One hundred and twenty 2-month-old Rex rabbits (60 males and 60 females) were randomly divided into four groups (n = 30, half male and half female), with two rabbits as replicate. The rabbits were housed in pairs in a wire cage measuring 60 cm×45 cm×40 cm with bamboo flooring. The rabbits were raised in a temperature range of 17°C to 24°C and under natural light conditions. They were fed pelleted diet ad libitum which 4 mm diameter, and water was available on demand from nipple drinkers. Fattening began at the age of two months, and they were reared for 44 days. During the experimental period, which lasted for 60 to 105 days of age, rabbits were inspected daily; no health problems were observed. The diet formula was prepared according to the dietary requirements by the Nutritional Research Council for rabbits (Table 1).

Before the start of the study, the rabbits had been housed for ten days without the provision of enrichment material. When the experiment started, three types of EE materials (WS, RD, and CB) were used in the enriched treatment groups (Figure 1). The RDs (Figure 1B) were hung at approximately 20 cm from the bottom of the cages, while the WSs (Figure 1A) and CBs (Figure 1C) were placed at the base of the cage. The beans content of the can made series of sounds when touched by the rabbit.

The body weights were measured every week. Feed intake (FI) was recorded throughout the experiment. The feed conversion ratio (FCR) and average daily gain (ADG) were calculated according to the procedure described by Jordan [8]. The methods for measuring body length, chest depth, chest circumference (heart girth), dorsum length (from the cervical part to the base of the tail), hind leg, and rump circumference were based on those proposed by Mcnitt et al [13,16]. The body index and chest depth index were calculated using the following equations:

Feed and water were withdrawn for 24 h before slaughter. The rabbits were stunned, slaughtered, immediately eviscerated, and then skinned. The carcass (without the head, legs, tail, viscera, fat deposits and blood), skin, liver, heart, kidney, and spleen were weighed. Dressing out percentage was calculated as follows:

The contents of both the stomach and cecum were removed before weighing. The skin area (cm²) was determined using the following equation:

Lightness (L*), redness (a*), and yellowness (b*) values were measured in the left thigh muscles and buttock parts within 40 min post-mortem using a chromameter (Minolta Chroma Meter, Hamburg, Germany) operating under D65 illuminant and 10° observer angle. Each part was measured three times, and the average values were calculated [17]. The Commission Internationale de l’Eclairage (CIE) L* refers to the measure of lightness, the larger the number, the lighter the color. CIE a* indicates a red (+) to green (−) color scale, and CIE b* denotes the yellow (+) to blue (−) color scale. The hue angle (Hue) which defines color (0°-red; 90°-yellow), was calculated as an arctangent (b*/a*). Chroma denotes the color intensity (0-dull; 60-vivid), and computed as follow:

Meat tenderness value was measured from the hind leg using a meat tenderness meter (TENOVO C-LM3, Beijing, China). The pH value of the hind leg was determined using a pH meter (SuYuan PHS-3C, Shanghai, China) within 45 min post-mortem.

Skin samples measuring 2 cm in diameter, were collected from the center of the buttocks of the experimental rabbits. After rinsing with normal saline, the samples were immediately soaked in a paraformaldehyde fixative solution.

The samples were dehydrated with alcohol gradient, embedded in paraffin, and sliced in 6 μm sections after 48 h, stained with hematoxylin and eosin and observed under a digital microscope (Moti, BA210, Xiamen, China). Seven images (magnifcation of 40×) of different view were obtained for each section, and the HF diameter and density were examined using the Motic Images Plus 2.0 system. The diameters of the PF and SF were measured, and the number of PFs and SFs were counted in each image.

Data were analyzed using the SPSS 20 software. Seven images were obtained for hair follicle numbers in each section. They were obtained repeatedly and analyzed by repeated measures analysis of variance (ANOVA). Other data were evaluated by one-way ANOVA. In the study, the Turkey's test was used to determine the significant difference, and p<0.05 was defined as significant.

