Ruminant nutrition knowledge（2）
3. Fats
– Lipids: refers to all substances that are soluble in fat-soluble solvents.
– Lipids include: monoglycerides, diglycerides, triglycerides, as well as phospholipids, glycolipids, free fatty acids, ethanol, fat-soluble vitamins, etc.
Common animal fats and vegetable oils are triglycerides.
– The effects of adding animal fats and vegetable oils to feed:
[1] Increase the energy of the feed and provide essential fatty acids.
[2] Improve the palatability of the feed and reduce heat stress.
[3] Reduce feed dust and act as a lubricant when mixing the feed.
[4] Regulate rumen fermentation, increase the solubility of fat-soluble nutrients in the small intestine, and improve feed utilization efficiency.
– Lipids in plants:
Plant lipids can be divided into the following three categories:
[1] Lipids stored in seeds, mainly triglycerides, are the main component of concentrate raw materials.
Oil seeds have the highest lipid content, ranging from 18% in soybeans to 35% in sunflower seeds.
[2] Lipids present in leaves, including glycolipids and phospholipids, are mainly found in forage grasses.
[3] Other compounds, such as carotenoids, essential oils, chlorophyll, and waxes.
Plant lipids are mostly composed of unsaturated fatty acids (PUPA) + glycerol, and contain a large amount of oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3).
– Animal fats and oils:
Natural feed raw materials, the lipid content generally cannot meet the energy needs of fattening beef cattle, so animal fats need to be added.
Animal fat and liquid yellow grease are the fat ingredients commonly added to animal diets. The latter is a mixture of waste fat from the food industry and animal fat.
Animal fat has a negative effect on rumen microorganisms, so the amount added needs to be limited:
[1] Adding 2% fat to ruminant diets has no adverse effects.
[2] When the addition reaches 3% or more, cattle need a period of adaptation.
[3] When beef cattle are fed high-concentrate feed, the upper limit of fat addition is 6%.
– Digestion and absorption of lipids:
Triglycerides are hydrolyzed by rumen microbial lipase to produce glycerol and free fatty acids. Glycerol is metabolized by microorganisms to form propionic acid and absorbed by rumen capillaries.
Glycolipids and phospholipids also form free fatty acids after hydrolysis in the rumen.
The lipids entering the small intestine mainly include saturated long-chain fatty acids and microbial lipids, and fatty acids are absorbed in the small intestine.
– Energy value of fat:
The total energy of fat is about 9.33 Mcal/kg, and the average digestibility is about 75%, so the digestible energy (DE) of fat is about 7 Mcal/kg.
The metabolic energy (ME) of fat can be regarded as = DE, that is, there is no urinary energy or gas energy loss, so the ME of fat is also about 7 Mcal/kg.
The net energy for maintenance (NEm) and net energy for milk production (NEl) of fat is about 5.6 Mcal/kg, and the efficiency of ME to NEm or NEl is about 0.8.
The net energy for weight gain (NEg) of fat is about 3.85 Mcal/kg, and the efficiency of converting ME to NEg is about 0.55.
– Essential fatty acids (EFA):
Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are essential fatty acids for many animals.
Essential fatty acids are common in natural feeds, so they are generally not easily deficient.
– Adding fat to feed:
Lipids in the diet can inhibit rumen fermentation, resulting in a lower consumption rate of non-lipid nutrients, especially structural carbohydrates, which also affects the digestion of protein in the rumen.
For finishing cattle fed a high-concentrate diet, as long as they have undergone a period of adaptation, adding 6% of tallow fat to the diet is acceptable and will not affect production performance.
For diets mainly fed with roughage, if the purpose is to maximize the use of forage, the amount of fat added should not exceed 3% of the dry matter intake (DMI). If the purpose is to maintain the intake of forage, the amount of fat added should be less than 2% of the DMI.
4: Anion and cation difference
– Reasons for adjusting the DCAD of the feed:
As time goes by after the cow gives birth, the concentration of HCO3 ions and local CO2 will increase, and the internal environment tends to be acidic, which requires that the feed should be alkaline. As the milk production gradually decreases and the metabolic rate weakens, the alkalinity of the diet should be appropriately reduced.
For growing and fattening cattle, due to their high metabolic rate, the DCAD of the feed should also be maintained at an alkaline level.
The reason for requiring feeding a lower DCAD during the dry period or before calving is to prevent postpartum paralysis in the early stage of lactation. Postpartum paralysis in the early stage of lactation is that a large amount of blood calcium is used to synthesize colostrum, resulting in the decomposition and utilization of calcium in bone tissue.
Feeding a lower DCAD diet before giving birth can effectively prevent postpartum paralysis, and the prevention effect will be better when the calcium content in the diet is high.
– How to adjust the DCAD value of the feed:
Adding potassium carbonate or baking soda to the feed can effectively increase the content of cations in the feed, thereby increasing the DCAD value.
Adding ammonium chloride, ammonium sulfate, magnesium sulfate, and calcium sulfate to the feed can effectively increase the content of anions in the feed, thereby reducing the value of DCAD.
The DCAD values of common feed raw materials for cattle are generally high. For example, the DCAD values of alfalfa and soybean meal are around 600m and 500Eq/kg DM, respectively. It is usually necessary to add anionic salts (ammonium chloride, ammonium sulfate, magnesium sulfate, etc.) to the feed to reduce DACD in order to meet the needs of acid-base balance in cattle.
5: Daily Weight Gain
– Fasting Daily Weight Gain: refers to the daily weight gain of fasting body weight, the unit is: kg/d, this evaluation indicator is only used for growing and finishing beef cattle.
– Fasting weight: usually 96% of full body weight.
– Metabolizable protein: refers to the protein that reaches the small intestine and can be digested and absorbed by beef cattle.
According to the US NRBC feed evaluation method, the calculation of daily weight gain of feed needs to be considered from the following two aspects:
First, according to the total daily supply of metabolic energy in the feed, the daily weight gain is calculated. This weight gain value is called “metabolic energy fasting daily weight gain” and is expressed in SWGme.
Second, according to the total daily supply of metabolic protein in the feed, the daily weight gain is calculated. This weight gain value is called “metabolic protein fasting daily weight gain” and is expressed in SWGmp.
Finally, the daily weight gain (SWG) that the diet is expected to achieve is the minimum of SWGme and SWGmp, that is, the daily weight gain that can meet the needs of metabolizable energy and metabolizable protein.
