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Milk is for mammals the main nutritive source for the young while
they have their greatest relative growth. Therefore the amount of milk and its composition produced by the lactating animal is very well adapted to the special needs of the young. The variation in milk composition between species is illustrated in the table below.
| ___________________________________________________________ |
| Species |
Water % |
Fat % |
Casein % |
Whey protein % |
Lactose % |
Ash % |
Energy (kcal/100 g) |
| ___________________________________________________________ |
| Human |
87.1 |
4.5 |
0.4 |
0.5 |
7.1 |
0.2 |
72 |
| Rat |
79.0 |
10.3 |
6.4 |
2.0 |
2.6 |
1.3 |
137 |
| Dolphin |
58.3 |
33.0 |
3.9 |
2.9 |
1.1 |
0.7 |
329 |
| Dog |
76.4 |
10.7 |
5.1 |
2.3 |
3.3 |
1.2 |
139 |
| Horse |
88.8 |
1.9 |
1.3 |
1.2 |
6.2 |
0.5 |
52 |
| Cow |
87.3 |
3.9 |
2.6 |
0.6 |
4.6 |
0.7 |
66 |
| Reindeer |
66.7 |
18.0 |
8.6 |
1.5 |
2.8 |
1.5 |
214 |
|
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Composition of milk in various species.
As an example, reindeer which are living in very cold areas, need to be provided with a thick adipose tissue under the skin whereby the milk consumed by the young has a high fat content in order to quickly develop the protecting adipose tissue. The pups of the rat are born naked and therefore need milk with a composition of the protein adapted to development of the fur coat.
The shape of the lactation curve differs among different species.
Larger animals such as cows and goats have long lactation curves with peak production in the beginning of lactation, whereas smaller animals such as rabbits and rats have a short and conical curve, demonstrated in the picture below.
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Lactation curves of various species (Adapted from Biochemistry of lactation, ed T.B. Mepham, 1983).
In spite of the different species having different demands on composition and yield of milk, the production capacity is almost the same for all animals. Calculated from the correlation between the amount of milk produced per day and body weight it can be summarised that the milk producing capacity is around 1.7 ml milk/gram milk secreting tissue per day, demonstrated in the picture below.
The modern lactating dairy cow has a much higher milk production
than the calf needs. This is the result of the genetic breeding programs and the great improvements in feeding and management. Furthermore, the consumer demands on milk composition are not fully related to the biological capacity of the ruminants. Consumers and the dairy industry prefer a milk with a low fat content and high protein content. Therefore efforts are made to produce such a milk through breeding and by feeding. This alteration of milk yield and milk composition puts special demands on the modern lactating dairy cow.
Relationship between daily milk yield and maternal body weight (Adapted from Biochemistry of lactation, ed T.B. Mepham, 1983).
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Is it possible for the cow to meet these demands? Let us first discuss what happens during gestation and lactation from a biological point of view. During gestation and lactation the animal is exposed to a very exceptional physiological situation. During gestation the fetus has to be provided with nutrients in order to develop and grow to be born and during lactation the female animal has to produce a lot of milk to feed the young. To manage this, the female increases her food intake, changes the metabolism/food digestion and starts to build up body reserves (adipose tissue). The gastrointestinal tract is growing and its function is optimised. During lactation the animal continues to have a high calorie intake and often even increases its intake a lot more during this period. The body reserves built up during gestation are now used and the metabolism is shifted to a situation where the stored body reserves are broken down and used to provide energy for milk production.
How is this process regulated? During gestation a lot of hormones are activated which are regulating the metabolism and preparing the mammary gland for the coming lactation. After parturition the lactation continues to be regulated by different hormones originating from the brain, stomach and endocrine glands, but also from the mammary gland. Suckling is now the important stimulus for the mammary gland, directly or indirectly influencing hormones controlling lactation, food intake and behaviour. The finding that the mammary gland, is an organ that controls and not only an organ being controlled, is a very important statement when discussing milking technique. It has indeed lately been found that the teats are provided with nerves of different origin which during suckling/milking can influence different organs in the body and thereby different physiological events.
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The modern dairy cow has much higher demands for nutrients compared to her ancient sisters not selected for high milk production. Cows some hundred years ago produced milk enough for the calf, i.e. maximum 2-10 litres a day. It is not unusual that cows today are producing up to 60 kg milk per day, which is a tremendously high production in an evolutionary perspective. This high milk production is therefore associated with exceptionally high nutritional demands. For comparison, just prior to parturition the foetal calf requires approximately 10% of the cow´s net energy intake, while the energy requirement for milk synthesis can approach 80% of the net energy intake.
