Effects of Compound Lactic Acid Bacteria Additives on the Quality of Oat and Common Vetch Silage in the Northwest Sichuan Plateau

Abstract

The objective of this experiment was to determine whether compound microbial inoculants could enhance the fermentation of oat and common vetch silage that were stored in the Northwest Sichuan Plateau for 60 days under extremely low temperatures. Oat and common vetch harvested from single and mixed artificially planted grassland of oat and common vetch were chopped into 2–3 cm (oat, S1; common vetch, S2; oat–common vetch = 2:1, S3), then sterile water (T1), Zhuang Lemei IV silage additive (T2), and Fu Zhengxing silage additive (T3) were added to the feed and ensiled at the local outdoor environment for 60 days. Data were analyzed as a 3 × 3 factorial arrangement of treatments with the main effects of the materials, additives, and their interaction. Interactions between the materials and additives significantly affected the fermentation quality and the content of DM, WSC, and NDF and the number of yeasts in forages. Treatments with S3 have significantly higher contents of lactic acid, acetic acid, and lactic acid bacteria in the feed than those in the S1 and S2 treatments, while the contents of AN/TN and propionic acid were significantly lower compared with the S1 and S2 treatments (p < 0.05). Concentrations of lactic acid, acetic acid, and propionic acid were significantly increased and the content of neutral detergent fiber in the T2-treated silage decreased compared with the T1 treatment (p < 0.05). The T3 treatment significantly reduced the number of yeasts in the silage but the compound lactic acid bacteria additive treatment (T1, T2) significantly decreased the butyric acid content and pH of the feed and increased the acid detergent fiber content and the number of lactic acid bacteria in the feed compared with the T1 treatment. Among them, the butyric acid content of the T3 treatment decreased by 63.64–86.05%, while that of the T2 treatment decreased by 36.36–83.33% (p < 0.05). The comprehensive analysis of the membership function revealed that the silage quality was the best after the S3T2 treatment, so the implementation of the S3T2 combination in the Northwest Sichuan Plateau can provide guarantees for the production of local high-quality forage grass and alleviate the shortage of forage grass.

1. Introduction

The Northwest Sichuan Plateau is one of the five major pastoral areas in China, which is rich in grassland and livestock resources, and animal husbandry is the local leading industry [1]. However, long-term overgrazing has led to the serious deterioration of the grassland ecological environment in this area, the productivity of natural forage has decreased and the quality has deteriorated, and the contradiction between forage supply and demand has intensified, which has greatly restricted the development of the local animal husbandry economy [2]. Oat (Avena sativa L.) and common vetch (Vicia sativa L.) are the main forage grasses in the Northwest Sichuan Plateau, which have the characteristics of strong acceptability, high nutritional value, and good palatability [3,4]. Studies have shown that planting oat and common vetch mixed grassland at a ratio of 2:1 in plateau areas above 3000 m above sea level not only makes full use of natural resources such as land and climate, but also improves soil fertility, increases forage production, and enhances forage quality [5,6]. In recent years, the area of oat and common vetch artificial grassland planting in the Northwest Sichuan Plateau area has been expanding year by year, but the harvest time of oat and common vetch is in the rainy season, the local temperature is low, and it is very easy for the traditional hay preparation to cause forage mildew rot and nutrient loss [1]. Therefore, the selection of a forage processing method suitable for the Northwest Sichuan Plateau is the key link to producing high-quality forage and alleviating the contradiction between forage supply and demand. Silage, as a forage preparation technology with lactic acid bacteria fermentation as the core, can effectively prevent the decay and mildew of forage grass, improve its nutritional value, and prolong the silage period [7]. However, forage silage is a complex biochemical process, and microbial community succession and external factors can affect the fermentation effect of silage [8,9]. The high altitude, strong radiation, and extremely arid and cold climate of the Northwest Sichuan Plateau significantly affected the microbial community attached to the surface of forage, resulting in a decrease in microbial metabolic activity, slow growth of lactic acid bacteria, and growth dominance of undesirable microorganisms, which increased the risk of silage failure [10,11]. Therefore, it is imperative to seek a good solution to solve the difficult problem of the unstable quality of the forage of oat and common vetch.

