Straw ammoniation is a process by which low-value forage such as corn stalks, rice straw, wheat straw, and straw of other crops is ammoniated. Adding liquid ammonia, urea, or ammonium bicarbonate as ammonia sources result in the straw lignin being completely degraded, while the nutrients are enhanced. It is made more easily digestible by rumen microorganisms, which increases the digestibility of forage.
Straw silage refers to forage that is prepared through the fermentation of chopped fresh green fodder, forage grass, and all kinds of vines and other materials by lactobacillus in the anaerobic conditions of an airproof silage container (tower or silo).
Straw ammoniation and silage is one of the technologies that can be considered as part of livestock management. An overview of livestock management and related technologies is given in the article 'Livestock management'.
The cellulose part of the straw can be digested and utilised by ruminant animals, while the lignin part cannot be digested. The main function of ammonisation is to generate ammonolysis reaction using ammonia and straw, by damaging the ester bonds between lignin and polysaccharide, so that it can contact with digestive enzymes more easily, with an improvement in digestibility of straw. The digestibility and feed intake of ammoniated straw can be increased by approximately 20%, and the content of crude protein in ammoniated straw can be increased by two to three times (Guo, 1996).
To make straw silage, fresh plants are tightly packed in the airtight container, and the sugar contained in the raw materials is converted into organic acids (mainly lactic acid) via the anaerobic fermentation of microorganisms (mainly lactobacillus). When the lactic acid in the silage material reaches a certain concentration (pH lower than 4.2), the activities of other micro-organisms are inhibited and the nutrients in the materials are prevented from being broken down or destroyed by micro-organisms. For this reason the nutrients in the forage can be retained. A great deal of heat is produced during the process of lactic fermentation. When the temperature of silage material rises to 50°C, the activities of lactobacillus stop, and the fermentation is over. As the forage for silage is stored under airtight conditions with no microbial activities, it can remain unchanged for a long time.
Straw ammonisation mainly includes the following procedures:
- Selection of raw materials: all kinds of crop straw with good quality, no mould, and a water content of no more than 13%, such as straw, corn stalks, and wheat straw and other agricultural by-products such as rice husk and cotton seed hull can be used as the raw materials for ammonisation.
- Ammonia source and its dosage: Ammonia source includes liquid ammonia, urea, and ammonium bicarbonate, of which urea is the most commonly used. The dosages of urea and ammonium bicarbonate are about 3-5% and 8-12%, respectively.
- Ammoniation container: Containers can be a cement ammonisation pond (cellarage), plastic (Figure 4.1), water tank, basin, etc.
- Ammoniation method: According to the ammonia source used, the ammonisation method is divided into liquid ammonia treatment method, urea treatment method, ammonia water treatment method, ammonium bicarbonate treatment method, urea and lime treatment method, and so on, of which urea treatment method is promoted with the fastest speed by China and other developing countries (See figure 4.1). The urea treatment method is more flexible, and it can be carried out in stacking, ammonisation furnace, and other ammonisation containers.
- Ammoniation time: The length of ammoniation time is closely related to the ambient temperature, which is generally 2-3 weeks in summer, 3-6 weeks in spring and autumn, and 8 weeks or longer in winter. It is stored in a bag at the temperature of 20-30°C for 7-14 days, however it takes only 5-7 days when the atmospheric temperature is higher than 30°C.
- The preservation and utilisation of ammoniated straw: Ammoniated straw can be stored in the stack or other containers for more than six months. If the straw is treated with urea or other ammonia sources, the moisture content of straw is relatively high. It should be taken out from the ammonisation container according to the desired feed quantity before feeding, and then dried in a well-ventilated place for 10-24 hours to let the remaining ammonia volatise. It cannot be fed until there is no ammonia odor to irritate eyes and noses, but it should not be over dried to avoid adverse influence on the ammonisation effect. The ammonisation container should be resealed each time material is taken out.
Straw silage production mainly includes the following procedures:
- Selection of silage materials: Forage for silage can come from a wide variety of sources. Generally gramineous crops, leguminous crops, root tubers, stem tubers, aquatic feeds, and leaves can all be used for silage. Currently the most frequently used material is silage corn (with ears), followed by the snapped corn, sorghum stalks, fresh sweet potato vines, wild grass, and alfalfa.
- Timely harvest: Corn stalks are reaped for silage when wax ripe and yellow-green leaves both account for half.the stalk itself. The method of reaping vines before frost and harvesting sweet potatoes after frost should be adopted for the silage of sweet potato vines. Gramineous and leguminous forage grass should be reaped at the heading and full flower stage, respectively, and then they are mixed into the green corn stalks with the proportion of 1:2 for silage.
- Regulation of moisture content: If the harvested silage materials have high moisture content, they can be appropriately dried in the field for 2-6 hours after being harvested to reduce the water content to 65%-70%. If the crop straw has low moisture content, they can be added with water or mixed and filled with the newly-cut green materials to adjust the moisture content.
- Silage container: It mainly includes silage tower, silage trench, silage pillar, silage bag, or bale silage. Silage sites should be chosen in high and dry places with good drainage and convenience to prepare and access silage (See Figure 4.2).
- Chopping: After the forage is transported to the selected place, it must be promptly chopped by the chaff cutter. The chopping length depends on the type of forage. Generally, green corn stalk is chopped to 1.5-2.5cm and fresh sweet potato vine to 2-4cm. The shorter the cutting length is, the tighter it can be compressed when filling, which is more conducive to eliminating air. This will also shorten the period of aerobic activities of microorganisms during the silage process to ensure the formation of an anaerobic environment.
