Over the past two decades, the world saw a general increase in per caput consumption of dairy milk, with more growth in developing country, led by Chinas new ferocious appetite, consuming 28.7 kg/ capita / year in 2007 (9.7% annual growth rate) up from 4.5 kg/capita/year in 1987 (FAO 2012).
Bovine milk is a nutritionally rich product, designed to nourish the young animal to adulthood and is an essential diet for human nutrition (Bus & Wosley 2003). Its yield and quality has benefitted much from understanding of nutritional requirements of dairy cows, better availability and development of feed inputs, supplements and breeding programs.
Cow Milk Composition and Factors influencing composition and yield
General constituents of cow's milk are water, fat, protein, lactose and ash; with milk fat being the most variable component and lactose the most stable component.
In general the average components can be summarised as follow:
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As dairy establishment tries to meet consumers' expectations of a consistent product all year round, standardisation and regulation of milk quality came into place on the basis of the fat and protein content. In Australia and New Zealand, the Code requires that the whole milk should contain at least 3.2% of fat and 3.0% of protein (FSANZ 2012). As milk is a biological process with an inherent natural variation, fluctuations in compositions of milk is to be expected. The average composition of milk from the main breeds of dairy cattle is given in Table 1.
Intrinsically, the physiology difference between herds, cows would yield different fat and protein composition of milk, as well as yield of the milk. Environmental and seasonal changes would also induce large effect and variation in the composition of milk as portrayed in Graph 2.
Usually inverse relationship can be observed from higher-yielding breeds and variety that produce higher content of fat and protein, i.e quality and quantity do not go in tandem. Holstein, the highest yield breed, on average gives out milk of protein content 3.5%, in contrast with Jersey cows that gives out 5.5% (reference). There is also positive correlation between hereditary traits of cow that produce high butterfat and protein content (reference). As such artificial fertilization and selection progress can be done to emphasize subsequent generation of breed with quality milk. Due to this hereditary nature, the difference in composition between different breeds is apparent.
There is also an overall decrease of 0.4% of Solid-not-Fat content of milk over the first five successive lactations. With age, fat content of milk reduce exponentially. (reference).
The protein content decreases during the first or second month of lactation after which it shows a gradual increase.
The milk production of cows increases after calving, to reach a maximum (peak) level during the second month of lactation. It then decreases again gradually as lactation progresses.
The butterfat percentage decreases during the first three months after calving, and then stays constant for three months. After this period of five to six months, more noticeable increase occurs at the end of the lactation period. Therfore, if a great number of cows in the herd calve simultaneously, the fat content of the herd milk may be as much as 0,5% lower two months afterwards, after which it will increase again to a percentage that can be higher than it was during the calving season
Milk protein values are usually lowest in summer month (December and January) for northern Australia; the seasonal trends in fat and milk protein concentration have been recorded consistently in UK (reference). It has been suggested that the main factors are attributed to pasture transition from temperate to tropical pasture systems, with nutritive tropical value (C4) having lower quality compared to temperate pastures (C3) (reference). This forage quality variation will result in lower levels of milk protein (Erasmus et al. 2001, Beever et al. 2001). Dietary nutrition as such has been identified as major factor influencing milk protein in the world (DePeters and Cant, 1992; Murphy and O'Mara 1993). The deviation from the seasonal variation can as well be attributed to photoperiod (Coulon, 1994) and environmental temperature (DePeters and Cant, 1992).
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Pasture feeding system possess natural nutrition variation and as such, supplements will improve the situation when lack of thereof. Literature has reported that a complete ration should contain at least 18% fibre or roughage to prevent significant reduction of fat content. Young, lush pastures, high concentrates, low-roughage rations, and finely-milled roughage (where physical fibrousness is lacking) are examples of rations which produce milk yields with low fat contents. In most cases, reducing the proportion of forage in the diet of a cow increases both protein content and yield. Milk protein content can be increased 0.4 percentage units or more if forage proportion in the diet is reduced to 10% or less of the dietary DM. Malnutrition in the dry period and/or at the onset of lactation reduces the milk yield and fat content of the total lactation period.
Disease normally has a disadvantageous effect on both milk production and milk composition. In cases of mastitis, the fat content decreases, while an increase in the whey protein and chloride content is noticed. Mastitis decreases both yield and SNF.
The fat content of the first milk extracted is 1,95% and that of the last milk 10%. Therefore, if a cow is not milked out fully, some of the fat remains behind and the fat test will be low.
An inefficient milker who cannot win the trust of the cow often causes her to be nervous and is not able to milk out a cow completely. This causes the milk flow to be retarded, and the butterfat test to be detrimentally affected.
Incomplete milking also occurs when the cow is upset during the milking process, if the milking machine hurts her, or if the last milk is left for the calf to drink.
Where the milking intervals are uneven, the cows give less milk after the shorter interval, but this milk will have a higher fat content.
Where cows are milked twice a day at regular intervals, there will be little difference between the fat percentage and milk production of the different milking times, even if the milk yield of the morning is a little greater with a slightly lower fat percentage. Where cows are milked three or four times per day, the milk which is milked in the middle of the day will contain a little more fat.
Over-exercise causes a considerable decrease in milk production. The higher butterfat percentage which is obtained as a result of exercise does not justify the loss in total butterfat.
Caseins and Whey Proteins
Caseins and Whey proteins are the 2 main protein types in milk. Casein proteins composed of about 80% in milk true protein, and are comprised mainly of alpha S1, alpha S2, beta, kappa and gamma caseins. Casein exists in the form of micelles, a macromolecular aggregates of proteins and minerals.
Generally, milks with high lactose and low Na values yielded casein which was comparatively rich in B-casein and comparatively poor in K and gamma-caseins (Davies & Law 1977)
In the dairy production chain, especially cheese production, milk protein composition and the seasonal variation seen in the casein fractions, alpha S1, alpha S2, beta and kappa casein are strongest predictor of cheese yield and quality. The seasonal variation in casein fractions in total casein of milk seems to correlate with periods of pasture transition and declining pasture quality.
Table 1. Composition of milk from the main breeds of dairy cattle