1. Introduction
Natural and chemical additives have been adopted widely in silage processing used as a means of improving silage quality [
1]. The mechanism of ensiling involves the conversion of water-soluble carbohydrates (WSC) by epiphytic and/or inoculated lactic acid bacteria (LAB) into lactic acid (LA) under anaerobic conditions [
2,
3].
The increased in LA content resulted in a reduction of pH value of the silage and a subsequent restriction in the growth pace of undesirable microbes which are deleterious to the fermentation process [
4]. It known well, a predominant aspect of grass-based materials application is how to preserve the original nutrients and ensure sustainable supply [
5]. An important objective in ensiling forage is to reduce extensive proteolysis, which increases the nutritional losses and leads to affect the quality of silage regarding protein quality and the intake [
6].
Successful ensiling process, resulted in a reduced storage loss, preserve substantial amount of nutrients and produced edible silage with an ideal dry matter content but unsuccessful ensiling process produced bad silage that contains undesirable microorganisms like yeasts, moulds, mycotoxins, pathogenic bacteria [
7,
8]. According to a microbiological point of view, fungi colonizing plants at various stages of development and produce mycotoxins (i.e.,
deoxynivalenol, nivalenol, zearalenon, fumonisine, vomitoxin, T2,
patulin,
aflatoxin,
ochratoxin) dangerous for humans, animals, plants and microorganisms. In addition, moulds can colonize plants as endophytes, which makes it very difficult to limit these microorganisms from silage [
9,
10,
11].
During the fermentation process some problems might appear such as insufficient lactic acid fermentation and bad smell of silage [
12].One of the most practices to improve the fermentation process its adding LAB which could be ensure the appropriate production of lactic acid and decrease the amounts of acetic, butyric and propionic acid [
13]. LAB inoculants have a long history and have been incorporated into silage making as an effective technique to increase LA production [
14].
Researchers have mentioned that LAB is considered safe for a wide range of reasons: like healthy, ease of handling, nontoxic nature and low cost, low energy required and high yields. LAB fermentation contributes beneficially in food processing technology and the quality of the end-products such as flavour, shelf-life and safety and sensory profile of the final products [
14,
15,
16,
17,
18,
19,
20]. LAB from the genera
Lactobacillus,
Bifidobacterium and Pediococcus are commonly classified as probiotics [
21]. The species
Lactobacillus plantarum characterized by rapid productive of lactic acid and tolerant to acidity which was wildly applied to enhance the forage fermentation. While
Pediococcusacidilactici is identified describes as rapid productive to lactic acid and faster growing than
Lactobacillus, though it adapts for a wild range of temperatures [
21,
22,
23].
However,
Lactobacillus spp, unlike
Pediococcusspp, has the potential of improving lactic acid production, lowering pH, competes with spoilage organisms, minimizes gas losses and proteolysis [
24]. Thus this study aims at evaluating the efficiency of LAB isolated from sweet sorghum silage on enhancing the fermentation quality based on a comparison between sweet sorghum and Napier grass silages.
4. Discussion
The sweet sorghum and Napier grass used in this study contains relatively low DM, WSC and crude protein. According to Herrmann
et al. [
36] forages with insufficient fermentable substrate or too low dry matter content have a FC of < 35, which indicate difficulty in the preservation of the biomass as silage. In the present study, the FC of both grasses was less than 35 and WSC/BC ratio was less than 3, therefore demonstrating the necessity of LAB addition. The DM is one of the most important factors for ensuring the success of the ensiling process [
37], the optimal DM content for grasses was determined to be ranging between 350–400 g/kg [
38]. In present study both grasses had a DM less than the ideal range and the LAB treatments had a minor effect on DM, NDF and ADF which was in close consent to the results achieved by [
39,
40].
It is well known that, a good preserved silage characterized by low pH, according to [
41] rapid acidification reduces the risk of undesirable microorganism growth during the early stage of ensiling therefore the rate of pH decline may be more important than the final pH. In this study, the pH of all treated batches continued to reduce the pH from the start till the end of the experiment and this finding in consistent with [
42]. The more rapid pH decrease when adding the LAB inoculant provide a clear indication regarding its role in the fermentation process stimulation. As the inoculants reduced the pH from 5.76 to 3.9 in sweet sorghum and Napier grass from 5.97 to 4.9 in first 5 days of ensiling, the reduction was commensurately with the LA produced. This findings are in agreement with [
43], the faster reduction in pH is correlated with the high LA content which would further lead to improved nutrients preservation. Yuan 2015, also found high LA when applied
Lactobacillus spp in total mixed rations among other chemicals [
44]. As the previous studies mentioned, the addition of LAB inoculants at ensiling is intended to ensure rapid and vigorous fermentation [
45].
