The host and environment both influence the microorganisms of the cecum, and environmental factors are more significant than the host [23, 24]. The FRS and NRS are two main methods of intensive farming of ducks. The FRS is the most primitive method of duck farming in China due to its low cost and high meat quality. However, FRS require a particular rearing area and frequent replacement of cushions, which are still challenges, and diseases occur easily in FRS due to direct contact with feces. Currently, most farmers build grid structures approximately 60 cm above the ground and lay metal nets to remove excreta, but the cost is relatively high, and the cleaning and disinfection of the nets are inconvenient. In this study, we reared ducks with the same density, intake and drink under unified management in both an FRS and NRS to determine the differences in the intestinal growth and microorganisms of the cecum.
The growth of the small intestine in the outdoor environment, such as grazing and artificial grassland, was significantly higher than that in the cage [21, 25], and a similar situation appeared in this experiment. Both intestinal relative length, which can reflect the intestinal capacity, and the ratio of relative weight to relative length, which could reflects intestinal motility, were higher in FRS, suggesting that ducks reared in floor system have stronger intestinal peristalsis ability and larger food digestion areas, and it may result from increased activity in a swimming pool. Many reports have shown that the length and weight of poultry intestines are affected by the level of fiber, for example, dietary sunflower hulls could increase the length of the small intestine of broilers . Considering the specific situation of this experiment, it may be because in addition to artificial feeding, ducks in the FRS also consume mattresses on the ground and algae in ponds, which increase their fiber levels. Therefore, it can be concluded that FRSs is more conducive to duck intestinal growth than NRSs.
The comprehensive characterization of duck intestinal microbial communities is a critical precondition to understand and predict how rearing systems alter these communities. In order to further explore the differences between the FRS and NRS, the sequences of the cecum content were detected. Although there are no specific microorganisms in cecum of ducks in both systems at genus level, the diversity showed a significant difference at 13 weeks. In general, the diversity of intestinal microbial composition of poultry gradually increases with the increase of age after birth. All kinds of microorganisms rise and fall one after another, and then tend to a relatively stable state in youth. Therefore, the difference in diversity of duck intestinal microorganism in this study is convincing. Intestinal microorganisms were affected by many factors, including age, gender and environment[27–29]. Different rearing systems provided different growth environment for ducks, which made the diversity in ground-based ducks higher, and the result is consistent with that of Dagu chicken . Considering that long-term stress could reduce the diversity of intestinal microbiota , this result may be because the ducks were raised on a net and unable to contact the natural environment. Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria were dominant in the cecum in both systems, and this result coincides with wild turkeys, captive broilers, caged Beijing ducks and floor-raised Landes geese [31–34].
The environmental adaptability pathways of the microorganisms in the cecum of ducks in the FRS at 4 weeks were higher, and the metabolic-related pathways were lower, including xenobiotic biodegradation and metabolism, amino acid metabolism and lipid metabolism, suggesting that the change in the living environment after the brooding period caused stress. The abundance of most pathways related to diseases, including cardiovascular disease pathways, substance dependence and viral infectious diseases, in the NRS was higher than that in the FRS at 4, 8 or 13 weeks, and these diseases cause serious harm to the body, which is consistent with previous studies [35, 36].. Of the 8 different metabolic pathways at 8 weeks, the abundance of 5 of them were greater in the FRS than in the NRS, suggesting that ducks in the FRS had adapted to the environment and needed more substance, improving the meat quality. Studies have shown that the meat of outdoor chickens is darker and has a better water-holding capacity [37, 38]. In addition, of the 25 functional pathways at 8 weeks, 16 pathways appeared at 4 weeks, and their abundances changed between 4 weeks and 8 weeks. Only the cofactors and vitamin metabolic pathways in the FRS were more abundant due to the higher abundance of Bacteroides and Ruminococcaceae-uncultured bacteria, while the abundance of the translation pathway in the NRS was higher at 13 weeks due to the presence of Subdoligranulum and Brachyspira, implying that the differences in the functional pathways of the microorganisms in the cecum of ducks in the FRS and NRS gradually decreased with time. As discussed before, the colonization of intestinal microflora is a process that changes with age, thus the differential metabolic pathways and microorganisms are also a gradually stable process. When ducks enter the youth period and the intestinal environment is relatively stable, the influence of FRS and NRS on duck intestinal development and microorganisms can be revealed.