The impact of palatability on pig feed utilization and improvement measures

　　Animals obtain the necessary nutrients through foraging to meet their needs for maintenance, growth, and production. Increasing feed intake can better utilize the animal's production potential. For pigs, this can maximize meat production and quality. However, under large-scale farming conditions, the actual feed intake of pigs is much lower than the ideal feed intake determined by the breed. Since the conditions of large-scale farming cannot be changed, to improve the production performance of pigs, ways must be found to increase the actual feed intake of pigs and improve feed utilization. Improving feed palatability is a direct and effective method; pigs must like the feed and be willing to eat it actively.

　　1. Feed Palatability and Animal Feeding Physiology

　　Palatability refers to the characteristics of feed, such as aroma, texture, taste, and mouthfeel. It is the basis for a whole set of actions and reactions in animal foraging, locating, and finally feeding, similar to human pursuit of delicious food. Palatability reflects the degree to which feed is accepted by animals, changing feed intake by affecting animal appetite. Since animal feed intake can reflect feed palatability, studying the impact of palatability on feed utilization requires first understanding animal feeding physiology.

　　Animal feeding is a complex activity, including foraging, identification, location perception, ingestion, and chewing and swallowing. It is regulated by the central nervous system as follows: food emptying in the animal's digestive tract—hunger—nerve conduction stimulates the lateral hypothalamic hunger center—efferent nerve conduction instructions—feeding—the gastrointestinal tract is full of food—satiety—transmitted through afferent nerves to the ventromedial hypothalamic satiety center—instructions issued through efferent nerves—stop feeding. When feed palatability is good, the pig's appetite is strong, leading to increased feed intake and improved feed utilization. When feed palatability is poor, the pig's appetite is also poor, and even when hungry, the feed intake is not large. Feed not eaten or eaten less due to palatability causes significant waste, and feed utilization is low.

　　2. Factors Affecting Palatability

　　2.1 Poor Flavor

　　The flavor of feed is the most basic and intuitive factor in evaluating feed palatability. Feed flavor mainly refers to the color, aroma, and taste characteristics of the feed. During the processing of compound feed, additives such as vitamins, minerals, or drugs may be added. While exerting their nutritional and medicinal value, they also bring about poor taste, i.e., poor palatability. As for pig feed, many small feed companies use poorly-tasting crops or by-products from the food industry as feed raw materials to save costs. The resulting feed has poor flavor and cannot stimulate pigs visually, olfactorily, or gustatorily. Pigs mainly rely on taste and smell to forage. Poor flavor and poor palatability directly lead to insufficient feed intake, low feed utilization, and affected production performance.

　　2.2 Antinutritional Factors

　　Antinutritional factors are substances that reduce the digestion, absorption, and utilization of nutrients in feed, causing adverse physiological reactions in humans and animals. Several typical examples are tannic acid, protease inhibitors, and lectins. In nature, plants contain many types of antinutritional factors. These antinutritional factors themselves serve as a "defense mechanism" for plants, protecting the plant and its seeds from damage by pests, birds, and livestock and poultry. Nowadays, 90% of the components of factory-produced compound feed are plant-based raw materials, and plant-based raw materials usually contain one or more antinutritional factors.

　　Tannins have a bitter taste and directly reduce the palatability of animals. At the same time, tannins form indigestible complexes with proteins, carbohydrates, and other components in the feed, reducing protein digestibility. Tannins directly reduce the palatability of pigs. When pigs reluctantly ingest them, the combination of tannins and feed protein further reduces protein digestibility. Therefore, feed intake is reduced on the one hand, and protein digestion is hindered on the other, both of which contribute to low feed utilization in pigs.

　　Protease inhibitors are mainly found in legumes and can inhibit the action of various proteases such as trypsin, pepsin, and chymotrypsin. There are many types of these factors, with trypsin inhibitor (KTI) and chymotrypsin inhibitor (BBI) being the most important. Trypsin inhibitors combine with trypsin in the intestinal fluid to form inactive complexes, reducing trypsin activity and protein digestibility. In addition, chymotrypsin inhibitors cause pancreatic hypertrophy, disrupting and disrupting digestive function, and hindering animal growth. In pigs, these two protease inhibitors interact, disrupting digestive function on the one hand and reducing digestive enzyme activity on the other, thus reducing feed digestibility and utilization, and affecting pig growth and production performance.

　　Lectins are proteins produced by plant seeds and animals that bind to specific monosaccharides or oligosaccharides. It has been reported that most lectins are not degraded when passing through the digestive tract and can bind to receptors on epithelial cells. These two reasons cause changes in the morphology and metabolism of intestinal cells, thereby affecting the digestive and absorptive function of the intestine. Therefore, when pigs ingest plant-based feed containing lectins, the digestive function of the feed is affected, and utilization is reduced. In addition to the above three, there are other antinutritional factors such as saponins, cellulose, lignin, and phytic acid. Saponins weaken the activity of digestive metabolic enzymes; cellulose and lignin themselves are difficult to digest and hinder the absorption of normal nutrients; phytic acid combines with proteins and trace elements to form complexes, inhibiting the absorption of trace elements.

