Cows are ruminants, meaning they have four stomachs. As an adult the rumen is the main stomach for digestion, however when they are a calf, this stomach is small and undeveloped.
It is important to aid calves in developing their rumen from a young age to ensure that they can efficiently obtain nutrients from feed and convert it into either milk or meat (muscle and fat) later in life. Animals with a poorly developed rumen will not efficiently gain weight compared to those that have a well-developed rumen.
In a calf, the abomasum is the main stomach which digests milk. Within three to four months, the rumen will develop into the main stomach for digestion (of feed). This means additional feed needs to be offered to the calf from one week of age. This will also ensure calves are easier to wean.
Pellets and meal
A quality meal that is high in starch is essential. Meal gets deposited directly into the rumen where it stimulates development by growing the population of microbes in the rumen. These microbes produce volatile fatty acids which stimulate the development of rumen papillae (finger like projections lining the wall of the rumen) which aid in digestion. The better developed these papillae are at weaning, the more energy the calf will get from grass and pellets.
Always feed pellets and meal according to package directions.
Hay is essential in developing the muscles surrounding the rumen. These muscles encourage rumen motility and feed movement. Hay needs to be of good quality and clean (no mould!). Hay racks are ideal as they keep the hay off the ground. Avoid overfeeding hay prior to weaning as this will result in a large rumen with underdeveloped papillae.
Calves need free access to water. Rumen microbes require water to survive. This water cannot be provided from milk or milk replacer as these bypass the rumen and go into the abomasum only (due to the suckling effect activating the oesophageal groove). Water also aids in the absorption of volatile fatty acids and stimulates feed intake. It is important that water is kept fresh and clean so calves do not get sick.
Figure from Dairy NZ site note:
Figure 1: Calf rumen development after 6 weeks, fed either milk, milk and hay, or milk and grain. Source Penn State University, USA