1 Selected varieties and individuals
There are great differences in milk yield and milk fat rate among different breeds of cattle. The milk yield of carefully selected breeds such as Holstein cattle is obviously higher than that of local breeds. There is a negative correlation between milk yield and milk fat rate. The higher the milk yield, the lower the milk fat rate. However, the milk swelling rate of high-yield milk varieties can still be improved through planned breeding. Although different individuals of the same breed are at the same growth stage, under the same feeding and management conditions, their milk yield and milk fat rate will still be different. For example, the milk yield of Holstein cows varies from 3000 ~ 12000kg, and the milk fat rate is 2.6% ~ 6.0%. Breeding excellent varieties and individuals and constantly improving them is an effective way for high quality and high yield of dairy cows.
2. Choose high-quality feed
The volatile fatty acid in rumen metabolites of cattle is acetic acid. The proportion structure of propionic acid and butyric acid has great influence on milk yield and milk fat rate of dairy cows. After being absorbed, acetic acid is transported to mammary gland through blood, and a series of short-chain fatty acids are synthesized in milk fat. Propionic acid is absorbed and transported to the liver to become the raw material for synthesizing glucose; Butyric acid is a short-chain fatty acid in milk fat, and can also be condensed with acetic acid coenzyme A to form higher fatty acids. Increasing the ratio of acetic acid to butyric acid is beneficial to improve the milk fat rate of dairy cows.
Milk fat is synthesized by two basic types of precursors, of which about 50% is synthesized by mammary gland with acetic acid and acetic acid produced by rumen fermentation as raw materials, and the rest is supplied by feed fat or fat accumulated in the body (directly forming milk fat in the form of long-chain fatty acids). Different feed ingredients have great influence on the in vivo synthesis of breast distension.
2.l crude fiber
Crude fiber content has the greatest influence on milk fat in feed. Fiber decomposes in rumen to produce acetic acid, while starch can enhance rumen fermentation, reduce pH value and promote the production of propionic acid. The milk fat percentage is positively correlated with the ratio of acetic acid and propionic acid in rumen. If the forage in the diet is less than 50%, or ADF (acid-washed fiber) is less than 19%, or the total feed crude fiber is limited to 13%, the ratio of acetic acid to propionic acid will decrease due to the decrease of dietary fiber content, thus reducing the milk fat content.
2.2 Fat
The positive function of fat is high energy, which can directly provide fatty acids for milk fat synthesis. About 50% of milk fat must be provided by dietary fat. When cows are in positive energy balance, it is difficult to utilize body fat and transport it to the mammary gland, so dietary fat is the only source of milk fat during lactation. The negative effect of fat is unsaturated fatty acid produced by unsaturated fat metabolism, which combines with hydrogen in rumen to promote propionic acid production and reduce milk fat content; Due to the addition of hydrogen, the generated transfer monounsaturated fatty acids can also inhibit the synthesis of fat in the breast. According to the above dual functions of fat, in order not to hinder rumen fermentation and other characteristics, it is appropriate to add 3% ~ 6% fat to feed. Free oil often reduces the milk fat content, but the whole oilseed has no such effect. Soybean oil or whole beans can reduce milk protein content, but cottonseed oil and cottonseed have little effect. Adding unsaturated fat or high dose of fat often leads to the decrease of milk fat rate, mainly because of its influence on rumen fermentation. Therefore, people try to reduce the negative effects of fat by some measures, such as feeding whole or crushed oil cakes, treating fat with formaldehyde to avoid tumor fermentation, and using calcium saponifiers. These technical measures have different effects in maintaining and improving the milk fat rate. It was reported by Gao (1998) that adding 300 g/d calcium fatty acid could significantly increase the milk fat rate (P < 0.05), and the contents of linoleic acid, linolenic acid and calcium in total fat and essential fatty acids were significantly increased (P
2.3 Energy and protein
When the dietary energy level increases, the milk protein content increases, but it may reduce the milk fat rate. Increasing the protein content in the diet usually does not increase the milk protein content, but when the protein is lower than the necessary amount, it may decrease the milk protein. Only when cows are in a state of extremely low nutrition or long-term malnutrition can energy and protein affect the milk fat rate.
2.4 inorganic salts
When the rumen pH value is kept in the normal range, the formation of acetic acid is easier than propionic acid. Due to the increase of acetic acid, the feed intake, milk yield and milk fat rate of dairy cows will be improved. Adding alkalizing agent to fermentable feed such as high moisture grain and silage corn can achieve ideal effect, but adding alkalizing agent to feed such as hay and silage fresh grass with ADF accounting for more than 20% has no obvious effect. The most commonly used alkalizing agent is sodium bicarbonate or sodium carbonate, and the ratio of magnesium oxide and sodium bicarbonate is 1: 2, so the alkalizing effect is better. Zeolite can increase rumen pH value, but at the same time it may reduce milk fat and milk protein content. Han (1986) reported that feeding sodium acetate can increase the milk fat rate during rumen propionic acid fermentation.
2.5 Other feed additives nicotinic acid can improve the milk fat rate or milk yield. It is reported that feeding 6 grams of nicotinic acid daily during postpartum 120 d can increase milk yield 1 1.9%, and the effect of adding nicotinic acid to dairy cows in the middle lactation period is far worse than that in the early lactation period, which may be due to the fact that dairy cows are often in a negative energy balance state in the early lactation period.