Exogenous addition of lactic acid bacteria preparation is one of the important means to promote the rapid dominance of forage bacteria [12]. The increase in the number of lactic acid bacteria can significantly improve the quality of silage, but different lactic acid bacteria have different functions, and the effect of lactic acid bacteria additives may vary depending on the fermentation substrate; in addition, when different types of lactic acid bacteria are compounded together in a certain ratio, their synergistic effects also vary [13,14]. Previous studies have found that the addition of Lactobacillus buchneri alone or in combination with Lactobacillus plantarum to maize and sorghum silage can improve the aerobic stability of silage under low dry matter content conditions, but the addition of Lactobacillus buchneri in combination with Lactobacillus plantarum also reduces the content of ammoniacal nitrogen and fermentation losses compared with the addition of a single Lactobacillus buchneri. When Lactobacillus plantarum, Pediococcus pentosaceu, and Weissella cibaria were used to ferment Hemarthria compressa, the single or compound addition of Lactobacillus plantarum, Pediococcus pentosaceu, and Weissella cibaria could improve the quality of Hemarthria compressa silage, but when lactic acid bacteria were added in pairwise, the effect on silage quality was different. Among all the additives, the compound addition of three kinds of bacteria had the best effect [15]. When Fu et al. [16] fermented Sorghum bicolor × Sorghum sudanense with Lactobacillus plantarum, Pediococcus pentosaceu, and Lactobacillus brevis, it was found that the combination effect of Lactobacillus plantarum and Lactobacillus brevis was the best, and the effect of the combination of the three bacteria was worse than that of Lactobacillus plantarum alone or its compound with Lactobacillus brevis and Pediococcus pentosaceu. It can be seen that the effects of lactic acid bacteria additives can vary depending on the interactions between the bacteria and the fermentation substrate.

At present, there are few reports on compound lactic acid bacteria additives in the Plateau of Northwest Sichuan, and we do not know whether compound lactic acid bacteria additives can also improve forage quality in the special climate of the Northwest Sichuan Plateau. Based on this, by adding different compound lactic acid bacteria additives, the purpose of this study was to explore the effects of additives on the quality of oat and common vetch silage in the Northwest Sichuan Plateau, in order to select the most suitable compound lactic acid bacteria additives for oat and common vetch, and to provide a theoretical basis and practical guidance for the production of high-quality silage in the Northwest Sichuan Plateau.

2. Materials and Methods

2.1. Materials and Silage Additives

The seeds of oat and common vetch were supplied by Sichuan Nongken Muyuan Paradise Agriculture and Animal Husbandry Science and Technology Co (Ruoerge County, Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province, China). The oat (Avena sativa ‘Qinyin No. 3’) and the common vetch (Vicia sativa ‘Ximu 324’) were harvested from the Tangke town, Ruoergai country, Sichuan province (102°48′ E, 33°49′ N, altitude 3450 m) on 28 August 2021. When the oat was in the filling stage and the common vetch was at the podding period. Tangke Town has a plateau frigid–temperate climate, with sufficient sunshine, concentrated rainfall, large daily temperature difference, and no absolute frost-free period. The characteristics of silage raw materials are shown in Table S1.

The compound lactic acid bacteria additive Zhuang Lemei IV (Lactobacillus plantarum 550, Lactobacillus plantarum 360, Lactobacillus buchneri) was from Sichuan Gaofuji Biotechnology Co., Ltd. (Chengdu City, Sichuan Province, China), and Fu Zhengxing (two types of Lactobacillus corn, Lactobacillus casei) was from Sichuan Fu Zhengxing Biotechnology Co., Ltd. (Meishan City, Sichuan Province, China).

