Effects of mannan oligosaccharides on growth performance, nutrient digestibility, ruminal fermentation and hematological parameters in sheep

Schematic overview of the experimental program

The experiments included sheep feeding trial and samples laboratory analysis. The sheep were fed for 7 weeks, the feed intake and growth performance were recorded, and the feces, urine, ruminal fluid, and blood samples were collected during the feeding trial. The blood samples were analyzed by a fully automatic hematology analyzer immediately after collecting. Then, the other samples were taken back to laboratory to analyze the nutrients concentration and ruminal fermentation parameters (Fig. 1).

Experimental design, animals, and housing

A single-factor design was used for this experiment. Four different doses of MOS (SCIPHAR^®; Sciphar, Inc., Xi’an, Shaanxi, P. R. China) were tested: 0%, 0.8%, 1.6% and 2.4% of basal diet (on an as-fed basis). Ninety-six healthy Hu rams with similar body weights (31.11 ± 2.69 kg) were chosen and divided into four treatment groups. Each group had twenty-four rams. The test period included a 2-week acclimation period and a 5-week feeding trial (including a 6-day digestion and metabolism experiment during the fourth week). All rams were loose-housed in 10 m² barns and 20 m² yards according to experimental design. Each loose-housing system was equipped with a feeder and a drinker, providing ad libitum access to feed and water.

During the feeding trial, the weights of feed intake and residues were carefully recorded daily in each replicate, and body weights were recorded weekly before the morning feeding.

Experimental diets

Four isoenergetic and isonitrogenous diets were formulated to meet or exceed nutrient recommendations for rams according to the China Agricultural Industry Standard (NY/T816-2004). Feedstuff’s compositions were adjusted to create four experimental diets containing different levels of MOS but the same nutrient compositions (Table 1).

Sample collection and analysis

Statistical analysis

Because the single-factor experimental design was used in this experiment, the data were analyzed by one-way ANOVA (SPSS 19.0; IBM Co., Chicago, IL, USA) using the following model:

X_ij = μ + α_i + e_ij

where X_ij is the observation of the dependent variable (i = 1 to 4, j = 1 to 6), μ is the population mean, α_i is the i^th treatment effect, and e_ij is the random error associated with the observation.

Significance was declared at P ≤ 0.05 and tendency at 0.05 < P ≤ 0.10 using Tukey’s multiple comparison test.

Effects of different doses MOS on sheep growth performance

There were no differences in final body weight, average daily weight gain, or average daily feed intake between treatment groups (P > 0.05, Table 2).

Effects of different doses MOS on nutrient apparent digestibility and retention rate

The DM and OM apparent digestibility of sheep fed the 1.6% MOS diet were higher than those in sheep fed the 0% MOS diet (P = 0.010, P = 0.016, Table 3), and their NDF and ADF apparent digestibility were higher than those in the other treatment groups (P = 0.003, P = 0.028, P = 0.001; P = 0.001, P = 0.029, P < 0.001). In addition, the CP apparent digestibility of sheep fed the 1.6% MOS diet was higher than those in sheep fed the 0% and 0.8% MOS diet (P = 0.040, P = 0.044, Table 4). There were no differences in the apparent digestibility of other nutrients or in the retention rates of sheep between treatment groups (P > 0.05).

Effects of different doses MOS on ruminal fluid and hematological parameters

Supplementation dietary MOS did not affect SCFAs concentration, ratios of individual fatty acids to total SCFAs, or C2/C3 ratios (P > 0.05); however, the ammonia concentration (NH₃-N) of ruminal fluid from sheep fed the 1.6% and 2.4% MOS diets were lower than that of sheep fed the 0% MOS diet (P = 0.035, P = 0.013, Table 5).

Similarly, supplementation dietary MOS did not affect the hematological parameters of sheep (P > 0.05), and there was only a tendency regarding MOS increasing the MCHC concentration in blood (P = 0.068, Table 6).

Effects of different doses MOS on sheep growth performance

Many studies have concluded that supplementation dietary MOS improve the performance of monogastric animals. Supplementation dietary MOS improved growth performance and gut health in broiler chickens (Soumeh et al., 2019), and increased feed utilization and amino acid digestibility in White Pekin ducks (Park, Jung & Carey, 2019), as well as improved growth performance and modulation of intestinal microbial populations in Japanese quail (Hazrati, Rezaeipour & Asadzadeh, 2019). Supplementation dietary MOS maintained the normal intestinal health of rats (Yazbeck et al., 2019), and effectively replaced antibiotics as growth promoters in raising guinea pigs (Minguez, Ingresa-Capaccioni & Calvo, 2019). Additionally, supplementation dietary MOS improved the growth performance, antioxidant capacity and immunity of aquatic animals (Lu et al., 2019; Meng et al., 2019; Mohammadian et al., 2019; Widanarni et al., 2019). However, the studies focused on MOS in ruminants have drawn inconsistent conclusions. Westland et al. (2017) reported that supplementation dietary MOS increased the colostrum yield of dairy cows, but had no effect on calf health and weight gain. Da Silva, Bittar & Ferreira (2012) observed that supplementation dietary MOS did not improve calf performance when added to a milk replacer or starter concentrate. Morrison, Dawson & Carson (2010) reported no effects on calf performance, even though supplementation with MOS increased concentrate intake in the early lives of calves. Similarly, supplementation dietary MOS have been shown not to influence body weight gain in lambs (Demirel et al., 2007). Consistently with some of these previous studies, supplementation dietary MOS did not impact average feed intake or average daily weight gain of sheep in the current study (refer to Table 2). This may be because ruminal microbes degrade MOS in the rumen (Dai et al., 2015; Wang et al., 2019), and then supplementation dietary MOS could regulate the composition of ruminal microbiota and chyme, even changed ruminal fermentation type and nutrients transfer in small intestine (Diaz et al., 2018) but the efficiency was not enough to change growth performance. This indicates that MOS are suitable for nutritional regulation purposes in adult ruminants, but not for growth performance enhancement in sheep.

