Difficulties And Treatment Methods Of Feed Conversion Of Cotton Straw

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At present, most of the cotton stalks returned to the field directly after harvest in Xinjiang, China. However, due to the drought of the soil in Xinjiang, it is difficult for the cotton stalks returned to the field directly to be degraded into soil organic matter. At the same time, because cotton stalk often carries the pathogen of fusarium wilt, direct returning to the field will easily lead to the aggravation of cotton field diseases in the next year. It can be seen that the efficient utilization of cotton straw resources has become an urgent technical problem for Xinjiang cotton fields. Cotton straw is rich in nutrition and has the potential to be made into high-quality feed for ruminants; However, the lignification degree of cotton stalk is relatively high, and there is free gossypol which is toxic to animals, thus hindering the development of feed. The existing feed processing methods of cotton stalk, such as expansion treatment, ammoniation treatment, ferrous sulfate treatment and micro storage treatment, were summarized and analyzed.

Difficulties and treatment methods of feed conversion of cotton straw

◆ Difficulties in feeding cotton stalk

There are two main difficulties in feed processing of cotton straw: first, cotton straw must be softened before it can be better eaten by animals. This is because cotton straw contains more lignin and cellulose than corn, rice and other straws, and is harder and less palatable, which will reduce animal intake, feeding efficiency and digestive efficiency; The second is to reduce the content of free gossypol (C30H30O8) in cotton straw, a yellow toxic polyphenol widely distributed in the pigment glands of the whole cotton plant. Free gossypol can combine with protein, iron and other substances after entering the animal body, causing gastrointestinal mucosa damage, iron deficiency anemia, increased vascular wall permeability, reduced amino acid utilization and other diseases of the organism, resulting in growth retardation, poisoning and even death, and also endangering the reproductive system of male animals, leading to infertility. In order to prevent free gossypol from accumulating in animals for a long time and causing harm, it is generally required that the content of free gossypol in ruminant feed should not exceed 200 mg · kg-1. China has carried out research on low phenol cotton for a long time, and has made certain research achievements, such as the low phenol cotton varieties cultivated by the Cotton Research Institute of the Chinese Academy of Agricultural Sciences, Hebei Academy of Agricultural and Forestry Sciences and other units. However, due to the lack of gossypol in low phenol cotton varieties, their resistance to diseases and pests has been significantly weakened, so their popularization and application are greatly limited. Therefore, special free gossypol treatment technology must be adopted for the current cotton stalks produced in cotton fields to meet the feeding requirements.

◆ Feed processing method of cotton stalk

In order to make cotton straw into high-quality feed, targeted treatment should be carried out on the existing problems of straw raw materials to reduce the content of lignin, cellulose and free gossypol. At present, there are mainly three kinds of treatment methods: physical, chemical and biological.

Physical treatment is the basis of chemical treatment and biological treatment. Common physical treatment methods include crushing, steaming, soaking, radiation and expansion. Chemical treatment can change the content of various components in cotton stalk fiber by adding chemical reagents, reduce the content of lignin, cellulose and gossypol, and also improve the final quality of feed by adding some specific nutrients. Common chemical treatment methods include acid treatment, alkali treatment, ammoniation treatment and ferrous sulfate treatment. Biological treatment is to treat straw with the life activities of microorganisms or the metabolites of microorganisms, and inoculate different microbial strains into straw raw materials according to different treatment purposes. Compared with physical treatment and chemical treatment, biological treatment has longer reaction cycle and better treatment effect. Biological treatment is generally regarded as the core step in a set of mature cotton stalk feed process. Common biological treatment methods include enzymatic hydrolysis, silage and micro storage.

In view of the two difficulties mentioned above in the feed processing of cotton stalk, several treatment methods that are widely used and relatively efficient are further analyzed and discussed.

(1) Expansion treatment. Compared with other physical methods, the expansion treatment can destroy the original fiber structure in the straw to a greater extent, separating the originally closely arranged lignin and cellulose, so that the texture of the expanded straw is softer, the degree of lignification is reduced, the volume and surface area are significantly increased, the fiber bundles are exposed, and the fiber crystallinity is reduced.

Expansion treatment can be divided into steam explosion (referred to as "steam explosion") and extrusion expansion. The principle of steam explosion is to place straw raw materials in an airtight container, raise the temperature and pressure in the container by heating, fill the plant fibers with high-pressure steam, and then suddenly release the pressure to make the steam inside the straw explode, so as to tear and expand the originally compact lignin and cellulose. The working principle of extrusion is that the metal screw is controlled by the motor and transmission device to carry out strong extrusion friction and shear action on the straw in the extrusion chamber, so as to generate a lot of heat to make the extrusion chamber reach the state of high temperature and high pressure; When the straw is squeezed out of the nozzle, the external pressure drops suddenly, and the volume of high-pressure air between the internal fibers expands rapidly, so that the straw is expanded.

When the expanded straw is directly used as roughage, the feed intake and digestibility of animals are significantly improved. If it is used as an intermediate product to continue biological fermentation, it can also improve the fermentation efficiency and shorten the fermentation cycle. Under the condition of pressure 2.5 MPa and temperature 220 ℃, the content of lignin, cellulose, hemicellulose and crude protein in treated cotton stalks decreased by 11.81%, 9.34%, 16.76%, and 9.38% respectively; Meat sheep were fed with a small amount of pre extruded and post extruded straw mixed with basic feed, and the overall digestion efficiency of expanded cotton stalk in rumen of meat sheep was higher than that of pre extruded cotton stalk. Some studies have also shown that the high temperature during expansion has a certain detoxification effect on free gossypol in cotton stalks. For example, after steam explosion under pressure of 2.5 MPa and temperature of 220 ℃, the content of free gossypol in cotton stalks can be reduced by 33.7%. In conclusion, steam explosion can not only reduce the lignin and cellulose content in cotton stalk, improve the palatability of feed, but also reduce the free gossypol content.

