Later weaning could translate to less risk of diarrhoea and more resilient digestive tract
The older the calf, the less permeable the digestive tract when weaning; lower permeability = lower chance of diarrhoea and other digestive issues
Diarrhoea and other digestive problems are the leading cause of calf death. According to the National Animal Health Monitoring System (NAHMS) of the United States Department of Agriculture, such issues account for over 56% of preweaned heifer deaths in North America alone. The causes are multifactorial and can include farm management issues, stress, sickness, a change in body energy, indeed anything that can lower the calf's resistance to pathogens can lead to diarrhoea and other digestive issues. It also has to do with the physiology of the calf itself. The Gastrointestinal Tract (GIT) has many challenges, and part of what it has to accomplish is to absorb nutrients from ingested food while preventing hazardous elements from entering the bloodstream. This takes place in the epithelium layer of the gastrointestinal barrier. Scientists have just begun investigating the effects of weaning on this barrier, to further understand the mechanisms of digestive problems, especially diarrhoea, during the stressful weaning period.
Stress on the digestive system at weaning
The pre-weaning period is indeed critical for calves. It's the time when calves are most susceptible to disease and yet in dairy calves, it's also the period that can set the tone for eventual higher milk yields. Rearing programs promote dry matter intake (DMI) as a way to stimulate ruminal epithelium development and to minimize the reduction in growth that can occur at weaning. But, since weaning also results in a shift in the site of nutrient digestion, these changes can, as seen in some studies, also affect GI barrier function. The permeability of the GIT plays a critical role in preventing disease and sustaining nutrient digestion and growth, thus alleviating stress during weaning. But is it the weaning itself, regardless of age, that negatively affects permeability and causes GIT stress? Or, does age at weaning make a difference? A 2015 joint study by the University of Saskatchewan, the University of Alberta, and Trouw Nutrition R &D, sought to find out.
Is it age or is it weaning itself? The answer seems to be both
In the study of 14 Holstein bull calves, half were weaned via a step-down process beginning at d 35 and were fully weaned by d 42, while the other half were not weaned. GIT permeability, and other indicators of GIT function, were measured for both groups on d 14, d 28 and d 42. The calves were slaughtered on d 44 and their GITs were studied at length for a variety of factors including multiple measures of permeability.
The researchers found that, while advancing age decreases the permeability of the GIT which enables calves to better fight off disease, the biological changes in digestive site that come with weaning actually increase that permeability. This increase was most pronounced in the rumen, the duodenum and the jejunum, thus increasing the susceptibility of calves to digestive tract problems. So, what does this mean for the age at which weaning should take place? It seems that weaning at a later age could indeed fortify the GIT, thus protecting it from the increase in permeability that comes during the weaning process itself, no matter the age. Protecting calves from GIT problems, including scours, could be yet another reason for later weaning.
Scientists have just begun this line of inquiry, and while more investigation is certainly warranted, one thing is for sure: Lowering the stress during the critical time of weaning would leave calves better able to fight off diarrhoea and other potentially fatal digestive issues.