Lambs are born with a digestive tract incapable of utilising pasture or other roughage. This is understandable because they initially consume only milk; their digestive tract has to undergo at least 3-4 weeks adaptation to develop the fore-stomachs that ultimately ferment solid feeds.
The development of the fore-stomachs involves the acquisition of a consortium of microbes to populate the rumen, along with the development of its lining epithelial papillae (for absorption of nutrients), muscularisation (for movement and mixing) and expansion of volume (for capacity). These developments can occur independently of each other and the important first step is a process called epithelialisation. Closure of the oesophageal groove allows milk to continue to by-pass the rumen and pass directly to the true stomach.
Diet has a dramatic impact on rumen development. Papillae develop under the influence of the products of microbial fermentation; namely volatile fatty acids, in particular butyrate and propionate. The microbes that produce butyrate and propionate proliferate in the rumen only when lambs are fed a grain-based diet, especially if it contains an ionophore such as lasalocid (Bovatec).
The early introduction of grain to the lamb sets it up for grazing. Pasture or dry roughage intake subsequently promotes muscularisation. Capacity is mainly a function of age rather than diet.
A lamb continues to have a rumen that is proportionately smaller than that of an adult until well beyond weaning. At the same time, the lamb has a high requirement for nutrients for growth and tissue turnover. The younger the lamb, the higher the quality of the diet required to meet this demand. Protein is particularly important in younger lambs, due to the limited capacity of the rumen to supply microbial protein. Energy, minerals, trace elements and vitamins are always important for supporting growth and a robust, resilient constitution.
It is desirable to have lambs weaned by 16 weeks at a minimum of 40% of their mature weight. It is also desirable for the transition to pasture to be seamless and a minimum growth rate of 50 g/d maintained through summer. Since pasture is the most cost-effective source of nutrients, early rumen development is critical to achieving maximum feed conversion efficiency.
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