2.2. Silage Preparation and Treatments

The experiment was a two-factor completely randomized design. Three raw materials: S1, whole-plant oat silage; S2, whole-plant common vetch single; S3, oat–common vetch mixed silage (weight ratio of mixed sowing of oat and common vetch was 2:1). Two silage additives: the compound lactic acid bacteria additive Zhuang Lemei IV (T2), the compound lactic acid bacteria additive Fu Zhengxing (T3), and an equal volume of sterile water control (T1). The harvested oat and common vetch were cut into 2–3 cm, and the compound lactic acid bacteria preparations were inoculated at a rate of 5 × 105 cfu/g FW, and then evenly sprayed into each silage material. The mixed samples of 300 g were immediately vacuum-sealed in 30 cm × 40 cm single threaded polythene bags and a total of 27 silage samples (3 materials × 3 additives × 3 replicates) were stored in the local outdoor environment. After 60 days of fermentation, the silage quality, nutritional quality, and the number of microorganisms were determined.

2.3. Measurement Items and Analysis Methods

2.3.1. Chemical Compositions Analysis

Fresh samples and silage materials were de-enzymed at 105 °C for 30 min, then placed in a blast dryer at 65 °C and dried for 3 days to determine the dry matter (DM) content. The dried materials were stored after grinding and filtering with a 1.0 mm sieve for subsequent analysis. The water-soluble carbohydrate (WSC) content was analyzed by the anthrone–sulfuric acid colorimetric method [17]. The crude protein (CP) and total nitrogen (TN) contents were detected using the Dumas combustion method [18]. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined according to the method of Van Soest et al. [19].

2.3.2. Fermentation Compositions Analysis

An amount of 20 g of fresh samples was evenly mixed in 180 mL of 0.85% NaCl sterile saline, extracted in a refrigerator at 4 °C for 24 h, and then the remaining liquid was filtered through 4 layers of gauze, and the extracts were stored in a −20 °C freezer for subsequent fermentation quality determination. The filtrate pH was determined with a glass electrode pH meter. The ammonia nitrogen (AN) content was analyzed by the phenol–hypochlorite method [20]. The contents of lactic acid (LA), acetic acid (AA), propionic acid (PA), and butyric acid (BA) were analyzed by high-phase liquid chromatography (HPLC, KC-811, Shimadzu Co., Ltd., Kyoto, Japan). The column used was a Shodex Rspak KC-811 S-DVB gel column (Lisennoke Scientific Instruments Co., Ltd., Shanghai, China) with an SPD-M10AVP detector (Shimadzu Co., Ltd., Kyoto, Japan). The setting parameters were as follows: the wavelength was set to 210 nm. The mobile phase was 3 mmol/L perchloric acid, the column temperature was 50 °C, and the flow rate was 1 mL/min [21].

2.3.3. Microbial Community Counting

The microbial community count was based on the method of Zeng Tairu et al. [21]. The specific method was as follows: the 20 g sample was mixed into 180 mL sterile saline (0.85% NaCl), placed on a 4 °C shaker for 1 h, then filtered, and then serial dilution was performed from 10⁻¹ to 10⁻⁷. Enterobacter were cultivated on Violet Red Bile Agar (Difco, Hopebil, Qingdao, China) and counted after 24 h of aerobic growth at 37 °C. Lactic acid bacteria were cultured on De Man, Rogosa, and Sharpe agar (Difco, Hopebil, Qingdao, China) and counted after 48 h of anaerobic growth at 37 °C. Molds and yeasts were determined by Potato Glucose agar (Difco, Hopebil, Qingdao, China) and counted after 72 h of aerobic growth at 28 °C. Yeasts and molds were distinguished by colony appearance and cell morphology observation.

2.4. Statistical Analyses

The data were collated and calculated using Excel 2019; the results of the microbial plate smear counts needed to be converted into log10 cfu/g of FM before statistical analysis. The one-way ANOVA and multi-way ANOVA were performed using SPSS 27.0 for the additives and materials. Duncan’s multiple tests were used to compare the mean values of the different chemical compositions and the fermentation qualities of the silage. Results were expressed as mean ± standard deviations. Significant differences existed if the probability level was below 0.05.

