Bloat is an over-expansion of the abomasum (abomasal bloat; typically, in calves five to 10 days old) or rumen (ruminal bloat; typically, in calves over three weeks of age) soon after milk feeding. It is due to gas (produced by the normal fermentation of feed) building up and being unable to escape.The onset of bloat in calves is a complex subject and often triggered by multiple risk factors. The exact cause is unknown, but it is clear that certain things must happen to produce the condition.

An excess of fermentable carbohydrate in the stomach (from milk, milk replacer or electrolyte solution);
A fermentative enzyme (produced by bacteria, many of which are present in the calf stomach all the time);
Something that slows the rate of abomasal emptying. The pre-weaned calf must have milk to grow and develop, but anything that slows down the rate at which milk empties from the abomasum will give the bacteria present more time to ferment this essential feed.

It’s often difficult to identify the specific reason why cases of bloat are happening but understanding the main risk factors for the condition, correctly and consistently mixing a proven milk replacer and always following best practice husbandry guidelines will help.

Key risk factors for abomasal bloat – and how to reduce their impact

Feeding irregular milk volumes

A large milk volume takes longer to empty from the abomasum giving bacteria more time to ferment the milk and this may cause an excess of gas. Milk feeds should be limited to 3L per feed (reduce to 2L per feed during a bloat outbreak).

Inconsistent milk mixing rate:

The second factor influencing how long the milk sits in the stomach is the osmolality of the milk (how concentrated it is). Always mix milk replacer according to manufacturer guidelines and mix at the chosen concentration consistently.The osmolality of good quality milk replacers mixed at up to 15% (150g of powder per litre of mixed milk) will be fine for the calf, but bear in mind that other on-farm factors can subsequently increase the osmolality of the mixed milk (e.g. poor calf health, such as scouring; use of soft water and low water intakes).

Insufficient water intake:

Water intake is key. A low water intake will increase the osmolality of the milk being fed and slow the abomasal emptying rate (in combination with any scours and incorrect mixing rate).Water from a private bore hole may contain a high microbial content. Always make fresh, clean, ad lib mains water available from the first day of life. If there are more than 20 calves in a group, provide two water points.

Poor colostrum management:

Always feed a minimum of 3L of quality colostrum within two hours of birth and a further 3L within 12 hours. Incorrect use of a stomach tube can damage the vagal nerve, which is responsible for abomasal motility.Consequently, always use a tube that is soft, clean and in good condition or switch to bottle feeding colostrum via a bottle and teat.

Feeding milk at different temperatures:

Variations in milk temperature will alter the rate of abomasal emptying and stop the closure of the oesophageal groove. Milk should always be fed at a consistent temperature of 38-39°.

Abrupt diet changes:

An abrupt diet change can slow the abomasal emptying rate and lead to excessive gas production – be consistent and make any diet changes gradually.

Feeding the wrong starter:

Introducing a starter feed containing too much readily fermentable energy can easily produce too much gas. Likewise, an unpalatable, dusty, old and/or wet starter feed will reduce intakes and delay rumen development.Always feed a bespoke calf starter feed (either pellet or coarse mix) with a crude protein level of at least 18% – and make sure it is always provided dry and fresh from day three.

Poor hygiene and dirty feeding equipment:

Poor hygiene can result in excessive growth of gas-producing bacteria. Always ensure feeding equipment and teats are in good condition and are kept scrupulously clean.

Compromised calf health:

Poor calf health due to infectious scour organisms can cause mucosal damage, which reduces lactase activity (lactase is essential to break down lactose). Cold and wet weather can also increase susceptibility to disease because energy is being diverted away from fuelling the immune system to simply keeping calves warm.Always discuss the health status of your calves with your vet. Rapid growth of certain types of bacteria on farm can result in excessive gas production. Mixing and feeding a good quality milk replacer correctly and following calf husbandry guidelines will reduce the risk of any bloat issues.

Reducing the risk of bloat in calves

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