Effect of Sorbic Acid, Ethanol, Molasses, Previously Fermented Juice and Combined Additives on Ensiling Characteristics and Nutritive Value of Napiergrass (Pennisetum purpureum) Silage

Abstract

The effects of sorbic acid, ethanol, previously fermented juice and their combined additives on fermentation quality and residual water-soluble carbohydrates (WSC) of napiergrass (Pennisetum purpureum) were herein investigated. There were two experiments in total. The treatments of experiment 1 were as follows: control (no addition); sorbic acid at 0.1%, S; molasses at 1.0%, M; previously fermented juice at 5 mL/kg, P; SP; SM; SPM. The treatments of experiment 2 were as follows: control (no addition); ethanol at 1.5%, E; molasses at 1.0%, M; previously fermented juice at 5 mL/kg, P; EM; EP; EPM. The laboratory silos (10 L) were kept at room temperature (~25 °C), and opened after 120 days of ensiling and the chemical compositions of silage were analyzed. Sorbic acid addition significantly (p < 0.05) decreased the silage pH and increased the residual WSC and lactic acid (LA) contents as compared with control. Ethanol addition decreased pH, acetic acid (AA) and volatile fatty acids (VFAs) contents, and increased the ratio of LA/AA, LA and residual WSC contents. Molasses addition increased ratios of LA/AA and enhanced WSC content used by lactic acid bacteria (LAB). The PFJ addition increased AA and ammonia nitrogen/total nitrogen (AN/TN) and decreased the LA contents. Comparing the fermentation quality among all silages in the present study, the combined additives SM and EM performed best in improving the silage quality of napiergrass.

1. Introduction

The grass is often ensiled in summer or autumn and the resultant silage is used as feed when fresh grass is unavailable in winter or early spring in droughty areas or cold periods. Compared to other feed-making methods, ensilage is the best method for preserving fresh forage with minimal losses, especially in moist and rainy areas. Napiergrass (Pennisetum purpureum) is one of the important forages in tropic or subtropic areas and is widely planted in the south of China [1]. The yield of napiergrass in summer is usually abundant and there is surplus fresh grass after being used as succulence. As tropic grass, napiergrass has a coarse and hard stemmy structure, and as a result, they are usually less dense and presumably more permeable, and relatively large quantities of air may be trapped in the forage mass than temperate grass just after ensiling [2]. Moreover, the unfavorable microorganisms growing on tropical grass, mainly including aerobic bacteria, yeasts and molds, may reduce nutrients, especially water-soluble carbohydrates (WSC) [3].

Silage quality and nutritive value are influenced by numerous biological and technological factors, and silage will have a high nutritive value and good quality with proper ensilage techniques and additives. However, the results in practice indicated that the fermentation quality is often poor or even unsatisfactory [4]. Recently, silage additives are usually used in ensilage according to the silage condition or the characteristics of ensilage material. Thus, silage fermentation stimulants or aerobic bacteria inhibitors are generally used to improve silage quality and aerobic performance, such as molasses, previously fermented juice, formic acid and sorbic acid et al. [5,6,7]. Previous studies showed that sorbic acid and ethanol had strong effects on inhibiting the growth of yeasts and molds, and sorbic acid is widely used as a preservative in the food industry or as a silage additive [8,9]. Adding previously fermented juice (PFJ) of epiphytic lactic acid bacteria (LAB) was effective in improving fermentation quality for increasing the numbers of LAB [10,11]. Molasses could be used to compensate for the WSC loss caused by the initial undesirable bacteria activity and increase the substrate for lactic acid fermentation during ensiling. Several studies have demonstrated the advantages of molasses as a silage additive to improve fermentation quality [12].

Therefore, this study aimed to evaluate the effects of sorbic acid, ethanol, molasses, previously fermented juice and combined additives on fermentation quality and nutritive value of napiergrass silage. The amount of additives was based on previous experiments or tests [9,13,14].

2. Materials and Methods

2.1. Previously Fermented Juice Preparation

The PFJ was prepared from napiergrass according to the following manners: fresh cut grass (50 g) was macerated with 200 mL of distilled water using a blender. The macerated sample was filtered through double layers of cheesecloth, and 200 mL of the filtrate was collected into a 500 mL glass bottle containing 4 g of glucose. The glass bottle was fitted with a gas trap and maintained at 30 °C for 3 days [14]. After 3 days of anaerobic incubation, the pH value and the population of LAB were determined just before silage treatment.

