Silage fermentation quality of garlic stalk
The silage fermentation metabolites and chemical composition of GS ensiled for 7, 14, or 28 d, are presented in
Table 1. Initial pH before ensiling was 8.06, which declined (p<0.01) as the length of ensiling prolonged (a 1.26-unit decline by d 28 of ensiling). Lactic acid concentration was greatest after 28 d of fermentation. This is associated with the development of LAB which tended to increase (p = 0.07) as fermentation duration continued. With ensiling, WSC content decreased while NH
3-N formation increased considerably (a 4.4-times increase after 28 d of ensiling), which indicates the microbial conversion of WSC into lactic acid and protein breakdown during ensiling, respectively [
5,
19]. The chemical composition of GS silage differed negligibly as ensiling duration progressed. Crude protein content declined slightly with ensiling (p< 0.002), which can possibly be explained by the protein breakdown (proteolysis) during ensiling, which releases NH
3-N [
5].
An approximate number of 8 log
10 cfu of LAB per gram of fresh biomass is the required LAB number to ensure the fast decline of silage pH [
20]. In the present experiment, although the population of epiphytic LAB was adequate (7.93 log
10 cfu/g) to promote fast acidification and efficient ensiling, the silage fermentation was not successful, as evidenced by high NH
3-N content and high silage pH after 28 d of fermentation. This suggests that GS is a difficult-to-ensile biomass. This can be justified by three explanations: i) The GS contains an inadequate content of WSC, which probably did not provide sufficient sugars for lactic acid production, and thus silage mass acidification [
5,
6]. ii) The GS contains a high ash concentration (approximately 15%), which would increase the buffering capacity, thus resisting silage mass acidification [
5]. iii) GS has a hollow and tubular structure [
4] that increases the porosity of GS mass, accelerates the aerobic microbial activity during the early phase of ensiling, and thus converts readily available carbohydrates into carbon dioxide and water [
5]. These factors collectively result in an inefficient anaerobic fermentation and slow acidification [
20]. Based on these assumptions, the follow-up experiment attempted to improve the fermentation quality of GS with LAB inoculation in the presence of molasses as a source of WSC.
The silage fermentation dynamics of GS inoculated with LAB in the absence or presence of molasses in relation to the length of ensiling (7, 14, and 28 d), are presented in
Table 2. Generally, the fermentation quality parameters of GS silage treated with a combination of LAB+M were preferable to those of untreated or LAB-treated silages. After 7 d, silage pH declined from the initial value of 7.7 in the control silage to 5.8 in the LAB+M silage, indicating that the combination of LAB and molasses promoted the favorable silage fermentation pattern.
Inoculation with LAB prior to ensiling has been successful in the promotion of desirable fermentation patterns. However, when the epiphytic population of LAB exceeds the inoculant application rate, the domination of inoculant bacteria in the silage is difficult [
21]. In the present experiment, the natural population of LAB in GS was close to 8 log cfu/g of fresh GS (prior to ensiling), which considerably exceeded the LAB application rate (10
6 cfu/g of fresh GS). Surprisingly, LAB+M treatment promoted the more favorable fermentative patterns (with respect to a lower pH and less NH
3-N production) than untreated or LAB-treated silage. After 28 d of ensiling, LAB+M-treated silage showed a 35% increase in lactic acid content and a 0.9-unit decline in pH, with respect to the LAB-inoculated silage. In support, Huisden et al [
19] reported that the addition of molasses to corn silage supplied an extra source of WSC for LAB metabolism, which possibly allowed their domination in the microbial community of the silage, thereby stimulating favorable silage fermentation patterns.
Earlier findings confirmed that the combined use of the LAB inoculant and molasses improved the silage quality indices of various crops [
7,
8,
22]. For example, our recent investigation [
8] found that the combination of LAB and 5% molasses efficiently improved the silage quality parameters of spent mushroom substrate in both laboratory-scale and ton-bag silos. Presently, when GS was ensiled without molasses or a microbial inoculant, silage pH slightly declined (6.80 after 28 d of ensiling), which was accompanied by the high NH
3-N accumulation (631 ppm). However, when GS was ensiled with LAB+M, the silage pH dropped to 5.3 (after 28 d of ensiling) and as anticipated, less NH
3-N was formed (
Table 2). This observation was expected, because when silage pH declines more quickly, excessive proteolysis is suppressed during ensiling, which contributes to less NH
3-N formation [
5].
The chemical composition of GS ensiled with LAB in the absence or presence of molasses in relation to the length of ensiling, is presented in
Table 3. No difference in the chemical composition was seen after 7 d of ensiling. However, as ensiling duration prolonged, the NDF concentration decreased in LAB+M-treated silage, which is possibly associated with the hydrolysis of cell wall fractions that are more digestible during ensiling fermentation [
19].
Co-ensiling of garlic stalk with citrus pulp
This experiment evaluated the co-ensiling of GS with CP as an alternative storage technology, which has been successful in improving the silage fermentation quality of several feedstuffs [
23]. For example, the recent co-ensiling of wheat straw with sugar beet waste has proved to be a successful storage method for their long-term preservation [
24]. The typical silage quality parameters and chemical composition of GS and CP, ensiled alone or together (GS 70%+CP 30%) after 7, 14, or 28 d of ensiling, are reported in
Tables 4 and
5, respectively. Generally, GS ensiled with CP exhibited more desirable fermentation characteristics in comparison to GS silage alone. For example, after 14 d of ensiling, the GS 70%+CP 30% silage exhibited a 56% increase in lactic acid content which represented a 0.4-unit decline in silage pH, with respect to GS ensiled alone. The nutrient composition of GS 70%+CP 30% silage was comparable to that of GS ensiled alone. However, after 14 and 28 d of ensiling, a greater NFC concentration was recorded for GS 70%+CP 30% silage, which is associated with an NDF content that diminished as ensiling time continued. For example, after 28 d of ensiling, NDF concentration decreased by 7.5% with respect to GS silage, which was possibly caused by the dissolution of the more digestible NDF fractions during ensiling [
19]. These findings showed promise in the possibility of the successful preservation of GS when co-ensiled with CP, leading to the follow-up experiments to determine the proper mixing level of GS and CP for efficient silage fermentation.