The four experimental groups produced no mortality. The slaughter weights (final live body weight) of the enriched cage groups were higher than those of the control group. The ADG of the CB group was the highest at 25.63 g, significantly higher than that of the control group (p<0.05), and the ADG of the enriched groups was also higher than that of the control group. The lowest FCR was observed in the CB group, and the WS and RD groups had lower FCR values than the control group. The body length of the control group was higher than that of the WS group. The chest circumference of the CB group was the highest. The highest body index (77.22%) was also found in the CB group. The chest depth index, rear leg and rump circumference, and body index of the control group were lower than those of the enriched cage groups (Table 2).

The carcass weight and dressing out percentage of the control group were the lowest (1,414 g and 54.93%, respectively). The skin area of the CB group was the largest, while the WS and RD group had greater skin area than the control group. The stomach weight of the control group was the greatest and was significantly greater than those of the WS and RD groups (p<0.05). The spleen weight of the CB group was significantly greater than those of the other groups (p<0.05). The cecum weight of the CB group was significantly greater than those of the WS and control groups (p<0.05) (Table 3). The EE exerted no significant effect on the liver, heart, and kidney weights and the small intestine lengths of the four groups.

CIE L* and CIE a* of the control group were the lowest and the CIE a* of the RD group was the highest (p<0.05).The Hue value of the RD group was significantly lower compare to the rabbits raised without EE (Hue = 2.42, p<0.05). The meat sample of the CB group was the most tender (5.72 N, p<0.05). No significant effect was found in the other measured parameters (Table 4).

The PF diameters (p<0.05) and SF diameters (p<0.05) of the WS and CB groups were lower than those of the control and RD groups. More SFs (p<0.05) and PFs (p<0.05) were also found in the CB group than in the other groups (Table 5).

The rabbits in the enriched cages had higher slaughter weights and FI than those of the control group (non-significant). The slaughter weight results were in agreement with those found by Maertens et al [19], Luzi et al [9], Princz et al [20], and Mohammed and Nasir [11], who reported that EE exerts a positive effects on the weight gain of rabbits. However, the results of the current work are contrary to those obtained by Jordan et al [8–10,21] who reported that EE did not have effect on the slaughter weight. They used gnawing sticks, wood, bamboo as EE for fattened or weaned rabbits. Such discrepancy may be attributed to the different enrichment materials and growth periods of rabbits. The enriched cage groups had lower FCR, and higher ADG (p<0.05) than those of the control group, and this supports the findings of Mohammed and Nasir [11]. This result could be attributed to the amount of time rabbits spend playing and gnawing. Gnawing is associated with digestive mechanisms and the nervous system, which could decrease stress and promote intestinal flow, digestion and overall animal health [21,22]. Decreasing stress might stimulate the appetite, increase the digestibility of dietary components, enhance the digestion of dry matter and energy release, influence hormones and the enzymes released, and consequently, improve the process of digestion [21,23]. In this study, the CB group had the highest ADG and the lowest FCR among all groups. This result may be attributed to the fact that the appropriate sound of cans could decrease stress by inducing playing behavior [24]. The CB exerted a considerable impact on the productive performance of the rabbits. In this study, the FCR in the RD group was higher than those in the other groups. This result may be attributed to the fact that the RD is yellow and soft and hangs in the cage. The rabbits are sensitive to the yellow color [25], and soft materials are easy for gnawing. The RD hangs in the cage, and sway when touched it. These characteristics made this enrichment material attractive to the rabbits and led to increased gnawing and playing, relative to the other groups. None of the enrichment materials showed harmful effects on the rabbits during the entire experiment.

In accordance with other studies involving the use of EE for rabbits, similar carcass trait performances with and without enrichment were obtained [9,20]. The carcass weight was higher in the enriched cage groups than in the control group. The RD group showed the highest carcass weight, which may be due to their heavier slaughter weights. The dressing-out percentage in this study was numerically higher (non-significant) in the enrichment groups than in the control group; this outcome is in agreement with the results of Kermauner et al [21] used wooden sticks for cage enrichment. The largest skin area was observed in the CB group, possibly because the chest circumference and chest depth of the CB group were larger than those in the other groups. The fur value of Rex rabbits depends on their hair quality and skin area. A large skin area is deemed highly valuable. Cage enrichment could increase the skin area, and this result may be related to the higher slaughter weight.

