Research On Animal Production

Extended Abstract
Background: The domestic rabbit (Oryctolagus cuniculus) is an animal of the phylum Chordata, class Mammalia, order Leporidae, and family Leporidae. Its original origin is in Western Europe and Northwest Africa, and different breeds are used for various purposes, including producing wool, skin, and meat, conducting biological research, and as pets. The rabbit is intermediate between ruminants and monogastric animals and can effectively use cellulose-rich foods with a diet containing less than 20% grain. Rabbits have a short reproductive cycle and high fertility. The complex microbial population of the rabbit's digestive tract plays an important role in food digestion, vitamin production, fermentative activity with the production of volatile fatty acids, and stimulation of the immune response. Many probiotics, such as Lactobacillus, Protoxin (multi-strain probiotic), and Saccharomyces cerevisiae, are used in animal diets. Among various yeast species, S. cerevisiae has been widely used as a probiotic supplement in animal production due to its abundant content of proteins, polysaccharides, small peptides, amino acids, vitamins, trace elements, nucleotides, and other growth factors. Probiotics improve feed conversion ratio and weight gain, reduce mortality, reduce disease infection, and stimulate the immune system. Prebiotics literally mean prerequisite for life, and are indigestible or poorly digestible food elements that have beneficial effects on health by stimulating the growth or activity of a limited number of probiotic bacteria in the large intestine. The limitation of using antibiotics has led to efforts to find a suitable alternative to them. Today, organic acids, probiotics, prebiotics, plant extracts, and also synbiotics, which are mixtures of probiotics and prebiotics, are used as alternatives. Therefore, the present study aimed to compare the effects of probiotics, prebiotics, and synbiotics on the performance of carcass characteristics, meat color measurements, and intestinal microbial population in growing rabbits. 
Methods: In this study, 32 weaned rabbits (56 days old, body weight 450 ± 50 g) were selected for the experiment. The rabbits were individually assigned to four treatments with eight replications in a completely randomized design. Before the start of the experiment, the rabbit housing room, feeders, and waterers were thoroughly washed, and then the walls and floor of the room were disinfected with lime water. To combat internal parasites, all rabbits were injected subcutaneously with 0.5 ml of ivermectin per rabbit head. The experimental diet included a basal diet (control), a basal diet with a commercial probiotic level (0.02%), a basal diet with a commercial prebiotic level (0.2%), and a basal synbiotic diet. The experimental diets were prepared according to the average age of the rabbits and based on the NRC standard tables (1977). The diets were completely mixed and given to the rabbits in a single morning meal of 200 g, and the remaining feed was collected and weighed from the feeder every morning before giving new feed. At the end of the experiment, four rabbits from each treatment were slaughtered to determine the carcass characteristics and perform a microbial culture of intestinal contents. Colorimetric tests of raw rabbit meat samples were performed using a Tintometer (model CAM-System 500 USER MANUAL, manufactured by Tintometer GmbH, Germany) on the first and third days and after one month of refrigeration. Color indices, including L* (lightness), a* (redness), and b* (yellowness), were determined for the meat samples. The skin, head, thighs, hands, whole carcass, digestive tract, liver, heart, spleen, and kidney were weighed to evaluate quantitative carcass characteristics. The intestinal contents were completely removed, and 1 g of feces was sampled for microbial culture. The samples were placed in tubes, and 9 times the weight of the samples was added with a sterile diluent solution. 
Results: The addition of growth-promoting additives did not significantly affect the measured carcass characteristics, except for the weight of the kidney, digestive system, and heart of experimental rabbits. Synbiotics non-significantly increased the amount of aerobic bacteria. Moreover, synbiotics significantly reduced the total population of Coliforms. In addition, synbiotics significantly increased the amount of lactic acid bacteria. Color indices, including factors L* (lightness), a* (redness), and b* (yellowness), were not significantly different on the first day. the factors L*, a*, and b* decreased significantly during the sample refrigeration period. 
Conclusion: The results of the present experiment show that probiotic, prebiotic, and synbiotic supplements can be substituted as natural growth stimulants in the diet of antibiotic-free rabbits. Furthermore, the use of probiotic, prebiotic, and synbiotic supplements during the fattening period improves carcass traits, increases the beneficial microbial population, and improves the quality of rabbit meat.