**4. Improving aerobic stability using forage inoculants**

Silage feedout is the final phase of the ensiling process. At that moment, oxygen can slowly diffuse inside the silage mass. Diffusion speed will be influenced by different factors, including the level of humidity, porosity, and temperature of the silage [25].

The process of aerobic deterioration of silage involves a shift to aerobic metabolism in some microorganisms and the reactivation of strict aerobes that were dormant. Reduce nutritional value due to oxidation of the fermentation products, of carbohydrates, amino acids, and lipids to H2O, CO2, and heat. Simultaneously, the higher metabolic activity will increase the silage temperature, accelerating microbial growth. Several microorganisms are involved, but yeast and acetic acid bacteria are adapted to tolerate the initially low pH conditions and thus able to exploit this niche before pH increases following the catabolism of the organic acids. Crops with higher levels of easily accessible carbohydrates are more prone to aerobic deterioration, i.e., corn, sorghum, and sugarcane, since these sugars can be readily fermented by spoilage microorganisms in the presence of oxygen.

Following the isolation of a *L. buchneri* strain [26], researchers described its unique metabolic pathway, which consisted of converting moderate amounts of lactate under low pH to equal parts of acetate and 1,2-propanediol [27]. The latter chemical is an intermediate in the potential synthesis of propionic acid. *L. buchneri* does not have the gene to complete the reaction, so another species of LAB has to be involved to convert 1,2-propanediol to an equimolar amount of 1-propanol and propionic acid [28]. This conversion was initially observed in silage by *Lactobacillus diolivorans* [29], but other members of the buchneri group also possess the genetic system [30], like *Lactobacillus reuteri* [28].

Compared to lactic acid, the key feature of acetic and propionic acids in improving aerobic stability of silage is based on the difference in p*K*a between these weak acids and lactic acid, which is a stronger acid, with a p*K*a of 3.86. At higher p*K*a, 4.76 for acetic acid and 4.86 for propionic acid, these weak organic acids will have a low dissociation level under most ensiling conditions, thus allowing for passive diffusion inside the yeast or other microorganism cytoplasm. Once inside the cytoplasm, propionic acid will dissociate to the corresponding salt since internal pH is above p*K*a value. The same process is also possible for acetic acid. Constant pumping of the protons released inside the cytoplasm causes physiological stresses impacting several metabolic pathways in yeast cells [31].

Length of fermentation and establishment of heterofermentative LAB population are now considered critical toward the establishment of a good aerobic stability level. The facultative, or obligate heterofermentative, strains of LAB have lower growth rates than homofermentative strains, including rods like *L. plantarum* or coccids of the genera *Leuconostoc*, *Enterococcus*, or *Lactococcus*. The growth conditions after several days of ensiling are also more restrictive for physiological activities considering the low pH usually encountered. The strains succeeding the earlier colonizer need to be more tolerant to both acidity and osmotic stresses, simultaneously. Observation of the succession of different species of LAB during the anaerobic stability phase often leads to high abundance of LAB belonging to the *L. buchneri* taxonomical group [32], leading to specific adaptation to this ecological niche by these strains. Although few physiological studies on *L. buchneri* strains had been published, Heinl and Grabherr recently published a complete analysis of the genetic potential of the strain CD034 compared to other genomes from public databases [33]. One of the comparisons performed aimed to describe how the genetic system of this species can cope with high concentration of organic acids, including lactic acid. The anaerobic conversion system of lactic acid to 1,2-propanediol

**161**

cially for larger producers.

*Lactic Acid Bacteria as Microbial Silage Additives: Current Status and Future Outlook*

(to acetic acid and CO2 under aerobic condition) represents one of those properties. It is possible to extend these observations to the results gathered from transcriptomic analysis on the strain *L. buchneri* CD034 [34] following the aeration of culture grown under anaerobic conditions. The team described the functions of 283 genes induced by the presence of oxygen. They also observed physiological adaptation related to changing oxygen concentration. Genes required by lactic acid fermenta-

