Production performance is one of the major concerns in poultry production as it is closely associated with the profits of animal products, while increasing focus on poultry health status, a reflection of physiological status, is not only due to its impact on production performance but also because of the raising attention to animal welfare issues. In this study, we compared the effects of FRS and NRS on production performance and health status of Nonghua ducks at different ages. Based on the higher final BW, ADG and lower FCR of waterfowls in NRS, studies on Yangzhou geese and Chaohu ducks reported waterfowls in NRS exhibited better growth performance than in FRS [9, 24]. Similarly, in Moulard ducks, though FCR was not significantly affected, the higher final BW and ADG in NRS in comparison to sawdust-FRS and sand-FRS also revealed that NRS resulted in a better growth performance . However, with a lower ADG but a higher FCR of Cherry Valley ducks in NRS, Chen et al.  reached an opposite conclusion of ducks in FRS exhibited better growth performance. These inconsistent conclusions may result from different breeds and slaughtering ages. To clarify the effects of FRS and NRS on meat ducks in different breeding stages, the current study selected three key weeks of age to study the interactions between RS and age. Results showed that BW and ADG were higher in NRS at 8w while no significant results were found at 4w and 13w, indicating a better growth performance of NRS ducks before the market age.
As for carcass traits, the average CY, EY and SEY of Nonghua ducks at 8w and 13w under either FRS or NRS were higher than 85%, 75% and 80%, indicating outstanding carcass traits compared with Pekin ducks, Cherry Valley ducks, White Muscovy ducks and Jingjiang ducks (a native Sheldrake breed in China) at similar ages [26–29]. Previous study on Chaohu ducks noticed the lower EY but higher AFY in NRS . Similar to the current study, results showed that CY of NRS ducks was lower than FRS ducks at 4w and 8w, while CW, EW, SEW, SFW, AFW, SFY, and AFY were higher in NRS at 8w, SFY and AFY were also higher in NRS at 13w, indicating that NRS was conducive to carcass traits to some extent. With the age increasing, the majority of significances in carcass traits faded, but the lipid deposition was enhanced consistently in NRS. This was in agreement with Liu et al. , who reported Yangzhou geese in NRS exhibited higher subcutaneous fat thickness and higher AFY than in FRS. The visceral organ development is closely associated with duck growth, development, and health . Results showed GW and GI were generally lower in NRS at different ages, indicated that FRS ducks had better gizzard development than NRS ducks. This was in agreement with Wang et al. , who also found a better gizzard development of birds in FRS compared with CRS and NRS. The reason can be concluded as NRS ducks cannot intake any grain of sand from the floor, because adding large particles and structural components to diets could stimulate poultry gizzard function and development [31, 32]. Moreover, LW and LI of NRS ducks were lower than FRS ducks at 4w, indicating a liver developmental retardation at the early stage of breeding in NRS. Therefore, NRS improved the carcass weight before the market age but had defects in carcass yield. FRS ducks exhibited better gizzard and liver development at the early stage of breeding, while NRS enhanced the lipid deposition at the later one.
Since the liver is the major site of metabolism and detoxification, this retardation in NRS ducks may influence the serum biochemical parameters of Nonghua ducks. Serum biochemical parameters of AST, ALT, and ALP were biomarkers to evaluate the health status of liver, because hepatocytes would secrete these enzymes into the blood once the liver was damaged [33, 34]. Results showed these parameters were coherently lower in NRS ducks at different ages, demonstrating NRS was better for duck liver health despite the lipid deposition enhancement and liver developmental retardation. And this also showed an advantage over CRS, as ducks suffered liver injury once they were just put into cages . Furthermore, antioxidant capacity prevents cell damage from free radicals and other reactive oxygen species (ROS) to avoid oxidative stress (the imbalance between ROS and antioxidants) [36, 37]. The concentration of MDA and activities of SOD, GSH-Px are effective indicators to reflect the antioxidant status of ducks [38, 39]. A study on Shaoxing ducks showed rearing systems did not have significant impacts on the activity of MDA, SOD, CAT, T-AOC, and GSH-Px in the liver . Another study though only determined the total antioxidant capacity (T-AOC) in serum, results showed no significant difference between FRS and NRS . Similarly in the current study, rearing system did not significantly affect MDA, SOD, and GSH-Px, however, it exhibited a trend of lower MDA content and higher activities of SOD and GSH-Px in NRS at 4w, which indicated that NRS ducks might have a better antioxidant capacity than FRS ducks at this early stage of breeding. The activity of SOD decreased in NRS at 8w and maintained relatively lower than FRS ducks at 13w, while GSH-Px activity was significantly higher in NRS and MDA content was also relatively higher than FRS ducks at 13w. These results implied GSH-Px was responsible for scavenging excess free radicals at the later stage of breeding.