(2) Ammoniation treatment. Ammoniation treatment is to soak straw with urea solution or ammonia water of certain concentration. Compared with some methods using strong acid and alkali solution, ammoniation treatment is more mild. This method can adjust the carbon nitrogen ratio of straw, improve the level of protein and other nutrients in feed, and increase the intake and digestibility of animals after intake; At the same time, the ammonia produced by urea decomposition can also make the free gossypol in the cotton stalk become combined with gossypol and lose toxicity under certain conditions, so as to achieve a certain detoxification effect.

Wang Qian et al. treated the crushed cottonseed meal with 5% urea solution, which increased the crude protein content of cottonseed meal by about 5%, and the free gossypol detoxification rate exceeded 40%. Zhang Guochen and others used a screw extruder to extrude the urea concentrate, and then mixed the urea before and after expansion into the feed to feed ruminant animals (cattle, sheep, etc.). The results showed that the hydrolysis rate of the urea concentrate in the expansion group was significantly lower than that in the non expansion group, and the hydrolysis process was more uniform. The feeding safety and utilization rate of the expanded urea were significantly improved. Therefore, the feeding effect of cotton stalk treated with ammoniation should be better if it is expanded after ammoniation or both.

(3) Ferrous sulfate treatment. As mentioned above, free gossypol can combine with iron in the organism; Therefore, the crushed cotton straw can be treated with ferrous sulfate solution, so that the free gossypol in it can combine with ferrous ion in the solution in advance to achieve the purpose of detoxification. In theory, the combination of free gossypol and ferrous ion should be equal to the amount of substances, but the actual operation is limited by the degree of crushing of cotton stalks, etc. The addition amount of ferrous ion is generally higher than the content of gossypol to achieve better detoxification effect.

Soak 500g cottonseed meal sample in ferrous sulfate solution, drain it after 3 hours, place it in an oven at 40 ℃ for drying for 4 hours, and then measure it. It is found that when the ferrous sulfate dosage is 3 and 5 times of the free gossypol mass concentration, the detoxification rate of free gossypol in cottonseed meal is 82.13% and 83.12% respectively, indicating that ferrous sulfate solution can effectively reduce the free gossypol content of the sample, It has detoxification effect.

The binding process of free gossypol with ferric ion belongs to exothermic reaction. The research of Ran Han et al. shows that when removing free gossypol with ferrous sulfate, it takes 2 hours to generate a stable gossypol iron ion complex. High acid environment will inhibit the binding reaction between free gossypol and iron ion, and may even lead to the re decomposition of some bound gossypol into free gossypol, thus enhancing the toxicity. Therefore, ferrous sulfate method is more suitable to be used in combination with ammoniation or alkali treatment to ensure that the treated cotton stalk feed can have appropriate pH value.

(4) Microstorage processing. Micro storage is a commonly used straw treatment method at present. The corn straw yellow storage mentioned above is applicable to the withered and yellow straw raw materials as well as the micro storage treatment of cotton straw. Due to the higher degree of lignification of cotton straw, more complex physical or chemical methods are usually used for pretreatment, and then certain microbial strains and nutrients are added for micro storage fermentation. There are many factors affecting the fermentation effect of micro storage, including temperature, humidity, sealing conditions, microbial species, nutrients, carbon nitrogen ratio of raw materials, etc. After fermentation, the cotton stalk was soft, acid and alcohol, and its palatability was significantly improved.

L (+) lactic acid bacteria, Bacillus subtilis, yeast and so on are commonly used in the research on the feed of cotton stalk micro storage. Lactic acid bacteria can reduce the content of cellulose and hemicellulose in cotton stalks, transform soluble sugars into organic acids that are easier to digest and absorb, improve the nutritional value and palatability of feed, and enhance the gastrointestinal immunity of animals; Bacillus can produce cellulase and glucoamylase, promote the decomposition of macromolecular sugars, inhibit the growth of some harmful molds and maintain the stability of the fermentation process by producing bacteriocin; Yeast can secrete cellulase, protease and other extracellular enzymes to improve the digestion efficiency of animal feed. The study found that specific Bacillus and yeast can also significantly reduce the content of free gossypol in cotton stalk. Therefore, in the specific practice of cotton stalk micro storage, appropriate microbial strains should be selected according to the different components of raw materials.

◆ Research and development ideas of cotton stalk feed technology

To sum up, there have been many studies on the feed technology of cotton stalk, and some progress has been made; However, most of them adopt a single technology and treatment method, which can not make cotton stalks into ideal high-quality feed. Therefore, the research on compound or combination technology of cotton stalks feeding is worth carrying out. If a complete technological system for feed processing of cotton stalks can be established, multiple technologies can be used for composite or combined processing, such as steam explosion, ammoniation, ferrous sulfate and other treatment technologies, that is, before steam explosion, the crushed cotton stalks can be fully mixed with urea and ferrous sulfate, and lignin and cellulose can be separated, fiber bundles can be broken For multiple purposes, such as deep fusion of urea and cotton stalk tissue components, full contact and reaction of urea and ferrous ions with gossypol, and then micro storage of the treated cotton stalk can improve the efficiency of cotton stalk feeding and further enhance the value of cotton stalk feeding.