In order to comprehensively evaluate the effects of different lactic acid bacteria additives on the silage quality of oats and common vetch silage, the membership function value method was used to analyze the measured indexes [20], in which dry matter, crude protein, water-soluble carbohydrate, lactic acid, acetic acid, and lactic acid bacteria were considered as positive indicators, and neutral detergent fibers, acid detergent fibers, pH, ammonia nitrogen/total nitrogen, butyric acid, and yeasts were considered as negative indicators. The values of the membership function of the individual indicators of the different treatments were calculated separately, and then the average membership function value of each treatment was calculated; the larger the value, the better the quality of the silage. The membership function value formula is as follows:

X$\left({\mathrm{\mu }}_1\right)={\displaystyle \frac{\mathrm{X}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n}}}{{\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n}}}}$ (positive correlation);

X(µ₂) = 1 − X(${\mathrm{\mu }}_1$) (negative correlation);

X_µ: the membership function value of a silage treatment under an additive;

X: the measured value of an index for a silage treatment;

X_min and X_max: the minimum and maximum values of an indicator in all processes, respectively.

3. Results

3.1. Fermentation Quality of Oat and Common Vetch Silage

The fermentation quality of silage treated with different compound lactic acid bacteria additives is shown in Table S2. The material, additives, and their interactions significantly affected the pH, LA, AA, and BA content of the silage (p < 0.05). In comparison with the T1 treatment, the pH of S1, S2, and S3 was significantly decreased in the T2 and T3 treatments (p < 0.05), and the pH was less than 4.2. The AN/TN of all treatments was lower than 28 g·kg⁻¹ DM, and the AN/TN of the S3 treatment was lower than that of the S1 and S2 treatments. Among the S1, S2, and S3 treatments, the contents of LA and AA in the S3 treatment were the highest. Compared with the T1 treatment, the T2 treatment significantly increased the LA content of the silage (p < 0.05), and the S3T2 treatment had the highest LA content of 40.86 g·kg⁻¹ DM (p < 0.05). In addition to the S1T3 treatment, the T2 and T3 treatments significantly increased the AA content of the silage compared to the T1 treatment (p < 0.05), and the S3T3 treatment had the highest AA content of 14.94 g·kg⁻¹ DM. The LA/AA ratio was higher than 2:1 for all treatments throughout the fermentation process. The T2 treatment significantly increased the PA content of the silage (p < 0.05), and the highest PA content was 2.87 g·kg⁻¹ DM in the S2T2 treatment. In this study, the BA content of all treatments was less than 0.9 g·kg⁻¹ DM.

3.2. Chemical Characteristics of Oat and Common Vetch Silage

The chemical characteristics of silages treated with different compound lactic acid bacteria additives are shown in Table S3. The materials, additives, and their interactions significantly affected the DM, WSC, and NDF content of the silage (p < 0.05). The DM content of all treatments ranged from 160.62 to 262.03 g·kg⁻¹ FM, and the S3 treatment had a higher DM content (p < 0.05). In comparison with the T1 treatment, the T2 treatment significantly decreased the CP content of the S1 and S3 treatments (p < 0.05). The T2 treatment significantly increased the WSC content of the silage compared to the T1 treatment, while the T3 treatment significantly decreased the WSC content of the S1 and S3 treatments, and the S1T2 treatment had the highest WSC content among all treatments at 61.27 g·kg⁻¹ DM (p < 0.05). Except that the ADF content of the S3T2 treatment decreased compared with the S1 treatment, the NDF and ADF content of the S3 treatment increased in other treatments. In contrast to the T1 treatment, the T2 treatment decreased the NDF content of the silage, while the T3 treatment only led to a reduction in the NDF content of the S3 treatment. Both the T2 and T3 treatments demonstrated a significant increase in the ADF content of the silage (p < 0.05), with the T3 treatment exhibiting a more pronounced increase (7.25~18.09% vs. 23.77~34.72%).

