Effects of agro-industrial byproduct on fermentation quality and nutritional composition of elephant grass (Pennisetum purpureum Schum) silage

Muhammad Rusdy

Department of Forage Science and Grassland Management, Faculty of Animal Science, Hasanuddin University, Indonesia
muhrusdy79@yahoo.co.id

Abstract

The effects of agro-industrial byproducts as additives  on fermentation characteristics and nutritional quality of elephant grass were reviewed. PH value, NH₃-N/TN, lactic acid, acetic acid and butyric acid contents were used to evaluate fermentation quality, while crude protein, NDF, ADF, cellulose and hemicellulose contents were used to evaluate the nutritional quality of elephant grass silage.

Molasses, cassava meal, maize meal and wheat bran produced well preserved silages as indicated by their pH, NH₃-N/TN, lactic acid, acetic acid and butyric acid contents.  Almost all  additives  increased crude protein and reducing NDF and ADF contents of elephant grass silage. Cotton seed meal and defatted rice bran produced silage with higher crude protein compared to the other additives. It can be concluded that many agro-industrial products can be used to produce well preserved silage and increasing the nutritional quality of elephant grass silage.

Keywords: agricultural-byproducts, ensiling, napier grass, nutritive value, preservation

Introduction

In densely populated area of Indonesia, scarcity of  feed for animals, especially during the dry season  has been a major hindrance to smallholder farmers to increase their animal production.  To alleviate the problem, the government has encouraged the farmers to grow elephant grass (Pennisetum purpureum Schum.), a grass that has  high production potential and good nutritional value. As results, at the present time, the grass has been the most widely cultivated fodder in Indonesia. Elephant grass has been the most promising and the high yielding fodder, giving dry matter yields that surpass most other tropical grasses (Humphreys 1994). Dry matter yield of 84,800 kg/year has been reported in Puerto Rico when the grass grown under natural rainfall of 2000 mm/year where 897 kg of N/ha/year  were applied  and the grass was cut every 90 days (Vicente-Chandler et al 1959). Its crude protein, cell wall constituents  and digestibility are fairly good, provided it is harvested at intervals of 6 – 8 weeks of regrowth (Rusdy 2016).

Although the grass has a high dry matter potentials, most of the yield is normally realized in the rainy season and the surplus  yield of this grass during the rainy season is generally  wasted. Stockpiling of this grass is generally not an effective storage method because  during the dry season, the grass becomes stemmy and its the nutritive value was  low.   Conservation of elephant grass harvested during the rainy season by hay making is difficult because of uncertainty of weather  condition during the rainy season in the tropics and the its large stem make it is  difficult to dry.  Conservation by silage making may be the best option to  supply  forage for the whole year and  provide forage of high nutritive value for animals during the dry season (Yitbarek and Tamir 2014). However , like most of other tropical forages, elephant grass is not suitable for making good silage because it has a low levels of  water soluble carbohydrates (2.7% DM)  (Yahaya et al 2004) and  high moisture levels (Yokata et al 1991),  which hamper the good fermentation and  often results in poor-quality forage. Therefore, to obtain a well preserved  silage, elephant grass should be ensiled with silage additives that contain low moisture but have high soluble carbohydrate levels.  

In some  rural areas in the   tropics, there are abundant silage additives from agro-industrial byproducts like molasses, maize meal, cassava tuber meal, rice bran and wheat bran that have been widely used and studied to obtain well preserved silage. These additives aregenerally suitable to preserve elephant grass silage  because they  can act as absorbent (except molasses) and contain  high soluble carbohydrate and nutrients.  There is limited information on efficacy of these agro-industrial byproducts on silage preservation. This paper attempts to review some work on the use of agro-industrial byproducts  as additives on fermentation quality and nutritional composition of elephant grass silage.  

Fermentation quality

Well preserved silage is characterized by lower pH, greater lactic acid content, lower contents of NH3-N/TN, acetic acid and butyric acid.  Additives had different effect on pH of elephant grass silage (Table 1).   pH values of  elephant grass silages containing molasses, cassava meal, maize  meal, and wheat bran ranged between 3.73 and 3.97, close to the 3.8 to 4.2 as ideal pH for a silage of good fermentation quality, as reported by McDonald et al (1991).

Conversely, pH values of the elephant grass silage without additives, silage with  plain rice bran or defatted rice bran 15%   were above the recommended pH standard of McDonald et al  (1991). This suggested that elephant grass silages without additives or added with  rice bran 15% increased the growth of clostridia and other aerobic bacteria. This  result might in part, be due to the lower soluble carbohydrate levels in control and   rice  bran compared to  molasses, cassava,  maize meal and wheat bran.  Epiphytic lactic acid bacteria are known to ferment the  soluble carbohydrates to lactic acid and to lesser extent to acetic acid under anaerobic conditions, resulting in lower pH values (Olubajo et al 1989). 

The concentration of NH₃-N/TN is indicative of protein degradation during the ensiling process. According to McDonald et al (1991), in order to be considered as having good quality, silage should have NH₃-N/TN content of maximum 10%. In the present study,  except for silage with  cassava scraping and  rice bran 15%,  all NH₃-N/TN values  were below this 10% value. This indicates that there was increased protein degradation  with addition of cassava scraping and rice bran.  The higher NH₃-N/TN content of elephant grass silage with  rice bran was correlated with the higher pH value (Table 1).  This is in agreement with  Muck (1996) that silage with high pH preserves less the protein compared with silage of low pH.  The relative high proportions of ammonia-nitrogen  in elephant grass silage with acassava  scraping as additive are difficult to explain, but it may relate to the high nitrogen levels of elephant grass at harvest. 

