After early weaning, the structure of the diet changed drastically. The energy content per kg of dry matter decreased from 22.18 MJ of breast milk to 13.8-14.2 MJ of grain-soybean meal. Dietary digestive energy concentration decreased, and appetite decreased and feed intake decreased due to weaning stress; fresh piglets had low glycogen storage, insufficient gluconeogenesis, and could not provide sufficient energy, and the body fat storage of newborn piglets was only weight. 1%-2%, impaired metabolic function, low fatty acid oxidation rate, body fat can not provide sufficient energy, these factors have led to insufficient energy intake of piglets, which has become an important factor restricting the normal growth and development of piglets. To this end, nutritionists add fat to the weaned pig diet to address the problem of insufficient energy. However, the relevant researches for many years have not reached a consistent conclusion. The following is a summary of the problems and the factors affecting the digestion and utilization of fat in piglets.
l. Effect of dietary fat on performance of weaned piglets
In recent years, many tests at home and abroad have shown that the addition of fat to the diet of weaned piglets can improve their performance. Dove (1993) added 5% oil to 540 diets of 25-day-old weaned pigs, and the daily weight gain of the experimental group was significantly increased (P<0.01). Verland et al (1995) added 6% of animal fat to the diet of weaned piglets. The daily gain was increased by Z1.4% (P<0.01) within 5 weeks after weaning, and the feed conversion rate was significantly improved. Dela et al. (2001) showed that dietary fat supplementation significantly improved the daily gain of 36-59 kg pigs, and pigs larger than 59 kg had no significant effect. Domestic Wang Youming et al (1999) used 270 heads of weaned piglets to be treated in five groups (control group, 3.5% palm tung oil powder; 2.8% feed grade mixed oil, 3.l% fish oil, 2.7% soybean oil) for a period of time. In the 45-day test, the soybean oil group, the mixed oil group, the brown tung oil group and the fish oil group were increased by 2.72, 9.39 (P<0.05), 1.90, -9.02 (P<0.05), respectively. The oil and fat group was 10.5%, 7.11%, 9.21%, and 2.63% higher than the brown tung oil powder group, the fish oil group, the soybean oil group, and the control group, respectively. Zhang Heliang. Pin Yuming et al. (1999) conducted a four-week trial with 96 35-day-old weaned piglets, replacing 0. 1.8%, 3.4%, and 5.2% soybean oil with a moderate amount of corn. The results showed that the addition of oil to the diet during the 1-2 weeks after weaning did not affect the performance of the piglets, which is consistent with many test conclusions (Cera et al., 1988; Tokach et al., 1989; Dove et al., 1993; Li Defa et al., 1990). However, 3-4 weeks after weaning, the daily gain, feed intake and daily intake energy of 3.4% soybean oil treatment group were significantly higher than the other three treatment groups (P<0.05). This indicates that piglets can make good use of a certain amount of fat during this period. Cera et al. (1988), Li et al. (1990), Dove et al. (1992) all proved this conclusion. This weight gain effect, in addition to the energy nutrition of fat itself, some scholars believe that there is "extra metabolic effect", that is, fat can delay the flow rate of food in the gastrointestinal tract, increase the digestion and absorption time of nutrients, thereby improving absorption. usage efficiency. However, the decline in production performance caused by excessively high proportion of fat addition may be attributed to the limited oxidative capacity of fatty acids in weaned pigs, and the excess fat stored in adipose tissue, leading to slow growth of muscle tissue (Zhang Heliang, 1999). In addition, some scholars (Jones et al., 1992; Verland, 1993; Tokach et al., 1995; Brown et al., 1998; Shi Zhaoshan et al., 1997) have shown that the addition of fat to weaned piglets does not improve the performance of piglets. For this different conclusion, the author believes that the effect of fat digestion and absorption, in addition to the effects of digestive physiology and fat digestive enzymes in piglets, is also affected by such types as test fats, diet structure and other factors. Impact.
Relationship between morphological structure, lipase changes and fat absorption of piglets after weaning
2.1 Changes in the structure of the small intestine after weaning
The normal structure and function of the small intestine is the basic guarantee for the full digestion and absorption of various nutrients including fat. However, after weaning, the small intestine showed severe villous shedding, lymphocytic hyperplasia of the intestinal mucosa and mitotic mitosis (HoppCll et al., 1982; Millsr et al., 1984). Hampson et al. (1986) pointed out that small intestine injury in weaned piglets has obvious site and age characteristics. Pekas (1990) believes that the development of the digestive tract of piglets must be healthy after 6-8 weeks of age. Therefore, after the early weaning of piglets, the volume and quality of the digestive tract and digestive gland are not mature except for the intestinal damage caused by changes in the structure of the diet, which greatly limits the digestion and absorption of nutrients such as fat. . The effect of fat on the small intestine villi varies with the type of fat and the amount added. Li Defa et al. (1990) reported that adding a large amount of fat to the diet would reduce the height of the small intestine villi and reduce the fat utilization rate. When the soybean oil and coconut oil were mixed in the diet 1:1, the shape of the small intestine villi was not affected. When used alone, the piglets' intestinal villi become shorter.