2.2. Silage Making

Napiergrass (Pennisetum purpureum) was cultivated in the experimental field of Nanjing Agricultural University (Jiangsu, China: N 32.04°, E 118.88°, annual mean temperature 16.5 °C and average annual precipitation 1090.6 mm), which contained 10 plots (180 m² per plot). The type of planting land was clay loam. Water and fertilizer management were normally carried out. Six plots of napiergrass were randomly harvested at the vegetative stage and chopped into about 1~2 cm in length with a laboratory forage cutter (65–300; Hedong Co., Ltd., Linyi, China). The chopped forage was mixed thoroughly with different additives before ensiling. The effects of different additives on fermentation quality were analyzed through two independent experiments. The additives and their adding amount (on basis of fresh weight) were as follows. Experiment 1: (1) control (no addition), (2) sorbic acid (S) addition at 0.1%, produced by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, (3) molasses addition (M) at 1.0%, produced by Lianyungang molasses factory, Lianyungang, China, (4) previously fermented juice (PFJ) addition at 5 mL/kg, (5) sorbic acid at 0.1% + PFJ at 5 mL/kg (SP), (6) sorbic acid at 0.1% + molasses at 1.0% (SM), (7) sorbic acid at 0.1% + molasses at 1.0% + PFJ at 5 mL/kg (SPM). Experiment 2: (1) control (no addition), (2) ethanol addition (E) at 1.5%, produced by Nanjing No. 1 Chemical Reagent Factory, Nanjing, China, (3) molasses addition (M) at 1.0%, produced by Lianyungang molasses factory, (4) previously fermented juice (PFJ) addition at 5 mL/kg, (5) ethanol at 0.1% + molasses at 1.0% (EM), (6) ethanol at 1.5% + PFJ at 5 mL/kg (EP), (7) ethanol at 1.5% + molasses at 1.0% + PFJ at 5 mL/kg (EPM). The control silage was sprayed with an equal volume of stilled water. About 7.5 kg of chopped fresh grass with additives was immediately packed into a plastic laboratory silo (10 L capacity) in triplicates, and stored in the room kept at ambient temperature (~25 °C). The packing density in the experiment was approximately 7.5 × 10³ kg/m³. The silos were opened after 120 days of ensiling and the chemical compositions and fermentation quality were analyzed.

2.3. Chemical Analyses

The chopped grasses were immediately collected for the determination of dry matter (DM), total nitrogen (TN), crude protein (CP) and WSC content. After the silos were opened, the contents were mixed thoroughly, and 35 g of the silage sample was taken from each silo. This was followed by adding about 70 g distilled water and macerating at 4 °C for 24 h. The extracts were filtered through two layers of cheesecloth and filter paper. The filtrate was stored at −20 °C before chemical analyses. The filtrate was used for determining pH, ammonia nitrogen (AN), lactic acid (LA) and volatile fatty acids (VFAs). The pH of silages was measured using a glass electrode pH meter (HI223, HANNA company in Italy). TN was analyzed by the Kjeldahl (AOAC) [15] method and CP was determined by multiplying TN by 6.25. The AN content was determined using the phenol-hypochlorite procedure. LA content was determined using the method of Josefa [16]. The VFAs content was determined with gas chromatography (Shimizu GC-14B, condition: column temperature at 135 °C, injection and detection temperature at 220 °C). The DM content of the fresh materials and silages was determined by drying in an oven at 65 °C for at least 72 h. The WSC content of fresh materials and silages was determined using the method of Murphy [17]. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents were analyzed according to Van-Soest et al. [18].

2.4. Statistical Analysis

The experiments were of a completely randomized design, with 5 replicates for each treatment, and the statistical analysis included a one-way analysis of variance (ANOVA) with additive treatments as a factor and Fish’s least significant difference test (p = 0.05). Data on the chemical composition of the silages were analyzed by ANOVA using the general linear model procedure of the Statistical Analysis System (SAS, 1999) [19].

3. Results

Table 1. Characteristics of napiergrass before being ensiled.

Dry Matter (g kg−1 FW)	Crude Protein (g kg−1 DM)	Water-Soluble Carbohydrates (g kg−1 DM)	Acid Detergent Fiber (g kg−1 DM)	Neutral Detergent Fiber (g kg−1 DM)
167.32	76.83	70.97	330.15	506.52

FW: fresh weight; DM: dry matter.

shows the characteristics of the initial napiergrass. It contained 167.32 g kg⁻¹ FW dry matter, 76.83 g kg⁻¹ DM crude protein and 70.97 g kg⁻¹ DM water soluble carbohydrate. The acid detergent fiber and neutral acid fiber contents were 330.15 g kg⁻¹ DM and 506.52 g kg⁻¹ DM, respectively.

