Two experiments were conducted to investigate the effects of different levels of Rapeseed Meal (RSM), as a source of protein, on digestibility (Experiment 1), performance and carcass characteristics (Experiment 2) in rabbits. Dietary Soybean Meal (SBM) was replaced with RSM at 0, 7, 14 and 21% (dry matter basis). No differences were obtained for Crude Protein Digestibility (CPD) coefficients of experimental diets. However, Dry Matter Digestibility (DMD) and Organic Matter Digestibility (OMD) decreased in diet containing 21% RSM. The daily weight gain values ranged between 27.1 and 28.5 g and tended to decrease as the level of RSM increased in the diet. The feed efficiency was not significantly affected by the inclusion of RSM, although rabbits on 21% RSM diet had the poorest efficiency (4.8) while the best feed efficiency (4.6) was obtained from the rabbits fed on RSM diet. The percentage values of liver and peripheral fat were not affected by the diet. The results showed that RSM could replace up to 14% of dietary soybean meal in the diets of rabbits without adverse effects on digestibility, performance and carcass characteristics.
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Many feedstuffs especially the agro-industrial by-products which are usually of no feeding value to humans could alternatively be fed at cheaper cost to animals. Rape Seed Meal (RSM) is a good source of protein, energy, calcium, potassium, phosphorus and magnesium for various species of animals (e.g., cows, sheep, fish, broiler, pig). However, it contains several antinutritional factors (e.g., glucosinolates, phytic acid, tannins, sinapine and erucic acid). Content of goitrogens (glucosinolates) has been markedly reduced through genetic selection, however at feeding higher levels of RSM, the higher concentration of glucosinolates can occur in the diets. These constituents can cause liver damage, decrease feed consumption and performance and can have a negative effect on reproductive performance (Tripathi, 2006). Hence, most reports recommended a limited inclusion level of RSM depending on the glucosinolate content of the meal. The aim of this study was to determine the effects of different levels of RSM in the diets of rabbit on digestibility, growth performance and carcass characteristics in rabbits.
MATERIALS AND METHODS
Animals and Treatments
Four diets were prepared in a pelleted form using the same batches of raw material. Diets were formulated to contain an increasing proportion of RSM: 0, 7, 14 and 21% DM for diets D1, D2, D3 and D4, respectively, by the substitution of a decreasing proportion (21, 14, 7 and 0%) of SBM. The diets did not contain any drug supplementation (antibiotic or coccidiostatic). The RSM (a double low-RSM varieties) had 34.9% Crude Protein (CP) and 13.7% Crude Fibre (CF). Ingredients and chemical composition of the diets are given in Table 1.
|Table 1:||Ingredients and chemical composition of the diets|
|*Rapeseed meal; ** Composition (per kg diet): CuSO4 (1 g); ZnSO4 (3 g); FeSO4 (16 g); KI (0.15 g).|
In experiment 1, 24 crossbred rabbits were randomly divided into four groups (6 rabbits per diet) and housed individually to determine the apparent digestibility of DM, OM and CP. Following a fifteen-day period of adaptation to metabolism cages, the total faecal output was recorded for each rabbit over a seven-day period. Faeces were stored at -20°C and afterwards dried at 80°C for 48 h and grounded through a 1 mm screen.
In experiment 2, 128 crossbred rabbits aged 28 days and weighing on average 1043 g were equally divided into four groups of 32 (four replicates of 4 male and 4 female rabbits each). The rabbits were placed in collective cages with 4 individuals each and fed ad libitum until 13 weeks of age, at which commercial slaughter weight in Tunisia (about 2.5 kg) was expected to be reached. The experimental period lasted 6 weeks. Since the rabbits were housed as subgroups, they were weighed individually however feed consumption per cage was recorded at weekly intervals. The average values for body weight gain, feed consumption and feed/gain on a per-rabbit basis were calculated for each cage. Mortality was recorded daily. Rabbits that died during the study were weighed and used to adjust feed consumption data. At the end of the experiment, 12 rabbits from each group selected randomly were weighed and slaughtered without fasting. After the limbs and head were removed, each carcass was skinned, the abdomen opened, gut and internal organs removed. The weights of warm carcass, liver, peripheral fat and gastrointestinal tract were recorded.