2.2 Changes in lipase after weaning
The total lipase activity of piglets before they were born colostrum was very low, and the activity increased rapidly after colostrum. Stable after a week of age. The amount of lipase secretion in piglets during lactation is insufficient, but the digestibility of fat is very high. The digestive enzyme activity of 4 weeks old weaned pigs decreased to about 1/3 of that before weaning. After 2 weeks, except for the activity of pancreatic lipase, which has not recovered significantly, other enzyme activities have recovered, even exceeding pre-weaning levels (Lindemann et al., 1986). Cera et al. (1998) found that the intestinal lipase activity increased almost every week during 0-5 weeks of unweaned piglets, but its activity did not increase after weaning at 3-5 weeks of age, and recovered after 1-2 weeks. . Domestic Zhang Zhenbin et al (1999) studied the changes of lipase activity after weaning in 14-day-old piglets. The results showed that the activity of pancreatic lipase decreased after weaning, while the activity of lipase in jejunum contents changed more smoothly. It began to rise on the 6th day, and increased to 44 on the 9th day before weaning, while the lactation group increased by 20 times from the original on the 9th day. These test results theoretically support the conclusion that diet supplementation of 1-2 weeks after weaning has no effect, while 3-4 weeks can improve the performance of piglets.
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3. Exogenous factors affecting the digestion and absorption of fat in piglets
3. l different fat sources
Different sources of fat, piglets digestion and absorption effects vary widely. Tests have shown that the various fat digestibility rates are coconut oil, soybean oil, and corn oil well oil (Cera et al., 1989; Jones, 1992; Chen Yuhui et al., 1997). For a single oil and mixed fat, the latter is considered to be superior to the former ( Li Defa et al., 1990). This difference is essentially due to the length of the fatty acid carbon chain and the fatty acid saturation.
3.1.l length of fatty acid carbon chain
Braude et al. (1973) found that the absorption rate of fatty acids is negatively correlated with the length of the carbon chain, and the shorter the carbon chain, the easier it is to digest and absorb. Chiang et al (1990) believe that the short-chain fatty acids are more easily absorbed than long-chain fatty acids. One is that the medium-short-chain fatty acid esterification rate is low, most of which can be directly absorbed, without the degradation of lipase, and long chain Fatty acids must be esterified to form fat particles before they can be absorbed. Second, short-chain fatty acids enter the liver directly through the portal vein, and long-chain fatty acids must re-synthesize triglycerides in the absorbed cells to enter the bloodstream through the lymphatic system. transport. Therefore, the medium and short-chain fatty acids are significantly higher in utilization efficiency and utilization rate than long-chain fatty acids.
3.1.2 Fatty acid saturation
Freman et al. have determined in 1963 that generally unsaturated fatty acids are more digestible than saturated fatty acids (except coconut oil), and there is a clear linear relationship between them. Stably et al. (1984) suggested that the digestibility of fat depends on the ratio of unsaturated fatty acid (U) to saturated fatty acid (S) in the whole diet. There is a clear linear relationship between the digestibility of fat and the U/S value of piglets. Piglets have higher utilization ability of vegetable oil than animal oils. One reason is that the proportion of unsaturated fatty acids in the former is higher than that in the latter. For coconut oil, although the content of saturated fatty acids is higher, the fatty acid carbon chain is higher. Short, it is easy to be digested and absorbed. Powles et al. (1994) considered U/S to be 5.71 pairs, and piglets had the highest digestive energy for fat.
3.2 Copper addition
Dove et al. (1995) reported that the simultaneous addition of copper and fat in the diet was better than the addition of both, and that Cu and fat levels were associated with daily gain (P<0.01) and feed-to-meat ratio (P<0.01). There was significant interaction with feed intake (P<0.01), while adding 5% fat and 250mg/kg copper compared with 15mg/kg and 250mg/kg copper alone, the former significantly increased the growth rate (P<0.05), feed Efficacy (P < 0.05) and feed intake (P < 0.01), but the results obtained by adding 5 to fat and 15 mg / kg of copper were reversed. Haydon (1992) and Sun Suling et al. (1996) also obtained the same results. It shows that there is a certain synergy between copper and fat. Adding fat to a diet containing 250 mg/kg of copper increased the fat digestibility from 60.8% to 85.1% (Dove, 1995). Luo (1996) also obtained copper to improve the apparent digestibility of fat.
