DING Lei-lei, WANG Pu-chang, ZHANG Yu-jun, FAN Guo-hua, WANG Zhi-wei, MO Ben-tian,CHEN Juan, XIE Cai-yun

Pratacultural Institution of Guizhou, Guiyang 550006, PRC

Abstract Corn silage is the most popular and important source of energy and fiber for ruminants. Much research focused on the chopping length, additives, sealing system, harmful bacteria and their toxins, and so forth, has been done on corn silage both at home in China and abroad. Some suggestions were presented in this review:(1) Corn silage modulation: Whole-plant corn harvested at the milk-ripe stage with a dry matter (DM) content of 30%～40% was selected as silage material and treated by rubbing and cutting. In the process of silage, commercial Lactobacillus buchneri was inoculated, exogenous protease added appropriately, cellulose enzyme added depending on the phenological period, the silage container and the species of the animals, and coextruded polyethylene?polyamide oxygen barrier film (OB) selected as the seal material. (2) Strengthening management: Aerobic exposure should be avoided at the storage stage and reduced during the feeding phase. The proportion of corn silage was increased depending on the type of animal and their management.(3) Suggestions for further research: Further studies would be needed for the effects of liquid and solid wastes on soil, water and vegetation, and the treatment of these wastes.

Key words Corn silage; Chopping length; Additive; Sealing system; Animal type;Harmful bacteria and toxins

1. Introduction

Corn silage is widely used as a major source of energy and fiber for ruminants, especially dairy cattle, due to its good palatability and high energy[1]. It has several advantages over other herbage silage. In regards to the dietary dry matter (DM) and starch intakes, the digestibility and milk production were increased with the increase of the proportion of corn silage in the diet[2-5]. Increasing the proportion of corn silage could improve N utilization, as reflected by the decrease of ruminal NH3-N,manure N and urea N, and the increase of dietary N used for secreting lactoprotein, suggesting potentially low NH3and N2O emissions[4-5].The loss of methane energy was decreased with the decrease of enteric methane production[3-5]. Recently many innovations have been made in corn silage technology, both at home in China and abroad. Much research has been carried out on the chopping length,additives, sealing system, harmful bacteria and toxins,and so forth, which has greatly improved the yield,quality, and aerobic stability of corn silage, as well as the performance of the animals in the diet of which it is included.

2. Chopping length

Whole-plant corn chopped short for silage was prone to compaction in the process of silage modulation. Its fermentation quality was better than that of corn chopped long for silage and the loss of nutrients from the silage material also was lower[6]. In general, the thinner the whole-plant corn was chopped,the better was the fermentation. Some studies have shown that the length of the corn silage could change the performance of an animal by regulating the animal’s feeding behavior. The neutral detergent fiber (NDF) intake was increased with the increase of the length of the corn silage and thus the chewing activity was increased[7-8], while the DM intakes and total volatile fatty acids in the rumen were increased with the decrease of the length of the corn silage,and the total chewing time, including foraging and ruminating time, varied as a single grain quadratic curve with the increase of the length of the corn silage.Increasing the total mixed ration (TMR) to a length greater than 19.0 mm could result in a high potential of total chewing activity and selective foraging[9].Short corn silage in the TMR could increase the production of milk, protein and lactose[9]. Therefore,increasing the length of the corn silage could increase the DM intake, improve ruminal fermentation and reduce selective foraging. However, the dairy cattle fed a TMR with short corn silage would change their feed preferences for long corn silage when they were nearly subject to subacute ruminal acidosis[10].The mechanical treatment of whole-plant corn can improve silage characteristics and reduce DM loss,because the fragmentation of corn cobs, skins and stalks enhances the digestibility of starch and fiber[11].It also can reduce the milk fat rate and yield, increase the total digestive tract digestibility of starch and reduce the total digestive tract digestibility of NDF[12].In comparison with treatment by grinding and cutting,the treatment of corn silage with rubbing and cutting produced the best fermentation quality and digestion effect[13].