　　2.3 Deterioration

　　Feed deterioration refers to the changes in feed due to microbial growth and nutrient changes during storage and transportation from processing to animal feeding. The main issues are mold and oxidation. According to the Food and Agriculture Organization of the United Nations (FAO), approximately 25% of the world's grains are contaminated by mycotoxins to varying degrees, resulting in millions of dollars in losses annually, significantly impacting crop producers, animal farmers, and food producers. The main molds contaminating feed include Aspergillus, Penicillium, Fusarium, and Mucor, followed by Cladosporium, Colletotrichum, Cephalosporium, Pyricularia, Paecilomyces, Trichoderma, and Rhizopus. Moldy feed shows significant changes compared to normal feed, mainly including clumping, heating, discoloration, unusual odors, and increased powdery substance. The growth of mycotoxins is closely related to moisture and temperature. Some studies indicate that mold is unlikely to grow when feed moisture is below 12.5%, while it can reproduce at 13%–16%, and mold inhibitors are ineffective when moisture exceeds 16%. Mold is unlikely to grow at temperatures below 10℃, while a large number of microorganisms will be produced at 28℃–38℃. Moldy feed can cause various diseases in pigs, mainly including: acute central nervous system symptoms in poisoned piglets, leading to death within a short period; slow onset in adult pigs, initially showing decreased appetite and skin rashes in multiple areas, followed by loss of appetite, abdominal pain, bloody stool, and eventually death; high rates of stillbirths and abortions in pregnant sows, and abnormal postpartum estrus; and gradual refusal to eat in lactating sows, showing continuous estrus or prolonged estrus cycles, affecting the survival rate of suckling piglets.

　　There are many reasons for feed oxidation, mainly the oxidation and rancidity of unsaturated fatty acids, producing unpleasant odors. Reports indicate that the products of feed oxidation and rancidity, such as aldehydes, ketones, acids, and esters, have adverse effects on animals, mainly manifested as: (1) unpleasant odors, animal refusal to eat, resulting in slower weight gain and low feed conversion rate; (2) feed heating and clumping, accelerating mold growth, and cellulose destruction; (3) lesions in the visceral system, digestive system, and circulatory system; (4) reduced animal immunity and weak disease resistance; (5) affecting cell structure, function, and the stability of biomembranes.

　　2.4 Unreasonable Processing

　　Feed processing from raw materials to finished products involves a series of processes such as crushing, mixing, and steaming, changing the physical form, chemical structure, and nutritional composition of the feed raw materials. According to different purposes, feed can be processed into powder, granules, flakes, etc. Earlier experiments showed that the digestibility of steamed and pressed compound feed in pigs was lower than that of steamed powder feed, while the digestibility of cellulose in conventional powder feed was significantly lower than that in steamed powder feed. With the same feed raw materials and the same formula, different shapes result in different palatability. The particle size of pig feed should be relatively fine. Too coarse feed has poor palatability, affecting digestibility and reducing pig production performance. However, excessively fine feed can easily cause gastric ulcers in pigs and increase production costs.

　　3 Improvement Measures

　　3.1 Use of Flavoring Agents

　　Feed flavoring agents, also known as palatability enhancers, utilize animals' pursuit of feed taste to induce feeding and increase feed intake. For palatability problems caused by poor feed taste, adding flavoring agents is a good solution. In rural areas, the pig farming environment is poor, and vitamins, minerals, and drug additives must be used to prevent diseases and promote pig growth. However, the unpleasant taste of these additives can cause pigs to refuse to eat and reduce feed intake. Adding flavoring agents significantly improves palatability and increases feed intake.

　　Studies have shown that pigs have a significant preference for sweets. Compound sweeteners (containing 66.7% sodium saccharin and other long-acting flavor enhancers and synergists) can increase piglet feed intake, but excessively high or low addition levels are not conducive to feed intake. In the piglet growth stage (1d–28d), adding 150 mg/kg of compound sweetener to the diet can achieve the maximum daily feed intake and the maximum daily weight gain in this stage. In addition, experiments have shown that flavoring agents have antioxidant functions, and as the storage time of the feed increases, flavoring agents can effectively alleviate the degree of feed oxidation.

　　3.2 Elimination of Antinutritional Factors

　　Antinutritional factors are the main factors restricting feed utilization. To reduce or eliminate the effects of antinutritional factors, various methods can be adopted. The main processing methods include physical processing, chemical processing, breeding new varieties, and biological processing.