As previously mentioned by [
3] using LAB to enhance silage quality by significantly improving the lactic acid concentration of silage, which is compatible with the present results, when the high LA concentration is produced by incubated LAB than control and commercial bacteria. On another hand, untreated sweet sorghum batch produced a lower amount of WSC this may due to the efficiency of epiphytic LAB to convert sugars and his results can be supported by the outcomes of Zhang (2015), mixed the sweet sorghum with alfalfa which revealed a pH decline [
46]. However, all strains isolated in currant study were a homofermentative LAB those bacteria are preferred during the ensiling process, because of their ability to rapidly convert forage sugars to lactic acid, which has a high acidifying potential [
3].
The other organic acids AA, BA and PA were produced when the undesirable bacteria fermented the WSC [
47,
48] and they are considered not desirable because of their a negative effect on the voluntary intake [
49]. In this study, the LAB inoculant succeeded to reduce the amount of these acids which may occur due to restricting the vigour of undesirable microorganisms. This results were in agreement with [
50], who reported that, soybean silage inoculated with L.
plantarum showed an acceptable reduction on AA and BA.
The fermentation process mainly depends on the availability of microorganisms in the silage, the water content of the crop and the type of substrate available for fermentation [
51]. In our study, the pH and WSC decreased on both treated and untreated grasses and this result agrees with previous studies by [
34]. The fast decline in WSC content and high production of LA in the first 5 days of ensiling could indicate by the ability of LAB to inhabit the other microorganisms. In this point, previously reported that, in addition to LAB, some other microorganisms in the fermentation process compete for available nutrients [
52].
Other fermentation factor related to
clostridial fermentation the ammonia N in silage which is predominantly a product of amino acids [
49]. Proteolysis occurs mostly when the pH is too high and in the presence of a large amount of soluble proteins. Our study demonstrated that LAB incubation could reduce ammonia N concentration and this might be due to their ability to compete with the undesirable microorganism activates. However, the poor silage quality occurred when LAB fails to produce sufficient lactic acid during fermentation to reduce the pH and inhibit the growth of clostridia [
53].
Successful fermentation process resulted in LAB domination among other micro-organism population, which produce lactic and other acids and due to presence of lactic acid, the pH drops to 5—3.8 [
37,
54] which in consist to present study. As shown in this study, the fresh grasses were rich in LAB, with a relatively high content of yeast and aerobic bacteria, this numbers of undesirable organisms was decreased in both treated and untreated silage. researchers mentioned that LAB could replace undesirable micrograms yeast and moulds, thus improving silage preservation [
55]. Unlike sweet sorghum, Napier grass silage had a high number of aerobic bacteria, mould and yeasts, according to the occurrence of a number of undesirable microorganisms at low levels in fresh plant materials, leading to their probable growth during the storage of silage, henceto anaerobic or aerobic spoilage [
56]. However the aerobic microbial decreases the nutritional value of the silage but also presents a hazard to animal health and the products quality and safety.
As known well, the quality of silage depends on the competition between different microorganisms [
56]. At the end of ensiling the strain HY1, which had been clustered into the Lactobacilli showed the best results in pH dropping in Napier grass compared to sweet sorghum. While strain HY2, which is of the
Pediococus acidilacticis species, recorded excellent results in reducing pH as well as in other fermentation parameters in sweet sorghum.
Pediococcuspentosaceus and
Pediococcus acidilactici are the main species used in fermentation processes as astarter (co-culture) for avoiding contamination [
21] even though the strain
Pediococcusclaussenii was not studied as a starter intensively and in our study showed good fermentation characteristics. Previously researchers found that the homolactic fermentation such as
pediccouss acidacticicii stable for low WSC forages [
53]. It is known that, adequate WSC and LAB counts are important for the rapid establishment and growth of LAB.