As for immune function, Xi et al.  reported that there was no significant difference in immune organ indexes of Cherry Valley ducks between NRS and FRS. Differently in current study, NRS ducks were higher in TW at 8w as well as SW and SI at 13w, which indicated a promotion on immune organ development in NRS. Whereas, the decreased BFI in NRS ducks at 4w and 8w implied a faster degradation of bursa. Current study also showed a trend of higher TI at 4w and 8w though the results were not significant, which was similar to Zhao et al. , who reported NRS ducks had lower mortality rate and higher TI than FRS ducks. Previous study also reported IL-1β and IgG profiles of Shaoxing ducks were not significantly affected by NRS , while immune cytokines were extensively affected by RS in current study. Generally, the tendency of higher profiles of IFN-γ, IL-1β, IL-4, immunoglobulins in NRS ducks indicated a better immune status, IL-1  and IL-4  contribute to T helper type-2 (Th2) cell activity, whereas IFN-γ contributes to T helper type-1 (Th1) cell activity . Higher levels of IFN-γ, IL-1β, and IL-4 during the experiment suggested the activity of Th1 and Th2 cells may be promoted in NRS. The enlargement of thymus at 8w may be associated with escalated IL-1β because IL-1 can act as a growth factor for thymocytes . Faster degradation of bursa and lower IL-6 content suggested NRS ducks may have lower immunological requirements. This could be explained by the direct dropping of duck feces in NRS, which makes ducks less opportunity to contact with their excreta. Wang et al.  and Almeida et al.  reported that FRS had higher microbial contamination and higher concentrations of NH3 and CO2 in the air than NRS, hence the better air quality in NRS also helped reduce the burden on duck immune system. Moreover, the elevation of immunoglobulins in NRS may be associated with escalated IL-1β, due to its promotion on IL-2 to enhance the secretion of immunoglobulins. In terms of serum proteins, NRS ducks showed higher GLOB and TP but lower ALB at 13w, this was in agreement with the study on Chaohu ducks , which also reported serum TP content was improved by NRS in comparison to FRS. Serum proteins change can be concluded as follows: First, FRS ducks took more exercise during rearing, which would enhance protein synthesis causing TP loss in blood . Second, due to the enhancement of lipid deposition in NRS, ALB would be used for lipoprotein synthesis leading to the lower ALB serum profile in NRS . Third, higher levels of immunoglobulins in NRS contributed to the change of GLOB in serum, especially the consistent elevation in IgA, IgG, and IgM in NRS at 13w. Based on the liver functions of synthesizing serum proteins including albumins , a coherent trend of lower ALB expression in NRS ducks at different ages might be responsible for the significantly lower serum ALB profiles at 13w. There was no significant change in SOD1, indicating this antioxidant was not significant in the change of two different rearing systems. IFN-γ is a pro-inflammatory cytokine, which participated in multiple inflammatory responses . Hence, the higher expression of IFN-γ as well as ALP in 13w-FRS than 4w-FRS and 8w-FRS indicated a potential more intense environmental stress in liver at the later stage of breeding in FRS. However, the specific regulatory mechanism needs further study to elucidate. Therefore, NRS could improve duck immune function by increasing the levels of certain immune cytokines profiles and promoting the development of immune organs, while liver gene expression indicated that ducks might suffer from a potential higher environmental stress in FRS.