Weaning is defined as the point at which calves transfer from a liquid to solid diet. Weaning is successful when the rumen has developed sufficiently to support the fermentation and digestion of solid feed.
How A Rumen Develops
A dairy cow has four parts to the stomach. They are: rumen, reticulum, omasum and the abomasums (as shown in Figure 1 below). When a calf is born, the rumen is not fully developed and hence the calf only has the ability to digest milk through its abomasum. Hence, it is vital that the rumen of the calf is developed early so that the animal is able to utilise feed more efficiently. Another benefit of developing the rumen early is so that the occurrence of rumen health issues will be minimised later in the animal's life.
At birth, the abomasum accounts for approximately 70% of the volume of the entire stomach of the calf. However, as the calf ages, she will be encouraged to eat grain and the rumen will start to function and the size of rumen will gradually increased while the size of abomasum decreases. By the time the animal reaches maturity, abomasum accounts for less than 10% of the total volume while the rumen, reticulum and omasum will make up the remainder.
Cross section of the rumen consists of three main layers:
a mucosal layer, which is the layer on the inside (as shown below).
a muscle layer, which helps to churn the contents of the rumen. This is vital as it keeps food particles in suspension, to expel gasses produced during digestion and to move digested material into the abomasum.
a tough, fibrous exterior layer to protect the whole organ.
Changing the rumen from a small inactive organ to an organ that functions efficiently enough in order to supply the calf sufficient nutrients for maintenance and growth requires many changes. Firstly, the rumen must be enlarged. The walls must thicken and the papillae which are tiny finger-like projections which become part of the mucosal layer, must also develop and elongate.
Research shows that the stimulus for these changes is the presence of butyric acid, which is an end product of the digestion of grain by micro-organisms in the rumen. Butryic acid is not produced by the digestion of forages such as hay which are commonly fed to calves as farmers believe that they will help stimulate the development of rumen. A good rumen development will turn the inside of the rumen from a smooth pale surface to a covulated, dark coloured surface with greatly increased surface area (papillae) and rich blood supply. This will help increase the ability of the rumen to absorb nutrients.
Development Of Rumen In Response To Different Diet
The figure above showed that forages - hay or straw, are not helpful in terms of rumen development in the early stages. In the pre-weaning phase, grain alone is sufficient to initiate the development process. After weaning, the forage becomes necessary for the musculature development of rumen when the calf is able to digest enough grain to survive and grow on.
Calves requires high energy to function relative to their ability to consume dry food. Hence, if a calf consumes too much hay, this will affect the intake of grain or pallets, which will slow down the growth and rumen development of the calf.
Water is crucial for the development of rumen. As seen from the figure above, milk does not help rumen development as it usually does not even enter the rumen.
Calves should have sufficient supply of clean and fresh water at all times. Take a 90kg heifer for example, she will need around 12 - 15 litres of water on a 25°C day. The requirement for water will increase as the temperature of environment increases. Without sufficient water, the rumen cannot function properly.