It is a well-known phenomenon in modern dairy production that the first part of lactation is associated with metabolic disturbances. During the first weeks of lactation the high-yielding cow is often in negative energy balance. The cow is producing high milk yields while she has difficulty to support herself with enough energy due to limited feed intake capacity, demonstrated in the picture below.
To be able to produce very high quantities of milk she therefore is
using her body reserves. In an evolutionary perspective it is not
strange with a short period of negative energy balance, the animal´s metabolism is shifted from an anabolic (build up of energy reserves) to a catabolic situation (break down of body reserves). Since the dairy cow is producing much more milk than she was created for from the beginning, the way to succeed with feeding her during lactation is a real challenge.
MIlk production, net energy intake and net energy balance during lactation in dairy cows (Adapted from: Bauman and Currie J. Dairy Science 63:1514, 1980).
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However, it is not only during lactation that the feeding of the animal has to be given careful attention. It is well known that rapid rearing can result in an earlier age of first parturition, but rapidly reared heifers often have inferior milk yields. The initial ductular framework on which secretory tissue is subsequently laid takes place before puberty. If the heifers are rapidly reared, this growth is reduced and the mammary gland has more fat tissue compared to animals reared at a slower rate. Furthermore, these heifers have reduced plasma growth hormone, a hormone that highly influences lactation capacity. Therefore, the maximum recommended growth rate for Holstein heifers and heifers of similar type, is a growth around 600 grams per day in the period between 90 and 325 kg body weight and growth around 800 grams per day in the period from 325 kg up to three months before parturition, demonstrated in the picture below.
Feeding during the dry period is also challenging. During this period feeding should be restricted to avoid fat cows which leads to metabolic disturbances during the beginning of lactation. It has been recommended that the pregnant cow at the end of the dry period should be adapted to high food intake in the beginning of lactation to avoid negative energy balance by a gradual increase of concentrate the weeks before calving. Research work is going on in this area and hopefully new light will be shed on the subject.
A B
Yield of milk (kg, FCM - fat corrected milk) during the first 250 days in first lactation, depending on growth rate (g / day), in period 90-325 kg body weight (A) and 325 kg body weight - 3 months before calving (B). (Adapted from Foldager & Sejrsen, 1989. In Mjölkkor, ed. Anderson et al. 1991, Stockholm.
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The dairy cow has, as a ruminant animal, special capacity to digest feeds with high amounts of fibres such as cellulose, feeds which generally are not very suitable for monogastric animals. Therefore, the feeding for dairy cows ought to consist of a ration with a high proportion roughage and low proportion concentrate. Today, to reduce the content of fat in the milk, farmers are feeding cows with high amounts of concentrate (including a high level of starch). A high level of starch and low level of fibres will alter the fermentation in the rumen. In the end this will influence fat metabolism in the mammary gland, resulting in lower milk fat content. Too high levels of concentrate might create metabolic disorders for the ruminants and lead to low milk fat syndrome.
What is then so special with a ruminant´s digestive tract? As
mentioned above the unique ability of the ruminant is its possibility to digest fibres. This digestion is taking place in the rumen, and a lot of different bacteria and other microorganisms are responsible for this process. During the fermentation process carbohydrates are digested to volatile free fatty acids (VFA) such as acetate, propionate and butyrate, which are mostly absorbed through the mucosa of the rumen. The degradation of protein in the rumen results in 20-80% in microbial protein while the rest, 80-20 %, is undegraded and will be digested in the abomasum or intestine together with the microbial protein. The diet for dairy cows usually contains a relatively low amount of fat, which is digested to glycerol and fatty acids. By products from the microbial metabolism are passed through to the other stomachs and absorption of different components takes place in different parts of the intestine. The rumen, reticulum and omasum can be compared to a kind of foregut fermentation chamber. It is important to have in mind that when we are feeding the cow we are first feeding the microbes, which in turn are feeding the dairy cow, demonstrated in the picture below.
Schematic picture of the digestive tract of the dairy cow.
How much feed does a high producing cow consume? The amount of feed the dairy cow consumes is dependent on factors related to the cow as well as to the environment. Basically the appetite of the cow is regulated by hormonal control and rumen fermentation acids. Furthermore, it has been observed that fatness reduces feed intake. Lactation stage, milk yield, feed composition, and frequency of feeding are also factors of great importance. Generally, a dairy cow of 600 kg body weight producing around 50 kg milk consumes about 25 kg dry matter of feed per day, or in other words a high producing cow must at least consume 4% of her bodyweight on dry matter basis per day. The total water consumption is between 3.5 and 5.5 litre/kg dry matter intake.
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