When calves are born the only compartment of their stomach that is fully functional is the abomasum. The omasum, rumen and reticulum compartments are present but not functional yet (See Fig. 1). Hence, newborn calves are not yet functioning as ruminants. That’s why milk or milk replacers are the most important feed in the first 3 to 4 weeks of life. Milk gets digested in the abomasum, providing the calf with all the essential nutrients they require during those first weeks. Around week 3, calves normally start eating dry food and water, so feed fermentation and rumen development start to occur and both will increase gradually until 4-6 months of life, when a fully functional rumen can be achieved. This is called the transition period from “monogastric” to ruminant. Multiple factors can influence this process, which in turn has a profound impact on the growth rate and health of the heifers, particularly after weaning, and this in turn to her future productive life as a cow. Yes, it is all connected. The objective on this initial stage is to convert the milk-dependent calf to a dry-feed-dependent heifer as soon as possible, as this is of substantial economic importance to the producer.
Rumen development refers not only to its size but to its functionality as well. The most important function that needs to be developed is the ability of this neo-rumen to absorb and to transfer the nutrients produced after fermentation to the liver via the blood stream. The absorption of nutrients inside the rumen is done by the ruminal papillae. Then, the specific objective during the transition to ruminant stage is to stimulate the growth and functionality of these papillae. That’s where the key is!
Over the years, extensive research has demonstrated the impact that different components in the diet have on the development of the ruminal papillae. For example, the classic study by Pennsylvania State University demonstrated the negative impact that milk by itself has on ruminal papillae development. Milk combined with hay helps developing the muscular portion of the rumen and to increase its size but does not stimulate the development of papillae either. In contrast, when starch (i.e. grain) is added to the diet, the development of the ruminal papillae and hence the absorption capacity of the rumen is at its best (see Fig. 2). The rumen papillae grow in response to the presence of volatile fatty acids (VFAs), particularly propionate, which is a by-product of fermentation of the grain in the rumen.
From the practical standpoint, the essential feeds to start a proper rumen development are water and a starter concentrate. The sooner the calves have access to these two feeds, and by sooner I mean the next day after birth, the sooner the whole process begins. Then the calves could be either weaned earlier or at a higher weight by the weaning time of your choice. Importantly, if using coarse-crushed grain mixes such as RSF Calf Rearer, hay is not necessary pre-weaning (No, it’s not!) Hay intake in the pre-weaned calf can restrict starter intake, negatively affecting dry matter intake and weight gain potential. As the calf approaches weaning and the rumen has become more functional, hay should be introduced to allow for adaptation to the post-weaning diet. Weaning can happen by the time the calves are eating consistently 1 kg of grain mix for at least three consecutive days. We will go in more detail about strategies and tips to wean calves in a future article.
Raising a calf is the making of the future cow. Most of the shortcuts, shortages and problems during the growth stage are actually collected when that calf/heifer gets into the milking shed. Think of the calf, particularly the first 12 months of age, as the time when the future cow is made or broken. Rumen development is the crucial single thing that can lead to one or the other way simply because this leads to good or poor nutrition. Nutrition will in turn have its impact on the development of all the other systems (e.g. immune, muscle-skeletal, mammary gland, etc.). If by the time the heifer is going to calve for the first time, she is not a 100% then forget about getting her genetic potential for the rest of her productive live. That is the compromise in play.