metabolic performance due to these antinutritional compounds (Choudhury and Khaled, 2014; Sinha and Khare, 2017; Nagraj et al., 2020). Further, the nutritional quality and the bioactive compounds in broccoli residues vary with broccoli cultivars, cultivation conditions, and pre-and post-harvest processing (Ilahy et al., 2020). Thus, further studies are warranted on the biological effects of different preparations of broccoli residues on animal performance and health.