3.3. Microorganisms of Oat and Common Vetch Silage

The number of microbial populations in the silage treated with different compound lactic acid bacteria additives is shown in Table S4. The materials, additives, and their interactions significantly affected the number of yeasts in the silage (p < 0.01). Compared with the T1 treatment, the T2 and T3 treatment increased the number of lactic acid bacteria in the silage, in which the number of lactic acid bacteria treated with S3 increased significantly, and the number of lactic acid bacteria treated with S3T3 was the highest (7.52 log₁₀ cfu·g⁻¹ FM). The T2 treatment demonstrated a reduction in yeast populations in S2 and S3, while the T3 treatment significantly decreased the number of yeasts in the silage compared to the T1 treatment (p < 0.05). Notably, no yeast was detected in the S1T3 and S2T3 treatments.

3.4. Comprehensive Evaluation of Silage Quality of Oat and Common Vetch with Different LAB Additives

Using the membership function analysis method, Table S5 shows the comprehensive evaluation of oat and common vetch silages’ quality with different compound lactic acid bacteria additives. The membership function analysis of each index of the tested treatment showed that the dry matter, crude protein, water-soluble carbohydrate, lactic acid, acetic acid, and lactic acid bacteria were positive indexes. The neutral detergent fiber, acid detergent fiber, pH, AN/TN, BA, and yeasts were negative indexes. According to the ranking of the membership function values of the above indexes, it can be seen that the average value of the membership function of the experimental group is higher than that of the control group, indicating that lactic acid bacteria additives can improve the quality of silage, and the ranking result of the membership function is S3T2 > S3T3 > S1T2 > S2T2 > S2T3 > S1T3 > S3T1 > S1T1 > S2T1. In general, the improvement effect of the T1 and T2 treatments on the S3 treatment tended to be the same, but the T2 treatment had a better-quality improvement effect on the S1 and S2 treatments.

4. Discussion

4.1. Effects of Different LAB Additives on Fermentation Quality of Oat and Common Vetch Silage

The pH is an important index to evaluate the fermentation quality of silage, and it is generally believed that the pH of high-quality silage should be less than 4.2 [22]. The soluble carbohydrate content of grass forage is rich, which provides sufficient substrate for the growth and reproduction of lactic acid bacteria, so after exogenous addition of lactic acid bacteria, the number of lactic acid bacteria in silage increases, accelerating the lactic acid production, and the pH value decreases. However, legume forage has a low content of water-soluble carbohydrate, a high buffering energy, and a high content of soluble proteins, which can be rapidly degraded to form NH⁴⁺, and inhibit the decrease in the pH value [23]. After mixed silage, it balanced the nutrient composition of both, such as the increase in water-soluble carbohydrate in silage compared with legume forage silage alone, which provided the conditions for lactic acid bacteria to convert soluble carbohydrate into organic acids, and the increase in the lactic acid content in the silage, which in turn led to the decrease in its pH value [24]. The ability of lactic acid bacteria to utilize fermentation substrates such as water-soluble carbohydrate varies according to the species of lactic acid bacteria, which may be the reason why in the present study, the Zhuang Lemei IV additive significantly reduced the pH value of the single-silage group of oat and common vetch, while the Fu Zhengxing additive significantly reduced the pH value of the mixed-silage group [13], but the specific reasons need to be explored in the near future. AN/TN reflects the proteolysis, and generally high-quality silage requires the AN/TN to be less than 100 g·kg⁻¹ DM [25]. In this study, the AN/TN of the treatments was all less than 28 g·kg⁻¹ DM, and the AN/TN content of the silage was decreased after the mixed silage of oat and common vetch, which may be due to the mixed silage balancing the characteristics and microbial communities of the two kinds of forage grass, inhibiting the reproduction of undesirable bacteria, and then decreasing the degradation of crude protein in the silage [26]. The content of organic acids can reflect the fermentation process of silage and is considered to be a homofermentation-dominated process when the ratio of lactic acid to acetic acid is at least higher than 2:1, which is a desired result in silage. Legume forages have acetic acid bacteria attached to themselves and this can produce acetic acid before anaerobic conditions are reached, thus allowing the accumulation of lactic and acetic acid in the silage [26]. The effect of two compound lactic acid bacteria additives on the accumulation of the acetic acid content of the silage was similar to its effect on the silage pH; when the Zhuang Lemei IV silage additive was added, it promoted the accumulation of lactic acid and acetic acid in the silage, but the addition of the Fu Zhengxing silage additive will cause most of the lactic acid in the oat and common vetch single-silage group and mixed-silage group to be isomerized to acetic acid, which will result in a relative decrease in the lactic acid content and an increase in the acetic acid content of the two groups [27], so it is presumed that there is also a competitive relationship between exogenous bacteria and microorganisms naturally attached to the surface of the forage, and that the addition of the Zhuang Lemei IV silage additive might introduce bacteria that have benign interactions with the forage microorganisms themselves, and that the addition of the Fu Zhengxing silage additive may have resulted in the dominance of heterogeneous fermenting lactic acid bacteria [13]. Propionic acid is a secondary metabolite produced by the fermentation of lactic acid by Propionibacterium. A small amount of propionic acid acts similarly to acetic acid and is effective in preserving silage nutrients. Butyric acid is produced by the decomposition of proteins and lactic acid by spoilage bacteria, the lower the content of which the better [28,29]. In this study, the contents of propionic acid and butyric acid in all treatments were less than 2.9 g·kg⁻¹ DM and 0.9 g·kg⁻¹ DM, respectively, which met the requirements of high-quality silage. However, the propionic acid content in the silage increased significantly after the addition of the Zhuang Lemei IV additive, which may be caused by the increased activity of Propionibacterium after the addition of the Zhuang Lemei IV silage additive [13], but the specific reason is still unclear and needs to be further explored. In general, the results of this study showed that the mixed silage of oat and common vetch and the addition of lactic acid bacteria could effectively improve the fermentation quality of the silage.

