Avoiding Osmolality Problems Will Reduce Scourings Losses

27 July 2020

Dr Tennant, our Ruminant Technical Advisor  says that AHDB calculate calf scours costs the UK cattle industry around £11M per annum in terms of deaths and lost performance.  Published research confirms digestive disorders to be the main cause of both mortality and morbidity in calves (see graph).

 “For these reasons dairy farmers really need to focus on managing calves to reduce the risk and extent of scouring,” she comments.  “To lessen the impact of scours it is vital to look after the gut wall and new developments show the importance of the concept of osmolality in achieving this.”

Osmolality is a measure of the concentration of particles in solution in a liquid and is calculated by adding the concentration of sugars like lactose and minerals including sodium, magnesium and chloride.  When a feed with high osmolality is fed it can result in damage to the gut wall, predisposing a calf to scours.  The higher the osmolality, the more damaging a feed can be to the gut wall. 

“Milk replacers with high levels of minerals and lactose tend to have higher osmolality. This can be a particular problem where lactose rather than fat is used as the primary energy source.”

She says the osmolality of cow’s milk is close to 300mOsm/kg which is optimal for the absorption and digestion of nutrients by calves.  However, many milk replacers when mixed have levels above 400mOsm/kg, increasing the risk of damage to the gut wall.  Some have levels closer to 600mOsm/kg.  For comparison, she points out that sea water has an osmolality of 1000mOsm/kg.

“Increasing osmolality from the target level can affect digestibility and increase the risk of gastrointestinal problems including nutritional scours.”

Dairy vet Emily Payne from the University of Nottingham says it is important to understand the impact of osmolality of digestion in the pre-weaned calf.  She explains the gastrointestinal tract plays two crucial roles. First it absorbs nutrients from the diet to fuel growth and secondly, but equally important, it prevents unwanted compounds and pathogens from entering the blood stream.

“This intestinal barrier plays an essential role in calf health. Anything that alters the permeability of the gut wall can facilitate the onset of disease and reduce the nutrients available for growth. Increasing osmolality affects the gut barrier function and integrity which can have a detrimental effect.

“Where osmolality in milk replacer is too high it is also likely there will be an increased incidence of abomasal bloat.  Finally, high osmolality levels can exacerbate the severity of scouring in sick calves.”

A better understanding of osmolality can help both avoid the risk of damaging the gut wall and also help improve the effectiveness of treatment.  If pre-weaned calf diets have a lower osmolality, Dr Tennent says they pose less of a potential problem.

“To help tackle the issue and reduce the risk of compromised gut wall integrity predisposing calves to scours, Trouw Nutrition developed Energized Calf Milk replacers using cow’s milk as the biological reference.  This is logical as this is what replacers are trying to replicate. 

“Based on 50% skimmed milk and including more fat and less lactose that traditional formulations, they are very close to the optimum osmolality level seen in cow’s milk and help support higher intakes and better growth rates.” 

Dr Tennant comments that an understanding of osmolality can also improve the effectiveness of treating scouring calves.  She says the common and sensible approach is to dose scouring calves with electrolytes to treat the dehydration resulting from scours.

“While the principles of electrolyte solutions are sound, it is vital that their use does not exacerbate the problem.  Many electrolyte supplements are sold as being suitable for inclusion in milk replacers, suggesting this is a quicker and easier way to supplement calves.  While there may be small practicality benefits, it can actually cause big problems due to the impact on osmolality.

“By definition electrolytes are salt solutions so will be relatively high osmolality.  While when fed mixed in water, the osmolality will be close to that of whole milk, but when added to milk replacer levels can be pushed to a point which is likely to impact on gut permeability and reduce the rate and effectiveness of rehydration.

“For effective oral rehydration of scouring calves, electrolyte supplements should always be offered mixed with water and not added to the milk replacer,” Dr Tennant urges.