This section has information on the role of various factors in milk yield and composition, including Genetics Stage of Lactation and Persistency, Milking practices, Age and Size, Estrous cycles and pregnancy, Environment, and metabolic diseases.
GeneticsA. Milk composition:
B. Breed differences - Fat is the most variable constituent of milk, minerals (ash) and lactose are the least variable. However, differences among individuals within a breed are often greater than differences among breeds. See Introduction and Milk Composition Lesson for more on differences between breeds in milk composition.
C. Diameter of the milk fat droplet - varies from 1 to 10 microns. Guernseys have the largest and Holsteins and Ayrshires the smallest fat particles. In general, the higher the fat percentage in milk the larger is the diameter of the fat particle. Also, its size usually decreases as the stage of lactation advances.
D. Carotene - is a precursor of vitamin A, and is a yellow pigment. Guernsey and Jersey cows convert much less carotene to vitamin A than other breeds of dairy cattle. Thus, milk from Guernsey and Jersey cows is yellow. Humans can convert the carotene to vitamin A; thus, milk from these cows provides as many vitamin A equivalents am milk from other breeds.
Stage of Lactation and Persistency
Colostrum vs. Milk
See also the Colostrum sections in the Mother & Neonate Lesson.
Colostrum is produced by the udder immediately after parturition. The composition of colostrum is considerably different from the composition of normal milk. Three to 5 days immediately postpartum is needed for the secretions to change to the composition of milk. During this period the total solids, especially the immunoglobulins, are elevated. Newborn calves are practically devoid of immunoglobulins, the antibodies against various disease organisms. Calves must ingest the immunoglobulins from colostrum to acquire a passive immunity against common calfhood diseases. Feeding colostrum after birth is especially critical during the first 24 hours of a calf's life. After this time, enzymes in the digestive tract degrade the antibodies and the permeability of the gut to antibodies decreases.
Lactose content is depressed in colostrum, whereas fat and casein percentage is rather variable. High lactose in the intestine can cause scours in calves, and presumably the reduced lactose content of colostrum helps to prevent this disease.
Calcium, magnesium, phosphorus, and chloride are high in colostrum, potassium is low. Iron is 10 to 17 times greater in colostrum than in normal milk. This high level of iron is needed for the rapid increase in hemoglobin in the red blood cells of the newborn calf.
Colostrum contains 10 times as much vitamin A and 3 times as much vitamin D as that found in normal milk. The newborn calf is practically devoid of vitamin A, and since it provides a degree of protection against various diseases every calf should be fed colostrum.
At parturition milk production commences at a relatively high rate, and the amount secreted continues to increase for about 3 to 6 weeks. Higher-producing cows usually take longer than low producing cows to achieve peak production. After the peak is attained, milk production gradually declines. The rate of decline is referred to as persistency.
After peak lactation, on average the decline in milk yield will be ~6% per month for first lactation heifers, and ~9% per month for mature cows.
Nonpregnant cows will decrease in production about 94 to 96% of the preceding month's yield after the peak of production is attained. Nonpregnant cows can continue to secrete milk indefinitely, but at a reduced rate.
Milk composition changes during lactation:
Fat percentage in milk decreases slightly during the early lactation and then increases as total production decreases with advancing lactation. Milk protein content gradually increases with advancing lactation. Lactose and mineral concentrations increase slightly during advancing lactation. Most of the increase in SNF components of milk is associated with advancing stages of concurrent pregnancy rather than stage of lactation.
A. Milking Interval and Duration - for more on this see the sections in the Lactation Lesson on Frequency of Milk removal.
B. During a Milking - see also the Introduction and Milk Composition Lesson.
Milk first removed from the udder contains much less fat (as low as 1 to 2%) than the milk removed at the end of the milking process (as high as 7 to 9%). The reason for this distribution of the fat globules is not known. Fat globules may aggregate in the alveoli which may retard their passage toward the teat, whereas the fluid portion more readily passes around the fat globules out of the alveolus. Immediately preceding milking, the milk in the larger ducts has less fat than milk in the alveoli.
C. Milking Duration -
Having a set milking time: Comparing 4 min. vs. 8 min. duration with machine on, the cows milked for 4 minutes for an entire lactation produced less milk, especially during early lactation, and were incompletely milked. The 8 minute group was somewhat over-milked.
Optimal milking time for most cows is just over 5 minutes to achieve maximal milk removal.
Leaving 4 lbs of milk in the udder after milking for 10 consecutive days permanently reduces milk yield for the entire lactation.
