6.8.2. Environmental Factor

The environmental factors that affect milk yield and composition are nutrition, stage of milk production, and season where the cow is lactating.

Lactation Curve

A normal lactation length for a dairy cow usually lasts around 10 months (305 days). The remaining two months are known as the “drying off” period. Colostrum is the first milk produced by the cow after calving. Milk yield will rise during the first months after calving, followed by a long period of continuous decline. The shape of the lactation curve will differ for each individual and also the breed of the animal. Feeding and management will also influence the shape and have a significant impact on the total amount of milk produced. Peak yield is the point where the cow reaches the highest milk production level during the entire lactation. Normally the peak is reached four to ten weeks after calving. Milk yield remains high for a while and then gradually declines in the later stage of lactation (Figure 8).

The dry period is the most important phase of a dairy cow’s lactation cycle. The dry period normally ranges from 55 to 60 days. This is meant for preparing the cow for the next calving or lactation. Therefore, any abnormalities during the dry period will have a negative effect on the cow’s health and milk production after calving.

Nutrition

Underfeeding cows reduces milk production and lactose percentage, but increases the fat, protein, and mineral content of milk. Feeding an adequate ration reverses these symptoms. As a general rule, any ration that increases milk production usually reduces the fat percentage of milk. The normal ration of the dairy cow usually contains 3 to 4% fat. Altering the type of fat in the ration of the dairy cow has not always produced consistent changes in the milk fat content. However, feeding cod liver oil and other highly unsaturated oils to dairy cows markedly reduces fat percentage without alteration of milk production. But until milk is priced on a basis other than yield and fat percentage, it seems unlikely that these practices will be followed.

There are a number of methods of feeding that depress milk fat percentage and simultaneously stimulate higher milk production. Perhaps the most common practice is restricted roughage, high grain ration. When roughage is restricted to 30% or less of the dry matter fed, fat percentage may be reduced to only 2% or less. Rations that depress fat percentage include: finely ground forage; high corn diets (especially if pelleted); flaked corn or heated starches; expanded grains; pearl millet; and lush spring pastures. Such rations obviously should not be fed for prolonged periods of time. Dairy cows in good flesh during the first one to 2 months post-partum yield milk that is richer in fat than cows that enter lactation in thin condition.

Cows fed large quantities of grain throughout the dry period will produce milk that is richer in fat and SNF for a few months post-partum than cows on a more modest feeding regimen. Restricting the protein content of feed reduces milk production and reduces the SNF portion of milk. Addition of protein in the diet above normal levels does not increase milk yield and only slightly increases the protein. Furthermore, when the total plane of nutrition is increased 25 to 35% above normal recommended levels the SNF content of milk increases only about 0.2%. Lactose is relatively insensitive to changes in the cow’s diet. About the only nutritional regimen that alters lactose is underfeeding, in which case lactose percentage is slightly reduces.

Vitamins A and D are not synthesized in the cow’s body; therefore, their levels in the milk are
influenced by the amounts in the feed and, for vitamin D, the amount of exposure the cow has
to sunlight. Vitamin A deficiencies in feed (especially forages) can be severe enough to reduce
the vitamin content of the milk to the point where calves cannot be raised on it. The only
minerals in milk that can be altered appreciably are iodine and iron. In fact the mammary gland
traps so much iodine from the blood that lactating cows may actually be in a thyroid deficient
state. Iron is found in only limited quantities in milk, but large quantities of iron can be fed to
increase the iron content of milk.

Stage of lactation

Colostrum is the secretion of the mammary glands during the first few days of lactation after giving birth. It differs from normal milk in composition, flavor, and odor. The odor is strong and the flavour bitter. A temporary increase in fat occurs shortly after calving. For about the first three days the colostral secretion will coagulate upon the application of heat. Normal composition of the milk occurs about five days after parturition. This gradual change apparently accustoms the suckling animals to the use of normal milk. Colostrum contains more solids than the subsequent milk. Colostrum is very rich in globulins that serve as the carrier of antibodies, which protect the sucking animal against disease-producing organisms.

Time of Milking

The time of milking has some effect upon the composition. If an equal length of time elapses between the morning and evening milking there is no consistent difference, but if the cow is milked in the morning and then late in the evening, the morning milk may contain from 0.5 to 2.0% more fat. If the cow is milked three times a day, the noon milking usually shows the highest fat content. The amounts of protein and lactose in the milk of an individual cow vary but little from one regular milking to the next.

Seasonal Variation

The temperature of the environment influences the composition of the milk as well. The fat
content of the milk decreases a s the weather becomes warmer and increases again with the
approach of the cool season. The fat content also t ends to increase towards the end of the
lactation period. If this comes in the cool season, the rise in fat content of the milk may be very
noticeable. The solids not fat content of the milk generally follows the variation for the fat
content.

Disease and Drugs

Many diseases, especially mastitis, ketosis, milk fever, and digestive upsets adversely affect milk production and may alter the composition of milk. For instance, Diseases like mastitis, lameness, and endometritis, as well as treatments like some antibiotics, can lead to reduced milk production. Regarding certain drugs such as Corticosteroids like dexamethasone, for example, can cause a substantial reduction in milk production and alter the concentrations of various milk components.

Milk yield and composition are the results of genetic differences of dairy cows and environmental factor where the cows are management.