Bacterial changes in soaked hay explored in British study

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Many people with obese or laminitis-prone horses or ponies soak their hay to reduce the level of water-soluble carbohydrates.
Many people with obese or laminitis-prone horses or ponies soak their hay to reduce the level of water-soluble carbohydrates.

Soaking hay to reduce dust and the levels of water-soluble carbohydrates can result in bacterial changes that may decrease the hygienic quality of the fodder, researchers have found.

The British study team says soaking hay is known to increase the bacteria loading in hay, but no information exists on how this process alters its bacteria profile.

Undesirable changes to this bacterial profile could pose a health risk or digestive challenge to horses by introducing foreign bacteria into the gastrointestinal tract and altering the normal gut profile.

Their preliminary study found that soaking hay not only encourages bacterial growth, but can cause changes in bacterial profiles that are unpredictable. Such changes had the potential to affect the fodder quality.

Researchers Meriel Moore-Colyer, Annette Longland, Patricia Harris, Leo Zeef and Susan Crosthwaite, reporting in the open-access journal PLOS ONE, also found that the soaking of hay before feeding, caused inconsistent leaching of water-soluble carbohydrates (WSCs).

For their study, they set out to map the bacterial profile of three different hays and determine how soaking altered it.

A perennial ryegrass hay and two meadow hays were used for the study, all harvested in July in southwest England.

All were analysed for WSCs and total aerobic bacteria, as well as DNA-based testing, to determine the bacterial families present.

They found that all three hays contained similar bacteria, but in different proportions. Indeed, Proteobacteria, Actinobacteria, Firmicutes and Cyanobacteria comprised 96% of the bacteria present in all three hay types.

Each hay was soaked for 1.5, 9 or 16 hours before further analysis to determine the loss of WSCs and any bacterial changes.

The average percentage loss of WSCs increased with soaking time, from 18% for 1.5 hours (representing 30 grams per kilogram on a dry matter basis); 38% for nine hours (representing 72 grams per kilogram); and 42% for 16 hours (representing 80 grams per kilogram).

The authors said that while the loss of WSCs was largely in agreement with earlier research by others, such losses cannot be predicted nor relied upon, as the variability of loss across the hays was high, from 6% to 63%.

The study found no additional benefit in terms of WSC leaching when hay was soaked for longer than 9 hours.

No pattern was evident in the present study between initial WSC content and the losses that occurred through soaking.

For now, WSC losses from soaking hay for a variety of times cannot be predicted according to hay species or levels of WSC content.

“It is important therefore, to highlight to horse owners that WSC losses from soaking hay cannot be set nor predicted according to either soaking time or hay species.”

The study team found that no relationship existed between leaching of the WSCs and bacteria growth or profile.

Grass type influenced bacterial profiles and their responses to soaking, but no differences were seen in richness or Shannon diversity indices — an index that is commonly used to characterize species diversity.

Analyses showed clustering of bacteria between meadow hays which differed from the perennial ryegrass hay, and this difference increased after soaking.

The authors found a highly variable response in bacteria growth in terms of their profiles across soaking times and hays.

They say more work needs to be done to determine if there is an optimum soaking time to maximise WSC loss without increasing bacteria loading.

“Little is currently known about the survival of feed bacteria through the equid gastrointestinal tract.

“The more hostile regions of the gut, for example, the low pH in the pyloric region of the stomach, may neutralise many feed bacteria but the fact that animals commonly suffer gastrointestinal upset post-ingestion of contaminated foodstuffs suggest that bacteria load may have a major impact on gut health.”

Bacteria growth was also not correlated with WSC content across the hays.

“Several studies have revealed that varying amounts of nutrients can be leached from leaves when the plant is exposed to water, but the plant features that may influence the degree of leaching are yet to be determined.

“It might be expected that leaching of WSC from plant material into a readily available solution would support bacteria growth, however, the correlation across all three hays showed that no relationship existed between the amounts of WSC leached from the hay and the total number of bacteria colony-forming units on the hay.”

While bacteria species appeared to alter upon soaking, variation between replicate samples resulted in no significant differences in the genera being present within the hays after soaking.

The response to soaking was however, individual according to hay type, and no relationship existed between WSC leaching, the quantity of bacteria present post soaking, or the alterations in bacteria profiles.

An increase was noted in the family of potential pathogens the Enterobacteriaceae in one of the meadow hays but this was not mirrored in the other meadow hay or the perennial ryegrass hay.

In their conclusion, they pointed to the highly inconsistent results in soaking hay to reduce WSC content. It can, they say, increase bacteria numbers and may cause a rise in potential pathogenic bacteria in post-treated hay.

“Testing a wider variety of hays may elucidate a response pattern and provide some insight into the behaviour of nutrients and bacteria when hay is soaked, but at present the factors influencing nutrient leaching and microbial growth during soaking are still unclear and unpredictable.”

Moore-Colyer is with the Royal Agricultural University; Longland is with Equine and Livestock Nutrition Services; Harris is a member of the Equine Studies Group, part of the Waltham Centre for Pet Nutrition; Zeef is with the University of Manchester; and Crosthwaite is with NIAB EMR, a horticultural and agricultural research institute in England.

Moore-Colyer M, Longland A, Harris P, Zeef L, Crosthwaite S (2020) Mapping the bacterial ecology on the phyllosphere of dry and post soaked grass hay for horses. PLoS ONE 15(1): e0227151. https://doi.org/10.1371/journal.pone.0227151

The study, published under a Creative Commons License, can be read here

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