Co-inoculation with different heterofermentative strains has recently been tested in the field or in commercialized conditions. This was the case for *L. buchneri* and *L. diolivorans*, tested on the fermentation of sourdough [35]. The authors showed an increase in the accumulation of propionic acid following the inoculation with both strains together. Co-inoculation of *L. buchneri* and *L. hilgardii* was tested in different ensiling trials [36, 37] inducing better fermentation and higher aerobic stability level. *L. hilgardii*, an obligate heterofermentative strain, was not only previously observed as a contaminant of wine but also represents one of the dominant LAB strains in water kefir [38]. Strains of this species are often observed in sugar cane silage [39, 40] and provide increased aerobic stability levels for this challenging crop. Improvement in fermentation and aerobic stability of sugarcane silage

Two recent meta-analyses [42, 43] provided a complete overview of the impact of inoculation of LAB and described the importance of fermentation and aerobic stability in relation to the specificities of the forages and the activity of homofermentative, facultative heterofermentative, and obligate heterofermentative strains. In particular, the meta-analysis of Blajman et al. [42] analyzed the role of inocula-

Improving aerobic stability to reduce overall losses during the storage and feedout is one of the main reasons to apply microbial inoculants on the forage at the time of ensiling. The value of silage inoculants is important, but optimal manage-

**5. Improving adoption of forage inoculant use by increasing awareness** 

According to the 2017 National Agricultural Statistics Survey [44] census report, approximately 120,000,000 tons of whole-plant corn alone was harvested for silage in the United States. Even with this huge quantity of silage, there is little reliable

Based on an independent market survey of U.S. beef and dairy producers, two thirds of respondents indicated that forage additives used on their operations are microbial based. The main reason for their use is to minimize mold and spoilage in silage. Other reasons cited include preventing heat damage and increasing herd productivity [45]. Most inoculant users plan on continuous using and investing in

Product performance, ease of use, and cost are the main influencers on the purchasing decision of inoculants. In addition, nutritionists and consultants are important sources for providing information on forage inoculants and the most involved outside sources in the purchase decision (personal communication). Most producers do not have a detailed understanding of the different types of inoculant products, but they instead recognize the value and return on investment (ROI) that these technologies can bring to their operation. Value-added services and education offered by inoculant companies are also reasons to purchase, espe-

*DOI: http://dx.doi.org/10.5772/intechopen.89326*

tion systems were hardly affected.

allowed increasing DM intake and milk yield [41].

tion on reducing the amount of yeast in silage.

ment of silos at all steps of the ensiling process is critical.

**of the economic value of forage inoculants**

this technology each year (personal communication).

survey data about the use of forage inoculants.

#### *Lactic Acid Bacteria as Microbial Silage Additives: Current Status and Future Outlook DOI: http://dx.doi.org/10.5772/intechopen.89326*

(to acetic acid and CO2 under aerobic condition) represents one of those properties. It is possible to extend these observations to the results gathered from transcriptomic analysis on the strain *L. buchneri* CD034 [34] following the aeration of culture grown under anaerobic conditions. The team described the functions of 283 genes induced by the presence of oxygen. They also observed physiological adaptation related to changing oxygen concentration. Genes required by lactic acid fermentation systems were hardly affected.

Co-inoculation with different heterofermentative strains has recently been tested in the field or in commercialized conditions. This was the case for *L. buchneri* and *L. diolivorans*, tested on the fermentation of sourdough [35]. The authors showed an increase in the accumulation of propionic acid following the inoculation with both strains together. Co-inoculation of *L. buchneri* and *L. hilgardii* was tested in different ensiling trials [36, 37] inducing better fermentation and higher aerobic stability level. *L. hilgardii*, an obligate heterofermentative strain, was not only previously observed as a contaminant of wine but also represents one of the dominant LAB strains in water kefir [38]. Strains of this species are often observed in sugar cane silage [39, 40] and provide increased aerobic stability levels for this challenging crop. Improvement in fermentation and aerobic stability of sugarcane silage allowed increasing DM intake and milk yield [41].

Two recent meta-analyses [42, 43] provided a complete overview of the impact of inoculation of LAB and described the importance of fermentation and aerobic stability in relation to the specificities of the forages and the activity of homofermentative, facultative heterofermentative, and obligate heterofermentative strains. In particular, the meta-analysis of Blajman et al. [42] analyzed the role of inoculation on reducing the amount of yeast in silage.

Improving aerobic stability to reduce overall losses during the storage and feedout is one of the main reasons to apply microbial inoculants on the forage at the time of ensiling. The value of silage inoculants is important, but optimal management of silos at all steps of the ensiling process is critical.