- Filling and compaction: The silage materials should be chopped and filled at any time, and compacted well at a depth increment of 20-30cm. If the compaction is interrupted for a long period due to other reasons, the upper layer should be compacted for a second time when it is refilled to prevent the forage from rebounding. The function of compaction is to exhaust air and create the fermentation conditions for anaerobic lactobacillus for silage. The tighter the silage materials are filled, the more thoroughly the air is removed, and the better the quality of silage is. The filling and compaction time should be short, which is generally less than three days.
- Sealing and management: The silage material should be stacked 50-60cm higher than the pillar (tank) mouth and covered with a layer of plastic film. Then it is covered and compacted with earth to form the steamed bun appearance. The soil sealing is about 40-50cm thick. There may be cracks that form about one week after storage, and they should be repaired quickly to prevent the leakage of water and gas. Sinking usually stops at the tenth day after storage. Then the cellar can be earthed up so that the pillar top is 30-40cm above the ground. The pillar top should be reworked into the shape of steamed bun, and then finished with plaster. In addition, we should prevent trampling by livestock, control rodents, and resist water filtration.
Advantages of straw ammonisation
- Saving grain, and reducing the dependence of animal husbandry on grain.
- Improving palatability and the feed intake of forage by livestock.
- Increasing the digestibility of organic matters in forage by 10%-12% and doubling the content of crude protein.
- The materials are easily accessed with simple methods.
- Reducing feeding costs and increasing economic benefits.
Disadvantage of straw ammonisation
- The ammonia utilisation efficiency is as low as approximately 50%. The surplus ammonia is discharged into the environment after the ammonisation facilities are opened, which causes environmental pollution and threatens the health of animals and human beings.
Advantages of straw silage
- Minimal loss of nutrients (generally by less than 10%), and effectively maintains the freshness of green feed.
- Fragrant, soft, and juicy, and therefore, highly palatable to livestock.
- Expands the application scope of feed sources.
- Easy to store in large quantities for a long time, as an economical and safe approach for silage.
- Less restricted by climate and season during storage.
- The preparation process of silage can kill pathogenic insects, weed seeds, etc.
- Improved feed digestibility and reduced methane emissions.
Disadvantage of straw silage
- The straw silage production process needs to be done quickly.
- The high degree of mechanisation requires a high investment cost.
Straw ammonisation and silage can significantly improve the digestibility of forage. One experiment indicated that the feed intake was increased by 53% and 32.8%. In addition, the average daily weight gain was increased by 126% and 97.4%, by feeding the beef cattle with ammoniated straw and silage, respectively, than those by feeding dry corn stalks (Wang et al., 2008).
The investment in straw ammonisation and silage technology is concentrated on expenses in construction of storage facilities, machinery, and covers. More economic benefits are reaped mainly by increasing daily weight gain and milk yield of animals fed with treated forage. Wang et al. (2008) showed through the experiment of beef cattle with corn stalks conducted with different treatment methods that the cost of coarse feed per head of cow increased by 45.5% and 51.6% with the use of ammoniated straw and silage, respectively. However, the corresponding revenues increased by 153% and 68.8% . The research result by Li Wenbin et al. (2010) showed that the profit of breeding beef cattle with silage increased by 51.5% more than that with dry corn stalks. It can be concluded that considerable economic benefits are achieved by feeding animals with ammoniated straw and straw silage.
Methane emissions of ruminant animals are produced through the normal fermentation of the feed taken by animals in the digestive tract. The energy loss in the form of methane by ruminant animals accounts for about 2%-15% of the total energy intake (IPCC, 2000). Generally, the amount of methane emissions by a single animal increases with the weight of the animal. Higher level methane emission are observed under greater the feed intake and with lower feed digestibility. Therefore, the improvement of feed quality and animal productivity is an effective approach to reduce methane emissions of ruminant animals (Dong et al., 2008).
Dong et al. (2004) calculated and compared methane emissions of ruminant animals after the straw was treated with ammonisation and silage technology using the IPCC method, The results showed that the methane emissions were reduced by 16%-30% by feeding treated straw than by feeding dry straw. Methane emissions of beef cattle that were fed dry corn stalks and corn stalk silage were 229L/d and 196L/d, respectively, under the conditions of identical energy intake level and the same ratio of fine feed to coarse feed; the methane emissions of the silage were reduced by 14.4% compared to the dry stalk (Fan et al., 2006).
Na Renhua et al. (2010) showed that corn straw after treatment of silage technology can help improve feed digestibility and reduce methane production through in vitro digestion test; with identical ratio of fine feed to coarse feed, the methane emission was decreased by 30% by feeding silage than by feeding dry corn. In China, the proportion of silage and ammoniated straw feeding is only 44% at present. Feed saving, improvement of feed conversion efficiency, and reduction in methane emission can all be achieved by constantly increasing the proportion of silage to ammoniated straw. The potential for methane emission reductions is also tremendous.
In China, the promotion and application of straw ammonisation and straw silage is mainly on large-scale cattle farms. Households and small farms are the main ruminant producers in China. Since these farms operate on a small scale, with no supporting ammonisation and silage facilities, the farmers cannot fully grasp the key technical points of scientific processing methods for straw ammonisation and silage, so further support on application of this technology is currently restricted.
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