In this study, no significant difference was observed in the other carcass traits (the heart, liver, kidney, and small intestine) of the groups with and without EE. This outcome is compatible with the results of Kermauner et al [21], Princz et al [10], Jordan et al [8], and Mohammed and Nasir [11]. The spleen and cecum weight of the CB group was significantly greater than that of the control group (p<0.05). There are no previous reports on the effect of EE on the spleen; therefore, we currently have no reasonable explanation for this result. Rabbits are true non-ruminant herbivores with a particular type of digestion called cecum fermentation [26]. The cecum is an important and large digestive organ in rabbits and is assumed to have a similar function as the rumen in cattle. It contains a rich population of bacterial that could aid digestion through fermentation and the synthesis of B vitamins. Additionally, the soft feces of rabbits are produced in the cecum, that is significantly for rabbits’ digestion. The weight of a rabbit’s cecum can reflect its developmental condition. In the current study, the daily gain of the CB group was the highest, and the FCR was the lowest, meaning that the rabbits in the CB group had better feed utilization, possibly because of the well-developed cecum.

Cage enrichment did not influence the pH, CIE L*, CIE b*, and Chroma; however, it increased the CIE a* value and decreased the hue value. Meat redness mainly depends on the activities of rabbits, with more activity resulting in a darker muscle color [27]. In this study, the rabbits in the enriched cages played with the enrichment materials and showed much activity. The RD is colorful, soft, and hangs in the cage, these characteristics made it attractive to the rabbits and led to increased gnawing and playing, relative to the other groups. This may explain the dark meat color of the enriched groups. It is in agreement with the result of Jordan et al [8], who added wooden sticks to rabbit cages. In contrast, Luzi et al [9] and Kermauner et al [21] registered higher CIE a* in animals without wooden stick. However, they did not consider the hue; therefore, they could not determine whether or not the meat of rabbits in conventional cages (without enrichment) is redder than that of rabbits in enriched cages. In our study, the hue value of the RD group was significantly lower than that of the control group (2.42, p<0.05), indicating that the meat color of the CB group was the best. Furthermore, the meat of the CB group showed the most tenderness. Therefore, improving meat quality by increasing the richness of the caged environment is beneficial. Overall, CB had a great positive effect on the meat quality of rabbits.

The fur quality of Rex rabbits depends largely on hair density [28]. Stress may cause hair loss and inhibit hair growth by activating the substance P dependence of macrophages or mast cells in the context of brain-hair follicle axis [29], which indicates that moods can affect hair growth. In this study, we found that the SF population of the CB group was greater than those of the other groups. Throughout the experiment, the rabbits in the CB group spent most of their time playing with the sound-producing cans. A common effect of EE is to reduce stress, therefore, this result indicates that CBs might have contributed to providing a less stressful environment and exerted a positive effect on the SF growth. Allison et al [30] reported that providing an EE material to C57BL/6J mice can delay the onset of alopecia and reduce its prevalence and overall severity. This result is similar to our findings and may indicate that husbandry methods promote the well-being of the animals and also improve their hair density. In this study, the PF density did not changed possibly because no additional PFs are formed after birth, although the size of hair follicles and strands can change slightly over time under the influence of androgens [28]. Intervention in postnatal mammalian HF had a relatively significant effect on SF only, thus in the present study, we only found that EE could affect the SF density. SF is an important micro-organ that grows the hair of underfur, therefore, the SF density could affect the underfur density and an improved SF density has a great effect on fur quality. From the study, we predicted that EE could be used to increase the SF density in Rex rabbits and among the three experimental EE materials we recommend the use of CB because it had the highest positive effect on the SF density.