The pH of GS co-ensiled with incremental proportions of CP is illustrated in
Figure 1. After 14 d of ensiling, the pH of GS 50%+CP 50% silage declined markedly, which is suggestive of successful silage fermentation that is known to slow down or inhibit the growth of fungi, molds, and yeasts [
25]. Consistent with our findings, Migwi et al [
23] reported that when 5% molasses was added to the mixture of wheat straw+ broiler litter, no effect on silage pH was detected. However, when the proportion of CP increased from 0 to 30% in the silage, the pH declined markedly [
23]. Similarly, when CP was ensiled alone, lactic acid concentration was negligible (1.02% of DM, after 14 d of ensiling;
Figure 2). However, when CP was co-ensiled with GS at a 50:50 proportion, lactic acid production increased markedly (6.0% of DM, after 14 d of ensiling).
The silage fermentation, sensory, and physical parame ters of GS co-ensiled with incremental proportions of CP are shown in
Table 6. After 2 weeks of fermentation, despite the substantial increase in lactic acid production with increasing CP proportion (
Figure 2), total viable colonies of LAB remained unaffected with increasing proportions of CP in the silage. This might be explained by the counting technique used, as this requires actively growing cells. It has been proposed that many LAB exist on the biomass surfaces that are viable but not culturable by using the traditional plating methods [
26]. Assuming this proposition, it appears that the increased availability of metabolic water provided by the increased proportion of CP into GS biomass supported the higher metabolic activity of the microbial ecology of silage, particularly LAB [
27]. In agreement, Muck [
28] ensiled alfalfa at different DM levels (17% to 73%) and found that the greatest levels of lactic acid were produced in silages of 40% to 55% DM. Likewise, Mthiyane et al [
29] found that when water was added to sugarcane tops silage, lactic acid production was restricted, which was explained by the multiplication and domination of heterofermentative LAB in high-moisture silage, as this generates the mixed metabolites of acetic acid, ethanol, and lactic acid. Essentially, silage making at the optimal moisture level should provide enough moisture for LAB growth and metabolism to decrease pH and prevent the growth of putrefactive microorganisms, while also providing enough dryness to minimize effluent production [
5,
13]. Presently, it appears that the 50:50 mixing proportion provided the appropriate moisture level that favored the metabolic activity of LAB in the mixed silage of GS and CP, and thus promoted substantial lactic acid production. However, the reasons why lactic acid production increased with moisture alterations of the silage are still not clear and necessitate more studies to identify the contributory factors to this observation.
As the GS:CP mixing proportion decreased, NH
3-N concentration lessened, which is indicative of diminished proteolysis and deamination during ensiling [
5]. After 14 d of ensiling, NH
3-N content accounted for 25.4% of the total N in GS 50% +CP 50% silage, which is approximately 70% less than the silages with 30% or 40% CP. The fast acidification of silage mass suppresses the growth of putrefying microorganisms, this probably occurred as the mixing ratio of GS:CP decreased, resulting in a lower formation of NH
3-N, which is associated with the improved utilization of silage-N and thus microbial protein synthesis in the rumen [
20]. Decreasing the porosity of forage silage mass using supplementary feed sources, such as corn meal, with fine particle sizes may accelerate the silage pH decline, diminish inefficient plant respiration (less heat generation), and thus promote effective anaerobic fermentation [
30]. This series of events is known to minimize proteolysis, which is accelerated at high silage pH and temperature [
20]. It appears that the co-mixing of CP and GS decreased the porosity of the GS mass, which thereby promoted efficient silage fermentation that minimized NH
3-N formation.
No putrid odor or moldy spots were detected in the ex perimental silages (
Table 6), which suggests that the silages underwent a successful fermentation. However, fermentation odor increased and garlic odor decreased as the incremental levels of CP were incorporated into GS silage. The absorption degree was highest when 40% or 50% CP was incorporated into GS, which indicates that the effluent lost from CP was efficiently assimilated into GS biomass. This finding is of great importance as CP has a high moisture content and its ensiling alone would cause problems associated with effluent and silage nutrient losses [
12], which can be minimized when co-ensiled with GS at the proper mixing ratio. Overall, this experiment found that when GS and CP were combined at a 50:50 proportion, the best ensiling and physical properties were achieved, as demonstrated by the domination of LAB, low final pH, substantial lactic acid formation, and negligible effluent loss, thus opening an avenue for the simultaneous and long-term conservation of these waste residues.
The chemical composition of GS co-ensiled with incremen tal proportions of CP is shown in
Table 7. As the proportion of CP increased in the silage mass the DM, NDF, and ether extract contents declined, and NFC content increased as expected. After 14 d of ensiling, GS 50%+CP 50% silage resulted in a 6.9% reduction in NDF content, with respect to GS ensiled with 30% CP, resulting in a higher NFC concentration, which is suggestive of the improved feed-nutritional quality of the mixed silage of GS and CP for ruminant feed.