B1, also called thiamine, is made in the rumen. When the rumen is unsettled, or cows are under heat or feed stress, B1 levels can drop fast. The first sign is often a sudden drop in milk, then the cows start showing strange behaviour.
Quick takeaways
- A sudden milk drop can be the first sign.
- Later signs can include stargazing (head held high), staggering, tremors and cows that lack energy.
- Triggers of B1 deficiency can be: High sulphur in feed, heat stress, poor water access, sudden feed changes or any other situation that upsets rumen function.
- If you see nervous signs, call your vet straight away. Early treatment usually works well.
- Prevention is about trying to read upcoming feed changes, heat stress conditions and adapting management and feed supplements to maintain rumen stability and performance.
What is happening in the cow?
The rumen does all the grunt work in converting feed into nutrients, and one of those key nutrients is B1. Synthesised by the fibre-digesting bacteria, these bacteria are particularly sensitive to pH drops in the rumen.
When cows are heat-stressed or the rumen is out of balance, rumen pH drops and the fibre-digesting microbes are the first to take a hit. B1 production can fall. At the same time, when under stress in the rumen, other microbes can start producing an enzyme called thiaminase that consumes rumen-available B1, driving down available B1 levels further.
Low B1 can affect the brain and nervous system. If it goes on too long, the brain can swell, and cows can be left with lasting damage or die.
What you might see on farm
- Sudden drop in milk with no obvious reason
- Cows are slower to come to the shed or not grazing down as hard
- Stargazing: head held high, neck straight
- Staggering or drifting, can look like grass staggers
- Muscle tremors, mouth flapping
- Cows lying down more than normal, very dull or slow to respond
What raises the risk?
- High sulphur in pasture, water or supplements. Problems are more likely when total sulphur in the diet goes above 0.5% of the total diet.
- Hot, humid days that cut grazing time and upset rumen function.
- Not enough water, blocked access at troughs, long walks between feed and water or competition at the trough.
- Sudden changes in feed or pasture quality, or variation between paddocks.
Practical steps to help prevent it
- Ensure cows have access to plenty of clean water without competition at the trough
- Avoid sudden feed changes. Step cows onto new crops or supplements gradually. The bigger the diet change, the longer the transition needs to be
- Keep the rumen stable during stress: a slow-release rumen buffer and a quality live yeast product can help, especially when cows are under heat and feed pressure.
- Be aware of sulphur levels in the diet: check high-risk feeds and supplements, and talk with your nutritionist or vet if you suspect sulphur is running high.
- If cows have been under heat and feed stress, extra liver support will help the cows deal with rumen & climatic challenges.
If you think your cows have B1 deficiency…
If you see a sudden milk drop followed by stargazing, staggering or tremors, treat it as urgent. Your vet can confirm what is going on and help you track down the source of the problem.
Treatment is usually injectable thiamine (B1). Vets often give it twice a day for around three days. Oral B1 can be added in feed for prevention once the immediate risk is under control. B1 breaks down fast in water and sunlight, so adding it to troughs is not the ideal mode of delivery.
If you’re seeing symptoms in the herd, keep records of when cases show up, what the cows were eating, water availability and weather. It helps you and your vet spot the pattern and reduce repeat cases.
Talk to your local Agvance Consultant for a tailored programme combining rumen stabilisers, liver support and management changes. Rapid response supports cow health, milk production and long-term herd performance.
