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Gut morphology

Are We Giving The Calf’s Lower Gut The Credit It Deserves?

Thinking about the nutrition of the neonatal calf we automatically tend to think about colostrum and basic nutrition including energy and protein as well as volumes of milk among other things. These are the things we hear about every day and so it is what we associate with baby calf nutrition. The reality is that there is a very complex discussion going on within the gut with major physical and microbial changes occurring. Gaining some understanding of this can help us focus on the key aspects to supporting the young calf’s development and therefore their future performance.

We are aware that a neonatal calf doesn’t have an adequate thermostat and is susceptible to disease due to passive immunity and an innocence to pathogens. However, we should take a minute to think more deeply about it. Ask ourselves why do cows produce colostrum, transition milk and whole milk? 

A good way to help improve our understanding of it is to think about it from the calf’s point of view. That is, to gain a greater understanding of what is going on within the intestine, including both physical and microbial platforms. This is an area with much more to unveil but thus far investigations have highlighted complicated changes in the gut morphology of the baby calf which begin to explain the production of three milk phases instead of two. 

It has become common knowledge that the calf has a porous intestinal wall to allow the absorption of antibodies (Immunoglobulin G; IgG) which will kick-start the immune system in the calf. This begins to close off and within 24 hours is no longer open to the absorption of these antibodies. It is now becoming more apparent that this is a simplified version of the small intestine which relates specifically to IgG absorption but not its overall porosity. The reality is that it is much more complicated than that. IgG is passed through the cells and the sealing of these sites is referred to as ‘closure’ while spaces between the cells still remain. 

The intestines in the calf, also known as the lower gut, have very few layers compared with the rumen. Rather than a multi-layer, it consists of 2 layers of epithelia with: absorptive epithelial cells, immune cells, and enteroendocrine cells (secretary cells). These have very specific functions such as absorption of nutrients, protection from disease and secretion of materials such as mucous and antimicrobial peptides. This is a relatively thin layer of protection between the inner space of the gut and the circulation systems. See Figure 1 for the comparison between the ruminal and intestinal epithelia.   

In the first weeks of life, the intestines are continually sensing nutrients while communicating with other relevant organs such as the liver and pancreas. The activity, diversity and site of enzymes not only vary with age but also with feed intake. Feed intake refers to both volumes and, more importantly, type. After birth, in the first days and weeks of life, the calf’s intestines must physically develop with the surface area increasing with intestinal villi growing in size and numbers as well as the total mass and overall functionality of the intestine. 

While this is happening, calves are also being exposed to many pathogens to which they do not have immunity. The quicker they can develop these characteristics the greater ability they will have to prevent the pathogens. Feeding the very nutritional transition milk encourages this development. The energy provided by the transition milk plays a major role in helping the gut to fight infection but there is also a bioactive component at work. This includes ingredients such as growth hormone, insulin-like growth factor, and glucagon-like peptide 2 which enhance the growth of epithelial tissue. oligosaccharides which promote the closure of the intestinal wall. 

As the cells in the gut are rapidly proliferating, it is important to provide transition milk for more than one feed. Repeated feeding continually helps to promote intestinal development. Furthermore, it provides a ‘white wash’ of sorts to help protect the calf physically. 

In addition to the physical development of the intestines, gut microbiota plays a significant role in its development. The issue is, that we still have a lot to learn about the microbial population in the lower gut. Most research investigating microbes focused on samples from the rumen or faeces but we have recently learned that the intestines need to be treated and investigated individually. They have their own microbial community which we need to support. As milk bypasses the rumen, the beneficial bacteria in it put a lot of focus on the intestines. When we think of supporting the intestines we automatically think of the ingredients we know most about, macronutrients and micronutrients as well as favourable microbes. Some substances such as oligosaccharides are very important in promoting the cells to bind together. 

While these are all very important, we must also consider the bioactive substances in transition milk that can help support these microbes. Bioactive substances are substances that cause beneficial responses in the gut rather than providing direct nutrition. These are in abundance in transition milk but not so much in whole milk. 

Providing the calf with a stable beneficial population of microbes, rather than allowing pathogens to take over, and promoting lower gut development can help prevent scours and improve the overall gut barrier. This is like a fence to keep out pathogens. The result is a happier healthier calf. 

Development and physiology of the rumen and the lower gut: Targets for  improving gut health - ScienceDirect

Image: Figure 1. Ruminal and Intestinal epithelia. Credit Mike Steele

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