The acids commonly used to evaluate the quality of  fermentation of silage are lactic acid, acetic acid and butyric acid.   Generally, in well preserved silage, at least 65 – 70% of the total acids will be lactic acid (4 – 7% in DM). The lactic acid should appear at higher concentrations than the others.  Acceptable silages generally contain <3% acetic acid, and <0.1% butyric acid (Ward and Ondara 2016). Lactic acid generally is associated with higher quality silage because this fermentation pathway results in the least loss of DM (McDonald et al 1991).  

In the present review,  additives of molasses, cassava, maize meal and wheat bran showed  a lactic acid  type fermentation with most of their lactic acid contents  above 4% (Table 1). This indicates that the silages  with  those additives were of high quality,  because fermentations that produce predominant lactic acids results in  the lowest losses of DM and energy (Kung and Shaver 2016).  The lactic acid is known to have greater acidification power in comparison with the other acids and responsible for the quick drop and maintenance of low pH. This might be related to  the higher soluble carbohydrate content of those additives.     

Both plain rice bran and defatted rice bran showed butyric acid and acetic acid type fermentations, respectively  (Table1). Silage that has undergone a long fermentation sometimes contain higher levels of acetic acid. A slow drop  to a low pH increases the amount of protein breakdown (Muck 1988).  Presence of acetic  acid producing bacteria in silage also is undesirable, as they are able to  oxidize lactic acid and  acetic acid to CO₂ and H₂O. In the present study, almost all additives yielded silage with negligible butyric acid levels, except untreated rice bran. High levels of  butyric acid (0.5% DM) in silage  are usually indicative of low nutritive value as many of the soluble nutrients, like crude protein are degraded to ammonia nitrogen by clostridial fermentation (Kung and Shaver 2016). This indicates that untreated rice bran is not not good to be used as additive for preserving elephant grass silage.

Nutritional composition

In general, addition of agro-industrial byproducts  increased crude protein content of elephant grass silage. The highest increase in crude protein was found in silages with added defatted rice bran and cotton seed cakes (Table 2).

The increased protein content of elephant grass silage with  additives  indicates that the additives preserved more protein compared with control. The high soluble carbohydrate contents in the additive might promote the lowering of  pH value, which inhibited protein degradation. The plants generally undergo less proteolysis if they contain high soluble carbohydrate levels. Once the pH has reached 5.0 or below, most plant protease activity ceases (Soderfund 2016). Besides, the higher crude protein contents in elephant grass silage with additives might be attributed to the high crude protein contents of some additives. The higher crude protein content of defatted rice bran (21%%)  and  cotton seed meal (25%) might be the reason of the high crude protein contents of elephant silages treated with both additives.

The contents of NDF and ADF of silages decreased with addition of the additives (Table 2). This is expected since all additives had a lower proportion of NDF and ADF than the elephant grass. Besides, the higher  soluble carbohydrate levels in the additives might promote the lowering pH  that enhanced hydrolysis of fiber component.   It is suggested that   in addition to soluble carbohydrates, cellulose and hemicellulose as product of enzymatic degradation of NDF and ADF by bacterial action can act as substrate for acid fermentation during ensiling (Yahaya et al 2002).

Jurjanz and Monteils (2005) noted that in maize,  hemicellulose tended to be degraded in the ensiling process while cellulose  was little degraded. In the present review, ensiling with additives did not consistently influence the hemicellulose content, except for silage treated with  cotton seed meal (Table 2). Even in some research, some additives like molasses (Bureenok et al 2012; Mtengeti et al 2006), wheat bran (Cheng and Peng 2004) and defatted rice bran (Yokota et al 1998) produced silages in which their  hemicellulose contents  were higher than control.    It is strongly suggested that, irrespective of chemical analytical methods used, in the ensiling of elephant grass with additives, both hemicellulose and cellulose  were degraded, as occurred in elephant grass silage added with cotton seed meal (Table 2). Which one of the two organic acids is more degraded in ensiling needs further study.

NDF values are important because they reflect the amount of forage the animals can consume. As NDF percentage increases, dry matterintake generally declines. ADF values are important because they relate to the ability of animals to digest forage. As ADF increases, digestibility of forage declines. In the present review, the  lower NDF and ADF values as affected by ensiling with additives indicate that agro-industrial products can be used to increase consumption of elephant grass silage with higher levels of digestibility.

Conclusion

• Except for  rice bran that produced  silage with  higher pH and NH₃-N/TN contents and cassava scraping that produced silage with higher NH₃-N/TN content, all other additives produced silage with good fermentation quality.

  
  

• Most additives increased crude protein content but the highest increase of crude protein was found in silage treated with cotton seed meal and rice bran. 

  
  

• All additives reduced NDF and ADF contents of elephant grass silages.

  
  

• Most agro-industrial byproducts are  suitable to be used to improve fermentation quality and nutrient recovery of elephant grass silage.

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Received 25 July 2016; Accepted 6 November 2016; Published 1 December 2016

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