Lesson 2. NATURE OF VARIATION IN MILK COMPOSITION, GENETIC, PHYSIOLOGICAL AND ENVIRONMENTAL AND THE SOURCES OF VARIATION
Module 1. Milk definition, composition and variation
It is an established fact that the composition of milk is not uniform in all the species. Depending upon the needs of the new born there will be variation in the proportion of the major constituents in milk. In addition to these differences several other factors also influence the proportion of various milk constituents.
2.2 Factors Affecting Milk Composition
The composition of milk will show variation due to several factors. These factors are grouped into:
- Animal factor
- Environmental and
- Miscellaneous factors
2.2.1 Animal factor
The milk composition being an important trait of milch animals and is basically a inherited character influenced by genes. This factor can be modified by proper planning of breeding practices It plays role in the quality and quantity of milk produced
The composition of milk varies between species and is designed to provide different amount of nutrients to new born of particular species to ensure optimum growth, and is an inherited trait for that species. The faster the rate of growth the more concentrated are the milk components needed for this growth. For instance Cat milk contains about 7.0% protein since the new born has to double its birth weight in 9.5 days, similarly the lamb will double its birth weight in about 15 days as such the sheep milk has only 4.8% protein. Human infants take substantially longer time (180 days) to double their birth weight, since the human milk least protein content (~ 1.0%).
There are several breeds in domestic animals which are reared exclusively for milk production. Milk fat being the costliest component of milk, there is considerable selection among the breeds to ensure milk with a considerably higher fat content. To a considerable extent this being an inherited trait selection of suitable breed will be based on the fat percentage in the milk produced by that breed. However, not much variation could be observed in the remaining constituents. Composition of the milk for some exotic and zebu cattle are presented for understanding this influence of breed on the milk composition (Table 2.1)
Zebu cattle also show variation in milk composition between the different breeds. The data of the milk composition due to breed of the cow is given here under ( Table 2.2).
Table 2.2 Average composition (%w/v) of milk from different breeds of indigenous cow
The variation among the milk constituents contributing to the SNF is very marginal the influence of breed on the fat and SNF composition is presented in the Table 2.3.
Table 2.3 Average composition (%w/v) of milk from different breeds of buffalo
d. Individual Cow
Although individual animals maintain the breed characteristic of milk produced by them. However, there will be a variation with the individual animals. This also depends on the genetic characteristics of the individual animals. By following good breeding practices it is possible to get higher fat per cent in the individual or milch animal. A similar variation could be observed in the protein and lactose levels in individual animals within a given breed. Thus, selective breeding can be used to upgrade milk quality apart from the inherited characteristics the environment and various physiological factors influence the composition of milk produced by individual animal.
e. Lactation Period
Similar to the variations observed in the breed and individual animals, variation during the lactation period is also observed in the fat and SNF percentage during a lactation period. Fat content is higher after parturition, which decreases to a low during the first and second months of lactation and then gradually increases during the remaining period of the lactation. The composition of the fat also found to be changing during the lactation. A slight decrease in Lactose percentage could be observed during the last stage of lactation. An increase in the chloride content was observed with the advancement of lactation, Calcium concentration reduces to a minimum and further increase whereas the total phosphorous declines throughout the lactation period. Natural acidity was observed to be at higher level at the beginning of lactation and will decrease to normal value during the first three months and remains fairly steady during the remaining lactation period till the last month it declines sharply.
220.127.116.11 Interval between milking
The fat content of milk varies considerably between the morning and evening milking due to shorter interval of time between the morning and evening milking than between the evening and morning milking. If cows are milked at 12-hour intervals, the variation in fat content between milkings would be negligible, however this is not practicable on most farms. Normally, SNF content varies a little even if the intervals between milking vary considerably.
Due to several physiological changes in the individual cow with advancing age, the composition of milk produced by these animals too shows variation in their composition. As cows grow older, the fat content of their milk decreases by about 0.02 percentage units per lactation. The fall in SNF content is much greater.
All the precursors of milk constituents are derived from blood of milch animal. As such providing the suitable and well balanced feed to milking animal,plays an important role in maintaining requisite levels of constituents in the milk produced. Underfeeding a lactating animal will only reduces the milk yield but also level of fat and SNF. However changes in SNF content are more sensitive to the feeding level than the fat content. Fat content and its composition are influenced more by the roughage (fiber) intake.
Milk production, being a physiological function of a lactating animal, general health condition of individual cow plays a very important role in maintaining good milk composition. Specific diseases have great influence on the composition of milk,both fat and SNF contents reduce when animal suffers from any specific disease.Mastitis, being a disease specific to the udder, has substantial effect on milk composition; it reduces lactose content with a corresponding increase in salt content of milk.
18.104.22.168 Completeness of milking
During the interval between the two milking, milk will be held in glandcistern and teat cistern, consequently there is possibility that fat being the lightest component of milk would be moving to the surface, resulting in the variation of fat percentage in different portions of milk during the process of milking. It is advised to draw the milk completely from the udder during each milking. This will not only ensure that all the milk produced by the animal is drawn but also the fat is not left in the udder. The last part of the milking designated as stripping has a higher fat content as compared to the fore milk which is drawn first during the milking operation. As such completeness of milking ensures uniform quality of milk.
There is wide variation in climate and also type of vegetation available in different seasons of the year. Due to seasonal variation in the availability of fodder, there are changes in the milk composition. In India,butter fat content in milk is maximum in the month of May and minimum in the month of November.Similarly the SNF is highest in October , and minimum in July and September . The variation in the milk production is dependent on several other factors viz., temperature, hours of sun shine,length of the day, exercise, rain fall, drought, stall feeding or pasture feeding, etc. that are associated with the season.
In dry months the total yields of milk tend to decrease along with a decrease in SNF and increase in fat percentage. In wet months there may be a decrease in both SNF and fat percentage along with or without the increase in the yield of milk which depends on the level of nutrition.
2.2.3 Miscellaneous factors
Subjecting the lactating animal to regular exercise will ensure good blood circulation through out the body and keeps the animal in good condition. In order to ensure sufficient reserve of energy such animals will eat more which would indirectly increase the milk yield. It was observed that the feed consumption of the animal will increase resulting in an increase in milk yield along with an increase in the fat percentage when compared with the period of rest.
Hormones play an important role in the animal, especially during the milking time. Favourable conditions during milking of animals result in good milk yield . On the contrary, harassment and frightening conditions such as barking of dog, presence of strangers, bursting of crackers,etc. disturbs the animal and reduces milk yield. A change in the milk composition may also be expected by these conditions. Animal in oestrus will also reduce the milk yield. However , these factors cannot be generalized as they maybe specific to certain individual animal at a given geographic location.