Effects of different doses MOS on nutrient apparent digestibility and retention rate

In this study, the DM, OM, NDF, ADF, CP, and ash apparent digestibility of sheep were significantly increased through supplementation with MOS, especially at a dose of 1.6% (refer to Tables 3 and 4). MOS are a type of carbohydrate that can be degraded by ruminal microbes (Dai et al., 2015; Wang et al., 2019). Indeed, supplementation dietary MOS has potential to maintain stable ruminal environment by regulating ruminal microbiota, then the composition of chyme from rumen and to duodenum changed, consequently, some oligosaccharides went to small intestine because of MOS not degraded 100% in rumen and nutrients digestion and absorption in small intestine regulated by MOS (Diaz et al., 2018). Because supplementation dietary MOS brought long villi and shallow crypts which provided a larger surface area for nutrients absorption in small intestine of animals (Chacher et al., 2017), and an our previous study also demonstrated that supplementation dietary MOS elongated the villus height and the muscular thickness, and decreased the villus width of lamb duodenum significantly (Zheng et al., 2020), so, the monitoring results about supplementation dietary MOS improved nutrient digestion of sheep in the current study were observed. And some previous studies have reported similar results, with MOS increasing the digestion of NDF and ADF (Zheng et al., 2018), and nitrogen retention in sheep (Zheng et al., 2018; Zheng et al., 2019), as well as improving the nitrogen, zinc, and iron metabolism of calves and lambs (Cole, Purdy & Hutcheson, 1992). However, another study reported that MOS did not affect nutrient digestibility in dairy cows (Moallem et al., 2009). Thus, future research confirming the effects of MOS on nutrient utilization by ruminants is warranted.

Effects of different doses MOS on ruminal fluid and hematological parameters

In the current study, MOS did not change the SCFAs concentration or ratios of individual fatty acids to total SCFAs; however, it significantly decreased the ruminal ammonia concentration (refer to Table 5). In the rumen, lower ammonia level indicates active microbial proliferation, and microbial protein synthesis and nitrogen metabolism are improved (Diaz et al., 2018). As a result, nitrogen utilization by the sheep improved, which is consistent with our observations of nutrient utilization. Previous reports have indicated that supplementation dietary MOS improve the ruminal microbial composition and ammonia levels: the supplementation of a high-grain diet with MOS decreased ruminal ammonia concentration in sheep (Diaz et al., 2018) and slightly reduced ruminal ammonia concentration in sheep rumens 1–5 h after feeding (Zheng et al., 2018); live yeast supplementation also reduced ruminal ammonia concentration in dairy cows (Moallem et al., 2009). Other oligosaccharides have shown similar properties: the addition of chitosan to a 50:50 concentrate:forage diet decreased ruminal ammonia concentration in sheep (Goiri, Oregui & Garcia-Rodriguez, 2010), and the supplementation with β 1-4 galacto-oligosaccharides also reduced ruminal ammonia concentration in sheep (Mwenya et al., 2004). In addition, although ruminal SCFAs concentration was not influenced by MOS significantly, but supplementation dietary MOS decreased SCFAs concentration. This may be supplementation dietary MOS could increase SCFAs concentration (Diaz et al., 2018), and then more SCFAs could stimulate the expression of transport proteins like monocarboxylate transporter 1 (MCT1) and Na⁺/H⁺ exchangers (NHEs) and free fatty acid receptors (FFARs), consequently, more SCFAs were absorbed by ruminal epithelium (Baaske et al., 2020). As a result, the decrease of SCFAs concentration in sheep rumen was monitored in the current study because the ruminal fluid was sampled 3 h after feeding. These results illuminate the potential of MOS to improve ruminal fermentation and nitrogen utilization in ruminants, but the further research is guaranteed to confirm the actual effects of MOS regulating rumen fermentation.

Hematological parameters are typically auxiliary indexes used to monitor health conditions or metabolic processes of animals. When used as an animal feed additive, supplementation dietary MOS improves digestion, the immune system, and the microbiota of the gastrointestinal tract. However, because their effects on the animal body are slight, basal hematological parameters are not greatly influenced by MOS. In the present study, supplementation dietary MOS did not influence the hematological parameters of sheep (refer to Table 6). Similarly, supplementation dietary MOS did not affect the WBC count, HCT, neutrophil count, mononuclear leukocyte count, or eosinophil count of dairy cows or their offspring (Franklin et al., 2005). Other studies on the effects of MOS in monogastric animals have reported similar results: supplementation dietary MOS did not impact RBC count, hemoglobin, HCT, or MCHC of weanling pigs in one study (Valpotić et al., 2017; Dos Anjos et al., 2019), or RBC count, WBC count, lymphocyte count, or diarrhea score in another study (Zhao, Jung & Kim, 2012), and did not influence monocyte, basophil, or eosinophil counts in Arbor Acres broiler chickens (Attia et al., 2017).