3. Additive

3.1. Bacterium

Improving the aerobic stability of corn silage through inoculation with lactic acid bacteria was more efficient than selecting the optimal stage of whole-plant corn or changing the mechanical treatment method[14].Homofermentative lactic bacteria can decrease the aerobic stability of corn silage, while heterofermentative lactic bacteria can increase the acetic acid production and aerobic stability of corn silage[15-16].Compared with the addition ofLactobacillus plantarumand distilled water in corn silage, the addition ofLactobacillus buchneriand propionic acid can reduce yeast counts and improve aerobic stability[17].Corn silage withLactobacillus buchnerihad a higher concentration of acetic acid and a lower concentration of ethanol than that treated with nothing orLactobacillus plantarum, and silage withLactobacillus plantarumhad the lowest aerobic stability[18]. Compared with corn silage treated with a high dose ofLactobacillus plantarumand propionic acid, corn silage treated with a high dose ofLactobacillus buchnerican achieve a high concentration of acetic acid and a low amount of yeast,resulting in more heterogeneous fermentation, thus,significantly improving the aerobic stability[19]. The effect ofLactobacillus buchnerion the improvement of the aerobic stability of corn silage was the best and was commercially available[15]. Compared with the commercialLactobacillus buchneri, localLactobacillus buchnerigave rise to a low concentration of lactic acid, a high concentration of propanediol and acetic acid, and a moderate level of aerobic stability[20]. Inoculation withPropionibacteriumalone or mixed with beer yeast had little effects on pH,water soluble carbohydrates, lactic acid and volatile fatty acids[21]. Increasing the amount of yeast can increase ethanol production[22]. Hence, the commercialLactobacillus buchneriwas recommended for use in corn silage, not localLactobacillus buchneri.

3.2. Enzyme

Supplementing corn silage with exogenous protease can enhance the in vitro digestibility of starch, as well as increase the levels of NH3-N and soluble protein, with the increase in the time of storage[23]. High doses of exogenous protease can cause rapid fermentation of corn silage with different dry matter contents, and produce more lactic acid and acetic acid, leading to an advanced peak of soluble protein content, but can accelerate the decomposition of protein, and the in vitro digestibility of starch,to reach the corresponding level ahead of time[24].Therefore, the amount of exogenous protease added to corn silage was an issue that needed to be addressed.

An increase in the cellulase dosage can linearly reduce the acidic detergent fiber (ADF) content,the neutral detergent fiber (NDF) content and the hemicellulose content of corn silage, but it can decrease the acid detergent lignin (ADL) content only at the milk-ripe stage and black layer stage[25].Corn silage supplemented with endoglucanase,exoglucanase and xylanase can increase the in vitro digestibility of ADF and NDF[26]. The addition of cellulase and hemicellulase can reduce the ADF and NDF content of corn silage stored in horizontal silos,but had no effect on the fiber content of corn silage in bags and on the the milk production of cows; it had little effect on fermentation and an inconsistent effect on different fibers; it can increase the deposition of N in lambs[27]. As a result, the addition of cellulase would depended on the phenological period, the silage container and the species of the animals.

3.3. Nitrogen

The addition of nitrogen such as urea, was a popular method for supplementing corn silage[28].Compared with no NH3-N, corn silage supplemented with NH3-N can rapidly enhance the pH and had no effect on lactic acid bacteria production, but it would delay the growth of lactic acid bacteria, slow down the intestinal bacteria decline rate, increase the yeast and mycete decline rate, and delay the growth of yeast and mycete after exposure to oxygen[29]. Corn silage supplemented with 0.3% NH3-N can significantly enhance the aerobic stability[29]. Adding whey products to corn silage supplemented with urea N can protect the urea nitrogen added to corn silage, because the lactic acid, acetic acid and pH of corn silage with urea were high and not affected by whey[30]. Therefore, N should be added to corn silage.

3.4. Acid

The combination of potassium sorbate and EDTA or only benzoic acid to corn silage as preservatives can achieve the highest concentration of water-soluble carbohydrates, dry matter recovery and aerobic stability, and the lowest concentration of ethanol[18,31].A high dose of potassium sorbate can reduce ethanol,ethyl acetate and methyl acetate concentration, but had no significant effect on methanol and n-propyl alcohol. A low dose of potassium sorbate can increase the ethanol concentration but had no effect on ethyl acetate. Therefore, a high dose of potassium sorbate can inhibit the production of volatile organics[32].Compared with benzoic acid, sodium benzoate gave rise to high soluble carbohydrate residues, high aerobic stability, low dry matter loss and high digestibility[20].A low dose of propionic acid would not affect the fermentation of corn silage, but would effectively reduce the heat of the silage and the TMR[33].Propionic acid can inhibit the production of lactic acid and maintain effective sugars, which can greatly increase lactic acid during the aerobic exposure stage and thus produce greater aerobic stability[34]. Mixed acid can improve corn silage quality. The growth performance of sheep was improved by feeding them silage treated with 0.6% mixed acid[35]. The mixture of sodium benzoate, potassium sorbate and sodium nitrite added to high moisture silage (DM＜63%), can significantly improve aerobic stability and dry matter recovery[36]. Therefore, acid can be appropriately added to improve the aerobic stability of corn silage.

3.5. Others

Adding molasses to corn silage can improve lactic acid production and aerobic stability. Adding dry by-products such as pistachios can reduce the gas production, the in vitro digestibility of the organism and metabolic energy. Increasing the amount of dry by-products can increase the propanoic acid content and reduce the NH3-N content and in vitro ruminal methane production[37].