Age and Size of Cow
Milk yield increases (at a decreasing rate) until about the 8th year of age and then decrease at an increasing rate.
Mature cows produce about 25% more milk than 2-year-old heifers. Increased body weight accounts for about 1/5 of this increase. The remaining 4/5 results from increased udder development during recurring pregnancies.
Heifers should be bred to calve at 24 months of age or earlier if they are of sufficient size to permit delivery of the calf. Although heifers will produce more milk during the first lactation if breeding is delayed to the point where she calves after 30 months of age, total lifetime production will be reduced.
Large cows generally produce more milk than small cows, but milk yield does not vary in direct proportion to body weight. Rather, it varies by the 0.7 power of body weight, which is an approximation of the surface area of the cow (metabolic body size). A cow which is twice as large as another usually produces only about 70% instead of 100% more milk.
Estrous Cycle and Pregnancy
A. Estrus - may temporarily depress milk yield, but this is not occur in all cases. This effect on milk yield is primarily from the increased physical activity of the cow in heat and lowered food intake, rather than an effect of elevated estrogen at estrus.
B. Follicular cysts - Cows with follicular cysts on the ovary produce significantly more milk (adjusted for days not pregnant) than normal herd mates. These same cows produced equivalent amounts of milk before the cystic condition was present. This suggests that circumstances associated with the cystic ovary condition caused the increased milk production and that high milk production does not cause follicular cysts. The longer that cows are cystic, the greater the production in comparison with normal herd mates. Anestrous cystic cows produce more milk than nymphomaniac cystic cows.
[Image kindly provided by D Kelser]
C. Pregnancy - reduces milk production during concurrent lactation. For example, a cow bred at 90 days of lactation will produce 750 - 800 lbs less milk in a 365-day period than if bred at 240 days. By the 8th month of gestation, milk production may be reduced 20% for that month in comparison with nonpregnant cows lactating the same length of time.
D. Twinning - in dairy cattle can result in increased dystocia, and decreased milk and milk fat yields.
A. Environmental Temperature -
C. Exercise - Moderate exercise is conducive to high milk production. Too much or too little is detrimental. Cows in stanchion barns should be turned out at least once per day for exercise and heat detection.
Metabolic Diseases Related to Lactation
Milk production starts suddenly and increases daily in early lactation. Each day greater amounts of nutrients are needed for lactation. Large quantities of nutrients are leaving the body, including: amino acids, fat, glucose, calcium, phosphate and water. The body has stores of amino acids and fat to draw upon, but not much glucose is stored in the body, and Ca stores are difficult to mobilize rapidly. Several metabolic diseases can arise under these conditions.
This section contains discussions on Ketosis and Milk Fever (Parturient Paresis)
Results from a rapid drain of blood glucose at exactly the same time that there is an underlying negative energy balance. Occurs usually 10 days to 6 wks postpartum. All cows are at least borderline ketotic, but only 4-12% develop clinical symptoms. Animals rarely die from ketosis, rather the symptoms persist until a new equilibrium is reached, of course this is at a reduced level of production.
Symptoms include: depressed appetite, decreased milk yield, loss of weight, listless behavior, increased milk fat %, acetone breath, and constipation, ketoneurea, and sometimes fatty liver.
How does ketosis arise? For each kg of milk produced, the cow must produce 50 g of glucose to make lactose, and production is increasing daily. There is an underlying negative energy balance during early lactation. The cow may not obtain sufficient feed to yield enough propionate to maintain blood glucose levels. This is sensed as an energy deficit and -
This results in over production of ketone bodies (and development of fatty liver). Other tissues can metabolize ketone bodies, but this is too much for them. Excessive prepartum conditioning can also accentuate the problem because the excess fat will tend to depress appetite, and will also lead to greater fat mobilization in early lactation.
Cause: unknown. A number of factors have been implicated, including: Insulin/glucagon imbalance, abnormal liver function, adrenal corticoids, thyroxine, and mineral and/or vitamin deficiencies. None of these have been proven to be the cause.
Prevention: Overfeeding during the dry period leads to excessive weight gain, and reduced ability to mobilize nutrients in early lactation. Therefore, moderate feeding during the dry period until 2-3 wk prepartum is recommended. Increase feeding rapidly during the postpartum period, but the cow has depressed intake and appetite.
Treatments: All are aimed at increasing blood glucose
Milk Fever (Parturient Paresis)
Concentrations of blood calcium are very tightly regulated, therefore difficult to change rapidly.
General control of calcium metabolism -