3.1. Experiment 1

The compositions of napiergrass silage treated with sorbic acid and its combination are shown in

Table 2. The effects of sorbic acid, molasses and previously fermented juice on DM, pH, lactic acid and acetic acid content of napiergrass silage.

Items		Dry Matter (g kg−1 FW)	pH	Lactic Acid (g kg−1 DM)	Acetic Acid (g kg−1 DM)	Lactic Acid/ Acetic Acid
Treatments	control	167.04 ab	3.62 c	82.37 ab	7.93 bc	10.77 ab
	S	166.42 ab	3.51 ab	88.79 b	4.59 a	20.46 cd
	P	165.34 a	3.73 d	60.95 a	13.31 d	4.71 a
	M	173.91 abc	3.54 bc	114.95 c	8.03 bc	14.51 bc
	SP	169.22 abc	3.57 bc	115.74 c	10.06 cd	12.97 b
	SM	176.03 bc	3.42 a	114.17 c	4.99 ab	23.54 d
	PM	177.50 c	3.50 ab	101.65 bc	6.47 ab	15.69 bc
	SPM	175.86 bc	3.42 a	109.77 bc	6.46 ab	16.96 bc
SEM		1.37	0.02	4.4	0.65	1.36
p Value		0.09	<0.001	0.001	0.001	0.003

S, sorbic acid; P, previously fermented juice; M, Molasses; SP, sorbic acid and previously fermented juice; SM, sorbic acid and molasses; PM, previously fermented juice and molasses; SPM, sorbic acid, previously fermented juice and molasses. FW, fresh weight; SEM, standard error of the mean. Values followed by different letters in the same column show significant differences among different treatments at p < 0.05.

and

Table 3. The effects of sorbic acid, molasses and previously fermented juice on volatile fatty acids, ammonia nitrogen/total nitrogen and WSC content of napiergrass silage.

Items		Propionic Acid (g kg−1 DM)	Butyric Acid (g kg−1 DM)	Volatile Fatty Acids (g kg−1 DM)	Ammonia-N (g kg−1 TN)	WSC (g kg−1 DM)
Treatments	control	0.24	ND	8.17 bc	90.49 bc	11.13 a
	S	0.21	ND	4.80 a	53.31 ab	14.35 abc
	P	0.27	ND	13.52 d	97.01 c	11.39 ab
	M	0.22	0	8.25 bc	69.74 abc	15.53 bc
	SP	0.21	0	10.27 cd	80.81 abc	13.42 abc
	SM	0.2	ND	5.19 ab	39.79 a	25.17 d
	PM	0.19	ND	6.66 ab	50.61 ab	16.98 c
	SPM	0.21	ND	5.62 ab	46.53 ab	23.57 d
SEM		0.01	0	0.65	5.87	1.11
p Value		0.993	0.466	0.001	0.04	<0.001

S, sorbic acid; P, previously fermented juice; M, Molasses; SP, sorbic acid and previously fermented juice; SM, sorbic acid and molasses; PM, previously fermented juice and molasses; SPM, sorbic acid, previously fermented juice and molasses. FW, fresh weight. TN, Total nitrogen. WSC, water-soluble carbohydrates. ND, not detected. Values followed by different letters in the same column show significant differences between different treatments at p < 0.05.

, respectively. Except for P treatment, other treatments decreased pH and increased LA contents. The pH in SM, PM and SPM were significantly (p < 0.05) lower than that in control. M, SP and SM significantly (p < 0.05) increased LA content as compared to control, while the LA content and ratio of LA/AA (lactic acid/acetic acid) in P was the lowest among all silages. The AA and VFAs contents in S were significantly (p < 0.05) lower than those in control, P and SP. The S, M and their combination increased the LA/AA, and the value of S and SM was significantly (p < 0.05) higher than that of control and P treatment. The addition of SM, PM and SPM showed lower (p > 0.05) VFAs content as compared with control, P and SP treatment. The propionic acid content and AN/TN tended to decrease treated with additives and BA was not detected in all silage. The AN/TN of SM was the lowest among all treatments, and the difference between control and SM was significant (p < 0.05). The WSC content of all treatments was higher than control and the WSC content of M and its combined treatments was significantly higher than that of control, S, P and SP. There was a significant (p < 0.05) increase in DM content in PM addition, and the other additives did not show a significant (p > 0.05) difference as compared with the control.