4. Sealing system

An anaerobic environment was the guarantee for good fermentation of silage such as corn. Sealing materials were crucial for maintaining silage quality,fermentation characteristics, digestibility and animal production performance. Some studies found that the oxygen permeability of coextruded polyethylene/polyamide oxygen barrier film (OB), coextruded PE/polyvinyl alcohol film (PVOH), polyethylene film(PE) and polyvinyl chloride film (PVC) increased,successively[38]. OB and PVOH films had better temperature and fermentation characteristics than the others. Moreover, OB film had the best effect in reducing DM losses, followed by PVOH film, PE film and PVC film, successively. There was no difference in pH value, lactic acid and aerobic bacteria quantity after 365 d silage sealed with the oxygen barrier film and the standard polyethylene film. However, mould was found in the silage sealed with the standard polyethylene film, suggesting that the oxygen barrier film may be more effective than the standard polyethylene film in inhibiting the proliferation of aerobic decay microorganisms[39]. The corn silage covered with high impermeability to oxygen film made with ethylene-vinyl alcohol had higher lactic acid content, lower pH, lower amount of yeasts,molds, and aerobic and anaerobic spore-formers,lower DM losses and higher aerobic stability than that covered with the standard polyethylene film[40].After silage was exposed to air, paenibacillus counts increased greatly. Oxygen barrier film can decrease the possibility of silage suffering from microorganism contamination resulting in milk spoilage by delaying aerobic spoilage[41]. Consequently, it was suggested that corn silage should be sealed with OB film.

5. Harmful bacteria and their toxins

The whole-plant corn for silage was infected by disease and the silage materials came into contact with yeast and fungi naturally. Both can affect the corn silage quality, thus leading to animal disease,production loss and animal production being subject to contamination, finally affecting human health[42]. For example, 6 kinds of fusarium and aspergillus found in corn silage ensiled for 11 months all can produce mycotoxin. Three kinds of different concentrations of mycotoxins were detected from the top to the bottom of silage, of which the trichodermin with strong immunosuppression capacity was up to 877 ppb[43].Listeria bacteria can be detected on the surface of the corn silage, as well as in the trough of cows, sheep and goats, in the milk of cows and sheep, but not in goats milk[44]. The aerobic exposure at the stage of storage and feed can increase aflatoxin[45]. The good fermentation of silage was related to additives that can reduce harmful bacteria and their toxins. The ochratoxin A, fumonisin B1 and the deoxynivalenol concentration of corn silage before fermentation were significantly reduced compared to that after fermentation[46]. The increase of rust infection degree will reduce the nutritive value and fermentation quality of corn silage, but inoculation with double fermentation bacteria can reduce the adverse impacts of rust infection on the fermentation quality, increase the NDF digestibility of corn silage with moderate rust infection and no rust infection, and reduce the aerobic spoilage and aflatoxin production of corn silage with high rust infection[47]. The corn silage with pH less than 4 can eliminateEscherichia coliand withlactobacillus buchnerior propionic acid can reduce or inhibit the growth ofEscherichia coliafter aerobic exposure[17].

Consequently, the harmful bacteria and their toxins of corn silage can be reduced by ensuring good fermentation of the corn silage, the appropriate addition of additives such aslactobacillus buchneri,avoiding aerobic exposure at the storage stage and reducing aerobic exposure at the feeding stage.

6. Animal types

Some research showed that increasing the dietary corn silage proportion in the dairy cow TMR can increase milk production, whereas that in the steer TMR can decrease the daily gain[5]. Increasing the ratio of feed to gain and supplementary feeding with corn silage forage for grazing dairy cows also can decrease the daily gain[48-49]. Therefore, the effect of increasing the dietary corn silage proportion on animal production performance also depended on the type of animals and their management.

7. Conclusion and prospect

In conclusion, the advice for silage modulation and use were as follows: rubbing and cutting were used to treat whole-plant corn. In the silage,commercialLactobacillus buchnerishould be inoculated, exogenous protease can be added appropriately, cellulose enzyme was added depending on the phenological period, the silos and the species of the animals. OB film was selected as sealing material.Aerobic exposure should be avoided at the storage stage and reduced at the feeding stage to strengthen management. The corn silage proportion in the diet was increased depending on the type of animal.

Corn silage is the most popular forage for ruminants and has been widely used in recent years.As an important means of adjusting grain feed, it is of great significance to agricultural structure adjustment and agricultural supply-side reform. On the other hand, the liquid and solid waste discharge from corn silage, especially corn silage with horizontal silos, is likely to contaminate the surrounding soil and water and poison vegetation, but little is known about it.Therefore, it should be further studied.