A huge blaze caused by a suspected methane explosion at a dairy farm in Texas has left 18,000 cattle dead and a worker critically injured.

The Animal Welfare Institute is calling it the deadliest barn fire involving cattle since the agency began tracking barn fires in 2013. The animals were valued around $2,000 each - meaning the total cost of the cattle destroyed was $36 million. 

The explosion at the South Fork Dairy in Dimmitt is thought to have been triggered by machinery overheating, causing methane to ignite, although the exact reason has not been confirmed. 

Methane is produced naturally on dairy farms by cows through enteric fermentation and manure storage. The barn is classified as a 'large dairy farm' because it had more than 15,000 cows. 

A female worker was critically injured after she became trapped inside the milking building during the blaze and had to be rescued by firefighters. As of Wednesday she remains in hospital. The farm is run by Frank Brand Dairies.