Carbohydrates include non-fiber carbohydrates and fibers. Among them, non-fiber carbohydrates include starch, sugar, and pectin. The proportion of non-fiber carbohydrates in the diet of dairy cows should be 20%-45%. This range is determined by the proportion of concentrate to roughage in the diet. For diets with less roughage, non-fiber carbohydrates are required to be 40%-40%. Feeding an appropriate amount of non-fiber carbohydrates can effectively improve the quality of milk, not only can improve the milk fat rate, but also can improve the milk protein rate. If overfeeding, the milk protein rate and the milk fat rate will decrease. Therefore, when feeding cereal feed, attention should be paid to limiting the feeding amount to prevent the occurrence of rumen acidosis, the decline of milk quality, and poor appetite of dairy cows which lead to the decline of milk production.
Dairy cows mainly feed on roughage, so the fiber has an important impact on the rumen health, milk production, and composition of dairy cows. The content of fiber can stimulate rumen fermentation and saliva secretion, and maintain the normal milk protein and milk fat content in milk is very important. Therefore, the lowest acid detergent fiber and neutral detergent fiber in dairy cows diet should be 19%-21% and 25%-28% respectively. If they are lower than the above levels, they will lead to low milk fat rate, acidosis and poor health of dairy cows.
However, it should be noted that the proportion of crude fibers should not be too high, otherwise the milk protein content will be affected. It is generally required to feed 40~50% of roughage. If the feeding quantity of roughage is higher than 65%, high-quality roughage should be selected to avoid the decrease in milk protein. Besides, the length of fiber is also very important, to ensure that the feed size is sufficient, the inaccessible is too short, or the powder is too fine, and the grass should not be less than 1 cm, otherwise, it will cause a sharp decline in milk fat rate.
The milk yield of dairy cows increases with the increase of crude protein content in the diet. The protein requirement of high-yielding dairy cows exceeds the synthesis of rumen protein. Therefore, in some farms, the crude protein content in the diet increased to 17.5% in the early lactation stage, and the amount of rumen-passing protein accounted for 35% - 37%. However, the milk production of dairy cows is not unlimited. For example, when cottonseed is used as a supplement of crude protein, milk yield and milk fat percentage increase with the increase of crude protein content in the diet. When crude protein content exceeds 17.5%, milk production almost ceases to increase. Besides, different protein supplements have different forms of addiction, milk yield, and milk fat rat.
For example, the use of crushed soybeans can increase the milk fat rate, while the use of soybean meal can improve milk yield, but the milk fat rate is not affected. The amount of degradable protein and rumen protein in the diet of dairy cows has an important effect on milk yield and milk composition. Rumen degradable protein and rumen-passing protein are generally required to account for 65% of the total protein in the diet. If they are too low, milk production and nutrient content in milk will be reduced.
The nutritional function of rumen-passing protein is very important for improving milk production and milk composition of dairy cows. But when adding rumen-passing protein, we should pay attention to whether the amino acids contained in the added rumen-passing protein match the amino acids lacked in the body. Otherwise, even if the amount of rumen-passing protein is increased, it will not increase milk yield and milk composition because the amount of amino acid needed by the body is not much or not increased at all.
For high-yielding dairy cows, there will be a phenomenon of negative energy balance in the early and mid-lactation period, and energy is difficult to meet the needs of dairy cows. To improve the energy level in the diet without reducing the fiber concentration in the diet, the way of adding fat can be used to increase the energy concentration in the diet. This is because fat can reduce microbial activity and cellulose digestibility. The added rumen fat or inert fat can enter the small intestine directly without hydrolysis in the rumen and be digested and absorbed. Therefore, it can not only provide energy but also avoid affecting the digestion and absorption of other nutrients. Some studies have shown that dietary fat can increase milk yield, but the milk protein rate will decrease, while the addition of vegetable fat will reduce the milk fat rate, and the addition of inert fat and rumen-protected fat will increase the milk fat rate.
In practice, some feed additives are often used to improve the milk yield and milk composition of dairy cows. The commonly used additives are mainly preservative amino acids. Studies have shown that dietary supplementation of protective methionine and lysine can improve milk protein, and the effect of both supplementations is better than that of single supplementation.
Buffer can buffer the increase of rumen acid caused by a high-concentrate diet, especially for high-yielding cows, when feeding low-fiber and high-grain diet, adding buffer can improve milk yield and milk fat rate. Buffers commonly used in production mainly include sodium bicarbonate, sodium carbonate, magnesium hydroxide, bentonite and so on.
5. Sodium acetate.
Acetic acid is the main component of synthetic milk fat, and the degradation of crude fibers in the rumen produces acetic acid, which is the main reason for the high milk fat rate of dairy cows fed with roughage. For high-yielding cows, to maintain energy intake level, the feed quantity of roughage is relatively less, and the milk fat rate can be increased by adding sodium acetate. Besides, the addition of sodium acetate also played a buffer role in heat stress.
Besides, the addition of some trace elements, such as copper, zinc, selenium, and iodine, can improve the milk production performance of dairy cows, and also has a certain role in improving the milk protein and milk fat rate. The use of total mixed rations ensure that the nutrients in the diet are adequate and the proportion of concentrate to roughage is reasonable,