Chemical analyses of diets were done with using the methods of the AOAC (1990) for proximate analyses.
The collected data were subjected to analysis of variance using the statistical analysis system (SAS, 1989).
RESULTS AND DISCUSSION
Diets were designed to be isonitrogenous. However, because the CP content of RSM was lower than of SBM, the CP content decreased with levels of RSM incorporation but still in agreement with the recommendations for growing rabbits (De Blas and Mateos, 1998). In contrast, the fibre content expressed in CF increased with levels of RSM incorporation.
|Table 2:||Effect of the inclusion of rapeseed meal on coefficient of total apparent digestibility of dietary compounds, growth performance, carcass yield and proportions of various organs of rabbits|
|1Rapeseed meal; *: p<0.05; NS: p>0.05, Mean values with the same superscript are not significantly different|
The RSM level in the diet affected (p<0.05) DMD and OMD (Table 2). The lowest DMD and OMD values were obtained from the rabbits fed on 21% RSM diet. This could be due to the fact that RSM dry matter is less degraded than SBM dry matter (Bravo et al., 2000). Lower digestibility of DM constituents was also reported by Sharma et al. (1980) in calves fed RSM diets compared to SBM diets. However, the results presented in Table 2 indicate that the replacement of SBM by RSM had no effect on CPD, probably due to similarity of the protein amount in tested diets. Generally there is a positive correlation between CP content and its digestibility.
The data on final live weight, daily gain and feed efficiency are given in Table 2. The rabbits on the four diets gained 27.1 to 28.5 g day-1. The final live weight attained during 6 weeks varied from 2471 to 2592 g. The values of daily gain recorded in the present study were similar to that of Tripathi et al. (2003) obtained with Soviet Chinchilla rabbits fed mustard (Brassica juncia) meal. The lowest final live weight (2471 g) and daily gain (27.1 g) were obtained from the rabbits fed on 21% RSM diet (Table 2). This result agrees with those of Esonu et al. (1996) who reported that the inclusion of 20% raw bean (Canavilia ensiformis) in the rabbit diets reduced the weight gain but cooking improved the weight gain. The inclusion of RSM did not affect feed intake and feed efficiency although rabbits on the control diet had the best feed efficiency (4.6) while the poorest feed efficiency (4.8) was obtained from the rabbits fed on 21% RSM diet. Similarly, Berchiche et al. (1995) found that feed consumption and feed efficiency were not different among group of rabbits receiving SBM (control), or faba bean meal with lucerne, wheat bran and barley, or sun flower oil meal.
The RSM level in the diet affected carcass yield and gastrointestinal tract (Table 2). Average carcass yields were 65.2, 65.3, 64.2 and 63.0% in D1, D2, D3 and D4, respectively. These values were slightly higher than that reported by Cobos et al. (1995) with New Zealand White x California growing rabbits, but much higher than those found in other experiments (Yalçin et al., 2001, 2003). This difference may be due to the dependency of carcass yield on breed, age and the number of internal organs left with the carcass of the rabbit at slaughter.
According to Tripathi et al. (2001), the presence of dietary glucosinolate increases peripheral fat content and induces metabolic disorders such as liver hypertrophy. In this study, there were no significant differences among the groups of rabbits neither for liver nor for the peripheral fat proportions. Also, there was no effect of replacing SBM with RSM on mortality. This could indicate that glucosinolate level in the tested diets is still within a tolerable level.
In conclusion, the inclusion of 21% RSM in rabbit diet had no adverse effects on animal health but resulted in lower DMD, OMD. Daily gain and carcass yield were also reduced but remained acceptable. However, the inclusion of 14% RSM gave a similar performance to that for the control diet (containing 21% of SBM). It is therefore, possible to reduce the use of expensive feedstuffs such as SBM for growing rabbits by replacing part of the diet with RSM. This will reduce the overall cost of feeding.
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