4.2. Effects of Different LAB Additives on Chemical Characteristics of Oat and Common Vetch Silage

The content of each nutrient composition is an important index to evaluate the nutritional quality of silage. In the silage fermentation process, lactic acid bacteria need to use forage water to promote their own life activities, so the dry matter content of silage has an important impact on fermentation [30]. This study found that the dry matter content of the oat and common vetch mixed silage was higher than when the two forages were silaged separately, but the dry matter content was consistently 259.73–262.03 g·kg⁻¹ FM, which may be related to the high dry matter content of the oats themselves. It meets the requirements of high-quality silage [31], indicating that the mixed silage of oat and common vetch can improve the feeding value of the silage. Crude protein is an important index to reflect the quality of silage, and its content is negatively correlated with AN/TN. In this study, it was found that the crude protein content of oat and common vetch increased after mixed silage without lactic acid bacteria preparation, but if exogenous lactic acid bacteria preparation was added, the change in the silage crude protein content varied with different additives. For example, the addition of the Fu Zhengxing silage additive had no significant effect on the crude protein content of the silage, but the addition of the Zhuang Lemei IV lactic acid bacteria additive decreased the crude protein content of the oat single-silage and mixed-silage group, which was similar to the change in AN/TN. This shows that the mixed silage of oat and common vetch can give full play to the characteristics of the two kinds of forage grass and balance their nutrients, and the difference in the effect after adding lactic acid bacteria may be related to the interaction between exogenous lactic acid bacteria and the lactic acid bacteria attached to the forage grasses themselves, as well as the different adaptability of different lactic acid bacteria to fermentation substrates [14]. Water-soluble carbohydrates are the material basis for lactic acid bacteria fermentation, and their content is an important factor in determining the fermentation process of silage. After utilizing the water-soluble carbohydrates in silage, lactic acid bacteria convert them into organic acids, which in turn inhibit the activity of undesirable microorganisms and reduce the consumption of water-soluble carbohydrate by undesirable microorganisms [32]. However, since the consumption of water-soluble carbohydrate in silage by lactic acid bacteria themselves is also larger, exogenously added lactic acid bacteria with relatively strong metabolic activity will accelerate the consumption of water-soluble carbohydrates in silage when they multiply in large quantities in silage, and the number of lactic acid bacteria under the treatment of the Fu Zhengxing silage additive is significantly higher than that under the other treatments, which explains the significant reduction in the number of lactic acid bacteria after the addition of the Fu Zhengxing silage additive in the water-soluble carbohydrate content in the oat and mixed-silage groups [16]. Fiber and its compositions are the key indexes for the routine evaluation of silage quality. The appropriate content of neutral detergent fiber can increase the saliva secretion of livestock, but too high a content will affect the digestibility of livestock and the lower the content of acid detergent fiber, the higher the feeding value [33]. The cellular respiration and rapid growth and reproduction of lactic acid bacteria in the early stage of silage will consume part of the nutrients in silage, which in turn leads to an increase in the proportion of acidic and neutral detergent fiber in silage [34], and the number of lactic acid bacteria in the mixed-silage group was higher than that in the single-silage group, which explains well the increase in the content of neutral detergent fiber and acid detergent fiber in the silage after the mixing of leguminous and grassy forage for silage. However, the effects of different additives on the forage fiber fractions were inconsistent and may also be related to the interaction between the bacteria and fermentation substrate as well as the characteristics of the two types of fibers [35], but the exact reason for this is not clear. Further exploration is needed.