　　3.2.1 Physical Methods

　　Most antinutritional factors are heat-labile, such as various protease inhibitors, lectins, and anti-vitamin factors, and can be denatured by heating. It has been reported that heating pigeon peas at 100℃ reduces the phytic acid content from 120.0 mg/100g to 100.2 mg/100g. Atmospheric steam treatment of soybeans for 30 min can reduce the activity of soybean trypsin inhibitor by 90%. High-temperature extrusion processing can also inactivate antinutritional factors. Extrusion processing at 100℃–140℃ can reduce the activity of soybean trypsin inhibitor by 74.8%–88.6%, and the degree of inactivation increases with increasing temperature.

　　3.2.2 Chemical Methods

　　Chemical treatment mainly refers to acid-base treatment, ammonia treatment, and the addition of special substances. These methods can remove antinutritional factors from feed. It has been reported that soaking cottonseed in 2% lime water or 1% sodium hydroxide solution for 24 h and then washing with clean water can remove most of the gossypol. Treating soybean meal with 5% urea and water for 30 d reduces urease activity by 90%. Adding 10 mol/L vitamin C and 0.5 mol/L copper sulfate to raw soybean meal and treating it at 27℃ for 1 h can inactivate more than 40% of the trypsin inhibitor (KTI), and treating it at 65℃ for 1 h can inactivate more than 90% of KTI and chymotrypsin inhibitor (BBI). Although chemical methods are effective and energy-saving, they may lead to chemical residue in the feed, affect the taste of the feed itself, and pollute the environment to some extent.

　　3.2.3 Breeding Methods

　　Modifying the nutritional components of feed materials through breeding, reducing or even eliminating antinutritional components in plants, is the most fundamental and effective method for removing antinutritional factors. Currently, 70% of barley production in China is used in the feed industry, so breeding high-quality feed barley is of great significance; for soybean meal feed materials, new varieties with low trypsin inhibitors, low saponins, and low phytic acid can be bred. We should vigorously cultivate and promote "double low" rapeseed varieties, characterized by a glucosinolate content of less than 2mg/g and erucic acid content not exceeding 5%. Although breeding a new variety takes a long time, once successful, the benefits will be substantial.

　　3.2.4 Biological Methods

　　With the development of science and technology, biotechnology methods have been increasingly applied to the feed industry, mainly using enzyme preparations and fermentation methods to process feed. In pig production, adding phytase to pig feed can increase phosphorus utilization by 50% to 70%, improve protein and amino acid digestibility by 2% to 5%, and reduce excretion accordingly. Fermentation is a traditional method for eliminating antinutritional factors. After fermentation, some indigestible substances in the feed become digestible, improving digestibility, palatability, and feed intake, thus improving feed utilization. Experiments show that piglets and fattening pigs fed with fermented feed using compound feed fermenters increased daily weight gain by 26.5% and 12.25%, respectively, and feed-to-meat ratio decreased by 20% and 11.7%, respectively, with a significant improvement in diarrhea. This shows that biological fermentation plays an important role in improving feed utilization and pig production performance.

　　3.3 Feed Mold and Oxidation Prevention

　　Mold and oxidation of feed directly lead to a significant reduction in nutritional value or make the feed unusable, so it is essential to prevent mold and oxidation. Necessary measures can be taken to prevent mold and oxidation: (1) The moisture content of feed entering storage should be controlled within a certain range, generally not exceeding 12%. Warehouse ventilation, insulation, and moisture-proof flooring should be in place, and the relative humidity should be controlled below 60%. High humidity will cause mold growth and is not conducive to the preservation of high-oil feed; (2) The optimal temperature for mold growth is 28℃～38℃, so the warehouse temperature should be controlled to prevent mold growth. Highly unsaturated fatty acids in feed are also prone to oxidation and rancidity at higher temperatures; (3) Appropriately use some mold inhibitors and antioxidants to deal with urgent situations of feed mold and rancidity. However, since this affects the nutritional value of the feed, long-term use is not recommended.

　　3.4 Reasonable Processing

　　Pigs prefer relatively small feed particles, with a general crushing size requirement of 0.5mm～0.8mm. Therefore, processing feed into appropriately sized particles is a prerequisite for increasing pig feed intake and improving feed utilization. It has been reported that processing crushed grain feed into pelleted feed can improve feed utilization by about 23%. During pelletization, the feed undergoes steam treatment and high-pressure extrusion, causing the feed grain cells to rupture and release intracellular substances, increasing the nutritional content of the feed. At the same time, pelleted feed has good palatability, reduces energy consumption during feeding, and ensures more uniform feed, preventing pigs from selectively consuming certain feed components.

　　4 Conclusion

　　Improving feed palatability is a feed-level method for addressing animal feed utilization issues. In addition to the feed palatability issues discussed above, there are other methods, such as those related to animal physiology, to improve feed utilization, covering a wide range. Practice makes perfect; only through extensive practice and continuous exploration and summary of experience can we find better ways to improve feed utilization. In short, everything is done to improve animal production efficiency and better serve humanity!