3.2. Experiment 2

The composition of napiergrass silage treated with ethanol and its combination were presented in

Table 4. The effects of ethanol, molasses and previously fermented juice on DM, pH, lactic acid and acetic acid content of napiergrass silage.

Items		Dry Matter (g kg−1 FW)	pH	Lactic Acid (g kg−1 DM)	Acetic Acid (g kg−1 DM)	Lactic Acid/ Acetic Acid
Treatments	control	164.30 a	3.58 a	97.17 a	9.91 ab	9.61 abc
	E	169.86 ab	3.55 a	104.76 ab	7.51 a	14.40 d
	P	174.63 bc	3.67 ab	95.29 a	12.68 bc	8.20 ab
	M	181.33 cd	3.57 a	117.77 b	8.50 a	14.22 d
	EP	181.61 cd	3.77 b	94.14 a	14.64 c	7.02 a
	EM	187.20 d	3.58 a	102.94 ab	7.89 a	13.10 cd
	PM	185.75 d	3.60 a	100.35 ab	9.66 ab	10.52 abc
	EPM	181.15 cd	3.61 a	102.67 ab	8.56 a	11.69 bcd
SEM		1.8	0.02	2.06	0.61	0.76
p Value		0.001	0.1	0.03	0.03	0.04

E, Ethanol; P, previously fermented juice; M, Molasses; EP, ethanol and previously fermented juice; EM, ethanol and molasses; PM, previously fermented juice and molasses; EPM, ethanol, previously fermented juice and molasses. FW, fresh weight; SEM, standard error of the mean. Values followed by different letters in the same column show significant differences among different treatments at p < 0.05.

and

Table 5. The effects of ethanol, molasses and previously fermented juice on volatile fatty acids, ammonia nitrogen/total nitrogen and WSC content of napiergrass silage.

Items		Propionic Acid (g kg−1 DM)	Butyric Acid (g kg−1 DM)	Volatile Fatty Acids (g kg−1 DM)	Ammonia-N (g kg−1 TN)	WSC (g kg−1 DM)
Treatments	control	0.26 b	ND	10.17 ab	78.81 abc	11.25 a
	E	0.22 ab	ND	7.73 a	75.59 abc	14.86 ab
	P	0.27 b	ND	12.95 bc	102.82 c	13.61 a
	M	0.18 a	ND	8.67 a	74.00 abc	18.01 bc
	EP	0.18 a	0.01	14.82 c	94.96 bc	12.62 a
	EM	0.21 ab	ND	9.76 ab	54.34 a	18.66 c
	PM	0.19 ab	ND	9.86 ab	61.35 ab	17.54 bc
	EPM	0.20 ab	0.08	8.84 a	59.68 a	17.51 bc
SEM		0.01	0.01	0.61	3.51	0.65
p Value		0.27	0.48	0.03	0.04	0.003

E, Ethanol; P, previously fermented juice; M, Molasses; EP, ethanol and previously fermented juice; EM, ethanol and molasses; PM, previously fermented juice and molasses; EPM, ethanol, previously fermented juice and molasses. TN, Total nitrogen. WSC, water-soluble carbohydrates. ND, not detected. Values followed by different letters in the same column show significant differences among different treatments at p < 0.05.

, respectively. All the silages showed similar pH with a small difference; however, the pH of EP was significantly (p < 0.05) higher than others except for P treatment. The pH of E was lower (p > 0.05) than control and other additions. There was a significant (p < 0.05) increase in LA content in M addition, and others were higher (p > 0.05) than control except for P and EP treatments. The AA and VFAs contents of P and EP were higher (p > 0.05) than that of control, and the other additives decreased the content. The E and M treatments significantly (p < 0.05) decreased AA, VFAs content, and increased ratios of LA/AA, but the combinations did not show a significant (p > 0.05) difference as compared with the control. However, The LA/AA value of P and EP was lower than other additions. Although all silages had low PA content, the control silage showed significantly (p < 0.05) higher content of PA than M and EP additions. BA was almost not detected in all silages. EM, PM and EPM decreased (p > 0.05) the value of AN/TN, while P and EP increased (p > 0.05) the value as compared with control. The additions of M and its combination showed significantly (p < 0.05) higher WSC content, but E, P and EP addition did not show significant (p > 0.05) differences as compared with control. The WSC contents of P, and EP were significantly (p < 0.05) lower than EM, PM and EPM additions. P, M and all combinations significantly (p < 0.05) increased the DM content.