At present, it is believed that the mechanism of action of copper affecting fat utilization has the following three aspects:
1. Copper can act as an activator of certain digestive enzymes to increase the activity of digestive enzymes. The addition of 25 mg/kg copper to the diet significantly increased the activity of lipase and phospholipase A in the small intestine and promoted fat absorption (Luo, 1996; Dove, 1996);
Second, the antibacterial effect of copper. Copper can reduce the pH value of gastric juice, inhibit the proliferation of pathogenic bacteria in the stomach, and improve the digestion and absorption capacity of the gastrointestinal tract;
Third, copper can promote the synthesis of lecithin. Phospholipids are essential for the absorption of fatty acids, and copper is a component of cytochromes and cytochrome oxidase, which promotes the formation of phospholipids. When copper is deficient, the synthesis of phospholipids is impaired, affecting the absorption and utilization of fat.
3.3 Addition of lecithin and choline
A large number of experiments have shown that the addition of lecithin to weaned piglets helps to increase fat utilization (Jones et al., 1992; R. D. Esouza et al., 1995; Brown et al., 1998; Zhang Geng Huaze, 1998). The reason is that lecithin can promote the transfer of lipids from the liver, and can also be used as an emulsifier to make up for the insufficient secretion of bile acids in piglets and promote digestion and absorption. However, it has been suggested that the improvement in performance of lecithin-added piglets is not directly related to the improvement of fat digestibility (Brown et al., 1998; Zhang Heliang et al., 1999). According to statistics, after the addition of lecithin, 81% of the reports reported that the daily weight gain of piglets can be significantly improved, and nearly 60% of the reports have improved the feed conversion rate (Zhang Geng Huaze, 1998). Choline, the only substance that synthesizes phospholipids and lecithins, also acts as a methyl group during fat metabolism. The lack of choline hinders fat metabolism and reduces the efficiency of fat absorption and utilization.
3.4 Addition of carnitine
In animals, only L-carnitine is physiologically active. The main function of L-carnitine is to transport long-chain fatty acids through the inner mitochondrial membrane for beta-oxidation. Its effect is related to the age of the animal, the composition of the diet (such as amino acid composition, especially lysine content), and the ambient temperature. In weaned pigs, the synthesis ability of L-carnitine is poor, and adding a certain amount of carnitine to the diet can improve the fat utilization efficiency. Hines et al. (1990) reported that the addition of carnitine to the basal diet increased the daily gain and feed-to-meat ratio (P<0.05) after 3-5 weeks of weaning, and improved the utilization of soybean oil. Qiao Qingyan et al. (1998) also concluded that the addition of carnitine can improve the conversion efficiency of fat-added diets.
3.5 Dietary factors
Pettigren (1989) reported that the use of fat in piglet diets has a greater improvement in production performance while maintaining a stable protein energy ratio. It indicates that the full utilization of fat needs to maintain an optimal protein energy ratio. Dong Guozhong et al (1999) showed that the combination of protein-based feed, low-protein amino acid balanced diet and antibacterial growth-promoting agent can improve the digestibility of protein and fat in early weaned piglets and improve production performance.
3.6 Other factors
In addition to the above several factors, the feed processing modulation process. The ambient temperature, the time when the diet is added with fat, and the lack of some nutrients will affect the use of fat by piglets. If the fat feed is added after puffing, the palatability and appearance are improved, and the feed fat digestibility is improved. Environmental factors, especially temperature, have a greater impact on fat metabolism. Herpin et al. (1987) reported that compared with normal temperature (23 °C), the white adipose tissue and cardiac lipoprotein lipase activity of piglets were significantly increased (P<0.05) under low temperature conditions (12 °C), thus affecting fat metabolism and utilization. .
4. Discussion
At present, the application of fat in the diet of weaned piglets can be recognized by most scholars. The effect of fat on piglets is poor after 1-2 weeks of weaning. After 2 weeks, the utilization rate is significantly improved. Whether the addition of oil at the beginning of the weaning will induce the development of lipase is still controversial. How to improve the activity of lipase in piglets after weaning as soon as possible is the key to the fat utilization effect, and it is worthy of further in-depth research. It is not difficult to see from the above that the digestion and absorption of fat is affected by various factors such as feed, animal's own physiological condition and environment, which is a complicated process. Therefore, when designing the diet formula, these factors should be fully considered to maximize the fat utilization rate and improve the performance of the piglets.
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