The gastrointestinal tract of the calf in early life undergoes rapid structural and microbial changes and is intrinsically linked with the immunity and health of the animal. Calf nutrition plays a key role in keeping the calf’s gut healthy at all stages.
In cow-calf beef systems, the weaning process is gradual and takes place over several weeks, with complete weaning occurring at six to nine months of age. This is very different to calf rearing operations, where calves are weaned at approximately 60 days of age (1). For these systems, improved weaning strategies are of great importance to economic success. Calf nutrition plays an important role here and optimising the diets for these young animals helps to increase post-weaning performance, while decreasing mortality and disease susceptibility at the same time.
During the first two weeks of life, calves are like monogastric animals, using the abomasum to digest colostrum, milk and/or milk replacer. The milk bypasses the rumen and directly enters the abomasum due to reflexive closure of the reticular groove.
During the transition from pre-ruminant to ruminant, the total volume of the rumen increases from 30 to 70% of the entire forestomach (2). Starter intake drives a rapid increase in ruminal fermentation stimulating ruminal papillae development as a result of production and absorption of volatile fatty acids (VFA’s). Forage intake promotes muscular development, expansion of rumen volume, rumination and saliva flow, with provision of free-choice forage being known to increase starter intake and performance (3) (read more about calf starter intake).
Although rumen development receives the bulk of the attention during the calf’s transition from pre-ruminant to ruminant, the lower gut (small intestine, cecum and colon) is also transformed during this time. After the first day of birth, the gut undergoes a “closure,” where large macromolecules, like immunoglobulins, are no longer able to be absorbed by the gut (4). This highlights the importance of providing colostrum soon after birth which, if successfully achieved within the first hour of life, accelerates the bacterial colonization of the small intestine (5). During weaning, the transition from milk to solid feed can increase gut permeability, compromising the integrity of the GIT making calves more susceptible to diseases, highlighting the importance of early life immuno-competence and colostrum.
The GIT development has a clear and major impact on the digestive capabilities and immune competence of the growing calf with long lasting effects on performance and health. Despite great attention to the weaning phase, calves only become fully functional ruminants at 4 - 5 months of age. Best practices and optimised nutrition during the transition to a fully functional ruminant is extremely important in order to achieve resilient and efficient calves that perform at their best in the feedlots (or feed yards).
