A calf with a visibly hollow or sunken flank is more than just a sign of an empty stomach. It can also be a practical visual indicator the calf is at greater risk of disease, poor performance, and even death.
The area in question is the paralumbar fossa, the triangular region between the last rib and the thigh. In a healthy, well-fed calf, that triangle is difficult to distinguish because the abdomen appears full. When the flank is noticeably hollow, however, it usually means the calf has gone too long without adequate nutrition.
Newborn Calves Lack Critical Energy Reserves
Newborn calves do not have much energy reserve to begin with. Calves are born with only about 2% brown adipose tissue, or brown fat, leaving very little stored energy available during periods without feeding.
Once those reserves are depleted, calves begin pulling energy from their own body tissues. This reduced energy availability likely contributes to poorer disease resistance, particularly during transport and the early receiving period.
The Link Between Sunken Flanks and Calf Mortality Risk
Research from the University of Guelph evaluating nearly 5,000 veal calves found about 20% arrived at the veal facility with a sunken flank. Calves showing flank hollowing had a fourfold greater risk of dying within the first 21 days after arrival compared to calves without a sunken flank. They identified sunken flank as one of several arrival findings associated with early mortality, alongside dehydration and abnormal navels.
Researchers suggested a sunken flank may reflect long periods between milk feedings and depleted energy reserves before arrival at the veal facility. The effect may be especially important in calves exposed to longer transport times, where energy demands increase while nutrient intake remains limited.
A hollow flank is not simply a sign a calf missed one feeding. It may be an indicator of prolonged nutritional stress and reduced ability to cope with disease challenges during one of the most vulnerable periods of the calf’s life.
Signs a Calf May Be at Higher Risk
A sunken flank is often one of several indicators a calf may already be struggling. Watch for:
- A visibly hollow paralumbar fossa
- Reduced alertness or a weak suckle reflex
- Signs of dehydration
- Delayed feeding after transport
- Cold stress or poor thermoregulation
- Reluctance to stand or move
- Calves arriving after prolonged transport times
A sunken flank alone does not diagnose disease, but it can act as an early warning sign that a calf has depleted energy reserves and may require prompt nutritional support and closer monitoring.
Transport Stress Accelerates Calf Energy Depletion
Transportation is one of the major periods where these issues can develop quickly. Calves may spend many hours moving through marketing channels, and the timing of the next feeding is often uncertain. Sunken flanks develop over time when there are long gaps between milk meals.
At the same time, calves are adapting to transport, new environments, social mixing and pathogen exposure. Long transport periods may further worsen energy depletion because calves expend additional energy trying to move and maintain body temperature, particularly during colder weather.
By the time the calf reaches its destination, the hollow appearance of the flank may reflect much more than temporary hunger.
Milk Feeding Protocols Before and After Calf Transport
Feed calves as close to transport as possible. Current calf care recommendations encourage feeding calves within one hour before leaving the dairy farm. Work evaluating feed management before and after veal calf transport has shown that calves fed two 3L milk feeds compared with one 2L milk feed before long-distance transport reduced the negative impacts of prolonged fasting, especially regarding energy balance and hydration status. However, even 6L of milk replacer pre-transport did not fulfill the metabolic needs of calves during long-distance transport.
Receiving protocols are also important. Calves with severe flank hollowing may require multiple feedings before the abdomen begins to fill out again. The gradual return of a fuller appearance in the paralumbar fossa can be a useful indicator the calf is recovering nutritionally.
Flank Fill Assessment is a Vital Calf Management Tool
Evaluating flank fill is inexpensive, quick and practical. Farm staff, transporters, and receiving personnel can all be trained to recognize the difference between a well-fed calf and one showing significant hollowing. That makes sunken flank assessment a useful welfare and management tool across the calf supply chain.
A sunken flank should not be dismissed as just a cosmetic observation. In many cases, it is a visible warning sign a calf has entered a higher-risk state. Recognizing that signal early, and responding with prompt nutritional support, may help reduce disease risk and improve calf survival during one of the most vulnerable stages of life.
