27/08/2018
PRINCIPLES OF FEEDING DAIRY COWS
Digestibility of feeds and feed evaluation
Not all the feed an animal eats is digested. The part what is not utilized will leave the body as faeces. The digested part of a feed can be expressed as a percentage of the total feed intake. This percentage is called the digestibility coefficient. Depending on the kind of feed the digestibility may vary considerable. Some products like young grass
and green fodders, which have a low crude fibre content, are very easily digested. Other products like overgrown fodders, straw and stover are very fibrous and have a very low digestibility coefficient.
The feeding value of a feed is mainly determined by its energy and protein content. Several feed evaluation systems take these 2 elements into consideration. In tropical countries most commonly the Total Digestible Nutrients (TDN) system is used for ruminants. The TDN considers the part of all the energy present in a feed, what can be digested by the animal. The protein content is expressed as Digestible Crude Protein (dcp). This is the part of the total protein content of the feed, which the animal is able to digest and to utilize.
Both the TDN and dcp values are usually expressed as grams per kg. When rations for the animals are being calculated, we want to know the feeding value of the feed as it is. So we express the TDN and dcp values as grams per kg feed.
In case we want to compare the feeding value of different feeds, we are interested in the quality of the dry matter. In that case we express TDN and dcp in grams per kg dry matter (dm).
Dry matter intake
To match the feeding value in a certain quantity of feed with the total requirement of an animal, we should have an indication how much the animal is able to eat. This quantity is expressed in dry matter (dm). The reason is that the quantity of water in a feed in fact can be considered as ‘drinking’ water for the animal. It does not contribute to the feeding value and to the intake of a particular feed.
The dry matter intake of an animal depends on a number of factors:
- the cow (age, weight, milk production, pregnancy, breed);
- the feed (quality, digestibility);
- the feed supply (grazing system, frequency of feeding, variation in the ration, water availability);
- the environment (temperature, rain).
When estimating the dm intake of an animal it is assumed that it receives an unlimited supply of roughage. This means that the animal has permanent access to the fodder. After eating as much as it likes, there should remain an edible left-over of about 10 % of the supplied forage.
It has been established that under those conditions a ruminant will eat between 1.5 % and 2.5 % of its bodyweight in form of roughage. The actual percentage depends heavily on the quality of the fodder which is supplied. When an animal receives poor quality roughage its intake is reduced. And since the fodder it eats contains only little nutrients, the total intake of nutrients becomes extra low. Therefore good quality roughage is the basis for a high milk production. In general we can say that a cow with a bodyweight of 450 kg is able to eat approximately between 7 – 11 kg dm roughage per day. However, it should be realized that in order to achieve this amount, the intake of young, fresh grass might easily reach over 60 kg per day. In case the farmer cuts and carries the grass, he should supply about 70 kg fresh material per cow per day to enable the animal to get fully satisfied.
On average we can work in our calculations with a dm intake out of roughage of 2 % of the bodyweight.
The total dm intake, including concentrates, of a normal producing dairy cow can be considered in the range of 8 – 13 kg dm per day. However, a well developed, high producing cow, which is supplied with good quality fodder and concentrates might even take in up to 18 kg dm per day.
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