By Mark Andrews 

Hay can be treated with steam to reduce the horse’s exposure to inhaled allergens that cause respiratory disease. Steaming kills potentially harmful microorganisms and binds fungal spores and dust particles to the hay, making them less likely to be inhaled.  

However, new research shows that steam treatment can have an adverse effect on the digestibility of protein in the hay.  

A team of scientists from Martin Luther University Halle-Wittenberg (MLU) has discovered that steam treatment causes a chemical reaction that damages the proteins in the hay and makes them more difficult for horses to digest. This can lead to signs of nutrient deficiency in the animals and, for example, impair growth or muscle development. A report of the work is published in the journal Animals. 

Professor Annette Zeyner from the Institute of Agricultural and Nutritional Sciences at MLU explains: “Many horses suffer from lung problems such as equine asthma. The steaming process virtually eliminates all of the living microorganisms and particles in the hay that could be inhaled during feeding and damage the lungs. In theory, the end result is a very good forage."  

However, her team discovered that the treatment also has its disadvantages as the steam damages the proteins in the hay.  

“A high proportion of the proteins, and the crucial amino acids contained in them, can no longer be digested by the small intestine — in other words, the horse lacks these proteins as a result of the steam treatment. However, some of these protein components are essential for horses and they cannot be absorbed in the large intestine,” Zeyner continues. 

The researchers demonstrated this by examining various hay samples collected from central Germany. In the steamed hay, they found an increased number of products that are generated by the Maillard reaction, an indication that the proteins in the hay have been damaged. (The Maillard reaction is a chemical reaction between amino acids and reducing sugars that also takes place when food is cooked, baked, or fried, and is responsible for browning or the development of flavours.) 

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“Proteins are composed of amino acids. The steaming damages them and they form new complexes with sugars in the hay,” explains the first author of the study, Caroline Pisch, from MLU. This makes them difficult for horses to digest. According to the researchers’ analyses, the treatment reduced the amount of protein that can be absorbed by the small intestine by almost half. The pre-caecal digestibility of the essential amino acid lysine was over 50 percent lower after steam treatment. 

According to Zeyner, this can lead to an undersupply of essential amino acids from the feed, which can be a problem for growing horses or lactating mares; young horses need proteins to grow, and mares need them to produce milk. To make matters worse, protein deficiency causes very unspecific symptoms in the affected animals. These include impaired muscle development and a dull or shaggy coat with so-called “hunger hair” — long isolated hairs in the horse’s coat.  

She suggests that horse owners can counteract this risk by enriching the animals’ diet with protein-rich single feedstuffs such as yeast and soybean meal or high-quality protein-rich compound feeds.  

The report concludes: “Steamed hay is still a proper and sometimes the only possible roughage for horses suffering from respiratory diseases such as equine asthma. Essentially, horse diets based on steamed hay should be balanced accordingly.” 

For more details, see: Effect of Hay Steaming on the Estimated Precaecal Digestibility of Crude Protein and Selected Amino Acids in Horses. 

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Published with the kind permission of Mark Andrews, Equine Science Update

Photo: Shutterstock/Luise123