4. Discussion

The DM content of a crop is crucial for the extent of ensiling fermentation, the DM of napiergrass material in the experiment is 16.73%, which was a limiting factor for good fermentation. The results showed that the main fermentation products of all silages were LA, AA, and the silage quality of control was good from the pH (3.58 and 3.62, pH of control), high LA content, and low AA content. It can be concluded that the silage fermentation was dominated by LAB, judging from the low pH, high LA content and LA/AA. This is probably due to the fact that the sufficient WSC content of napiergrass leads to LA fermentation. The WSC content of grass is one of the significant factors for good fermentation and determining whether or not to apply additives to ensilage materials [20]. Napiergrass, as a gramineous grass, was of low buffering capacity in general. The WSC content of fresh napiergrass in this experiment exceeded the minimum requirement of WSC content for well-fermented silage (5% of dry matter) [12,21]. However, controls in the two experiments had higher AN/TN and lower WSC content. These indicated that a part of WSC had been consumed by undesirable bacteria during the fermentation period. This was attributed to the high moisture content in ensilage material, which were conducive to undesirable microbial development during the period of ensiling [22].

As aerobic bacteria inhibitors, sorbic acid addition decreased the silage pH and increased LA/AA, and trace amounts of PA and BA of silage as compared with control. These indicated that sorbic acid was effective in the inhibition of aerobic bacteria activity and stimulation of the homo-fermentative LAB growth. Sorbic acid is used as a food additive and can restrain aerobic bacteria effectively. This is in agreement with the findings of Shao et al. [4], who reported sorbic acid effectively inhibited clostridia and other aerobic bacterial activity. There were higher residual WSC and LA contents in sorbic acid treatment as compared with control. These indicated that sorbic acid decreased the loss of WSC during the fermentation period, and resulted in high utilization efficiency of WSC by epiphytic LAB. Although the numbers of yeast, mold and other bacteria during ensiling were not counted in the experiment, it was suggested that sorbic acid might effectively depress the losses of WSC by inhibiting aerobic microorganisms during the early stage of ensiling and preserving more WSC content for LAB flourishment. This was well reflected in sorbic acid-treated silages with lower AN/TN and higher residual WSC content. These results suggested that sorbic acid inhibited the activity of certain microorganisms causing a higher level of AN/TN and more losses of WSC, which was in agreement with other researchers [9]. It is suggested that 0.1% sorbic acid addition is effective in improving the silage quality of napiergrass.

In the experiments, PFJ additions increased pH, AA, VFAs and AN/TN of silages, and decreased the LA content and LA/AA as compared with control. The microbial species in PFJ were complicated, including LAB, aerobic microorganisms and other unknown. The objective of adding PFJ was to enhance the numbers of LAB, stimulate the LA fermentation and improve the silage quality, so the key factor for PFJ as a good silage additive is the microorganism whose number dominates all [9]. The PFJ addition in this experiment increased the AA and AN/TN contents and decreased the LA content, which indicated that adding PFJ increased the numbers of aerobic microorganisms and others. The result of another experiment showed that adding more PFJ resulted in more loss of crude protein and dry matter of silage [11]. However, LA fermentation still dominated the ensiling fermentation process. The produced LA amount was absolutely larger than AA content, but the WSC content of PFJ addition was lower than sorbic acid and other additions. There might be certain aerobic microorganisms of PFJ causing a higher level of AN/TN and larger losses of WSC. The PFJ of the experiment did not effectively improve the quality of napiergrass silage, which was in disagreement with other results [9]. PFJ made from forage oat and Italian ryegrass decreased AN/TN and total VFAs contents and increased the ratio of LA/AA [4]. The studies of inoculating different LABs at ensiling have been carried out by many researchers with inconsistent results. The species of LAB varied with PFJ prepared condition, and raw material and might change with the fermentation process; therefore, the effect of PFJ on silage fermentation was not stable, mainly influenced by microbial species, quantity and activity of PFJ [23].