4.3. Effect of Different LAB Additives on Microorganisms of Oat and Common Vetch Silage

Silage is a technology based on the vital metabolic activities of microorganisms that enable the long-term preservation of forage, and the type and number of microorganisms directly affect the fermentation quality of silage, and lactic acid bacteria are the core microorganisms of silage fermentation [36]. In this study, the mixed storage of oat and common vetch increased the number of lactic acid bacteria in the silage, indicating that the mixed silage played the complementary effect of the two kinds of forage grass, making the silage nutrition more comprehensive and providing more abundant and diversified sources of nutrition for the reproduction of lactic acid bacteria [26]. In addition, the number of lactic acid bacteria in the silage was increased by the addition of exogenous lactic acid bacteria preparations, among which the number of lactic acid bacteria treated with Fu Zhengxing additive was the highest, which indicated that the added lactic acid bacteria would multiply in large quantities in silage, but the degree of reproduction varied with different types of lactic acid bacteria [15]. Yeasts are the main microbial responsible for the aerobic deterioration of silage, which affects silage quality mainly by decomposing sugar and lactic acid [37,38]. Yeast can utilize lactic acid for raw reproduction, and the mixed silage of oats and arrow end peas increased the yeast population of the forage, which was attributed to the higher lactic acid content of the mixed-silage group, which increased the supply of substrate needed for yeast growth and promoted yeast reproduction [39]. The addition of the Fu Zhengxing treatment significantly decreased the number of yeasts in the silage, and the treatment with the Zhuang Lemei IV additive decreased the number of yeasts in the single silage of the common vetch and mixed-silage group. This is because the addition of lactic acid bacteria decreased the pH of the silage and inhibited the growth and reproduction of yeasts [40], but it is doubtful that the addition of the Zhuang Lemei IV silage additive significantly increased the number of yeasts in the oat single-silage treatment, which may be the result of bacterial interactions [13], but the specific reasons need to be further explored. Molds and Enterobacter are undesirable bacteria that affect silage fermentation, and their excessive growth and reproduction will lead to silage failure.

5. Conclusions

In this study, we compared the effects of different compound lactic acid bacteria additives on the quality of oat and common vetch silage quality. It was found that oat and common vetch mixed silage balanced the nutritional characteristics of the two kinds of forage grass and made the nutrients of silage more comprehensive compared with the silage of the two forages alone. Both the Zhuang Lemei IV silage additive and Fu Zhengxing silage additive were beneficial in improving the quality of the silage. Combined with the comprehensive analysis of the affiliation function, the use of Zhuang Lemei IV compound lactic acid bacterial additive is recommended for oat and common vetch silage under the special climatic conditions of the Northwest Sichuan Plateau, which has a better effect on quality enhancement and efficiency.