Ethanol addition decreased pH, AA and VFAs contents, and increased LA and residual WSC contents and LA/AA. This indicated that ethanol had a certain effect on the inhibition of aerobic bacteria activity and stimulated the homo-fermentative LAB growth. There were higher residual WSC and LA contents in ethanol treatment as compared with control. These indicated that ethanol addition decreased the losses of WSC during the early stage of ensiling and enhanced the WSC utilization of LAB. The results were similar to that of Ohba et al. [8], who found that ethanol increased LA and residual WSC contents of Italian ryegrass silage. The AN/TN level in ethanol treatment was slightly higher than control, which indicated that 1.5% ethanol could improve the silage quality, but the level of ethanol in this experiment did not effectively restrict the activities of unfavorable microorganisms during ensiling, which was reflected in the silage treated with EP. EP addition markedly increased the pH and AA content of silage and decreased the LA and WSC contents as compared with control and ethanol addition. However, sorbic acid and PFJ (SP) significantly increased LA and decreased AA and AN/TN level, indicating that 0.1% sorbic acid depressed the activity of aerobic microorganisms and stimulated LA fermentation. Other studies have shown that sorbic acid and ethanol additives are harmless to animals and have been widely used as silage additives, improving the palatability of silage [9,13]. (Shao et al., 2007, Zhang et al., 2011).

Molasses addition decreased pH, AA and AN/TN, and significantly increased LA, residual WSC content and improved the fermentation quality in two experiments. These results were similar to that of Chen et al. (2016), Yunus et al. and Huisden et al. (2009) [14,24,25]. Molasses increased LA/AA and the result indicated that molasses enhanced WSC content by LAB and stimulated the homofermentative LAB growth. The effect of molasses addition on the inhibition of aerobic bacteria activity depended on LAB growth, producing a large amount of LA and decreasing silage pH quickly to restrict the activity of unfavorable microorganisms. There were higher residual WSC and LA contents in molasses-treated silages as compared with control and the level of AN/TN was lower than molasses addition. Although the WSC content of napiergrass material in this experiment was sufficient for LA fermentation from the residual WSC and low pH (less than 4.2), which is thought to be the critical pH to depress clostridia [26,27]. The silage quality treated with molasses was better than the control. Plant sugar was the substrate for the fermentation of LAB, so the concentration of raw material has a major influence on the extent and type of fermentation during ensiling. The addition of soluble carbohydrates may be expected to improve silage quality and some additives are used in practice to overcome the low content of sugars in tropical grasses or other forage. The concentrations of lactic acid in the nettle (Urtica cannabina) silages added with molasses were greater than in the control silage while the levels of acetic acid in those silages were lower [28]. Our results showed that the VFAs content and level of AN/TN silages treated with molasses were higher than sorbic acid addition. This can be partially attributed to the consumption of aerobic bacteria activity, which suggested that adding fermentable substrate to grass with high moisture content was not the most efficient in improving the fermentation qualities. These results were similar to the findings of Hashemzadeh–Cigari et al. [29], who reported that the effect of molasses on lucerne silage quality was inferior to control.

Of all combinations, the silage quality in SM and EM treatments was superior to that in single additive and other combination treatments. In experiment 1, combinations decreased pH, AA and AN/TN, and increased LA/AA and residual WSC content. In experiment 2, combinations decreased AN/TN and increased LA/AA and WSC content. Sorbic acid and ethanol belong to aerobic bacteria inhibitors, and molasses belongs to fermentation stimulants, enhancing soluble carbohydrates for LA fermentation. Sorbic acid and ethanol depressed aerobic microorganism activities and conserved WSC for LA fermentation, so SM and EM additions can further improve silage quality. However, the silage qualities of SP and EP silage decreased WSC and increased AN/TN, inferior to other combinations. This may be attributed to the PFJ addition increased aerobic bacteria numbers causing extra WSC loss than sorbic acid, ethanol and SM, EM addition. The pH and ratio of AN/TN content of silage treated with SP addition were lower than EP, and LA and WSC contents were higher. The quality of silage with SPM addition was better than with EPM addition. These results indicated that the effect of 0.1% sorbic acid addition on inhibiting aerobic bacteria was more effective than 1.5% ethanol. Other researchers found that sorbic acid and other aerobic organism inhibitors effectively improved maize, lucerne, Italian ryegrass and forage oat silage quality [30,31].

Comparing the fermentation quality among all silages in the present study, the effect of combined additives SM and EM on improvement in the silage quality was the best. This suggested that enhancing silage fermentation substrate (soluble carbohydrate) and aerobic bacteria inhibitors to grass, which contain sufficient WSC and optimal DM content are important for improving the silage quality.

5. Conclusions

Sorbic acid and ethanol additives can be used to improve the fermentation quality of napiergrass silage. The effect of sorbic acid + molasses and ethanol + molasses on improving the fermentation quality of napiergrass was the best among all treatments. Sorbic acid and ethanol are effective as silage additives. This study compared their effects in silage, studied the effect of combined additives composed of sorbic acid, ethanol and other additives, and provided a theoretical basis for the development and utilization of silage additives.