LC-MS Reveals Hay Steaming's Chemical Effects

Steaming hay is known to cut down on the number of bacteria and mold in it, but scientists are not entirely sure how this process affects the hay's nutritional value, or what happens to microbial levels if the hay sits in storage for a while after being steamed.

A team of researchers from Leipzig University and the Dresden University of Technology, both in Germany, set out to test two ideas: first, that steamed hay could be stored for up to 24 hours without bacteria and mold levels creeping back up, and second, that the steaming process itself might trigger an increase in certain byproducts (called Maillard reaction products) that form when heat causes sugars and proteins to react with each other. The researchers took hay that had gone bad and steamed it, then stored it in different ways—in hay nets, buckets, or left in the steamer itself—under conditions mimicking both summer and winter weather. To check for bacteria and mold, they tested the hay before steaming, right after steaming, and again after it had been stored for 12 and 24 hours. They also analyzed the hay's nutritional content before and after steaming, using standard testing methods. On top of that, they measured two specific compounds—CML and furosine—that can form when food is heated, using liquid chromatography-mass spectrometry (LC-MS) and ion exchange chromatography (IEC) to detect them. A paper based on this research was published in the Journal of Equine Veterinary Science.¹

Why is Hay Steaming Important for Horse Health, and What Gaps Remain in Understanding How Steamed Hay Holds Up During Storage?

Making sure hay is clean and free of harmful bacteria and mold is important when it comes to feeding horses properly, especially for horses with asthma. One way to improve hay's cleanliness is by steaming it, which helps cut down on mold, dust, and allergens. When done right (meaning the steam is hot enough, applied for the right amount of time, and spread evenly throughout the hay steamer), this process can eliminate 98-99% of the bacteria and mold present.^2,3In real-world situations, it is not always possible to feed hay to horses right after it's been steamed. Because of this, it is important to know whether the improved cleanliness of steamed hay holds up over time when stored the way it normally would be. Earlier research looked at how well steamed hay maintained its microbial quality after being stored either in lab equipment (like a drying cabinet) or simply at room temperature. Those studies found that when hay was stored at either 50°F or 77°F (10°C or 25°C) for 24 hours, no new bacterial or mold growth occurred.^4,5

The researchers report that what has yet to be studied is how hay holds up when it's stored under normal, real-world barn conditions, including how different seasons, changes in temperature and humidity, or even dust from the stable itself might affect it. However, recent research has shown that hay is more likely to grow bacteria and mold when it holds more moisture. Specifically, hay is at greater risk of spoiling once its dry matter content drops below about 85% (or 850 grams per kilogram).^5-7

What Did the Researchers Find When They Tested the Effects of Steaming and Storage on Hay's Microbial Content and Nutrient Quality?

The research team found that steaming caused the hay to lose some of its dry matter content, but that level bounced back up after the hay had been stored for 24 hours—whether it was kept in a hay net (during summer conditions) or a bucket (during winter conditions). Steaming also did a great job of killing bacteria, wiping out between about 96% and nearly 100% of the total bacteria present. After 24 hours of storage, bacteria and mold levels stayed within safe limits, according to established standards. However, in some of the experiments, certain specific types of bacteria and mold crept above those safe limits after just 12 hours of storage. As for the heat-related changes in the hay, the team saw levels of furosine roughly doubleafter steaming, and levels of CML nearly triple; both changes were considered statistically meaningful.¹

“Steamed hay,” report the authors of the paper,¹ “can be stored under various practical conditions for 24 hours without significant microbial recontamination. Hay steaming leads to a loss of preceacal digestible protein and an increase in Maillard Reaction product.”

References

1. Kummer, L. K.; Schrödl, W.; Schnabel, C. L. et al. Effects of Steaming and Storage on Microbial Quality and Nutrient Losses of Hay. J Equine Vet Sci. 2026, 106146. DOI: 10.1016/j.jevs.2026.106146
2. Moore-Colyer, M. J. S.; Lumbis, K.; Longland, A. et al. The Effect of Five Different Wetting Treatments on the Nutrient Content and Microbial Concentration in Hay for Horses. PloS One2014, 9 (11), e114079. DOI: 10.1371/journal.pone.0114079
3. Moore-Colyer M. J.; Taylor, J. L.; James, R. The Effect of Steaming and Soaking on the Respirable Particle, Bacteria, Mould, and Nutrient Content in Hay for Horses. J. Equine Vet. Sci.2016, 39, 62-68. DOI: 10.1016/j.jevs.2015.09.006
4. Glatter, M.; Bochnia, M.; Wensch-Dorendorf, M. et al. Feed Intake Parameters of Horses Fed Soaked or Steamed Hay and Hygienic Quality of Hay Stored following Treatment. Animals2021, 11 (9). DOI:10.3390/ani11092729
5. Humer, E.; Hollmann, M.; Stögmüller, G. et al. Steaming Conditions Enhance Hygienic Quality of the Compromised Equine Hay With Minimal Losses of Nonfiber Carbohydrates. J.Equine Vet. Sci. 2019,74, 28-35. DOI: 10.1016/j.jevs.2018.12.024
6. Harris, P. A.; Ellis, A. D.; Fradinho, M. J. et al. Review: Feeding Conserved Forage to Horses: Recent Advances and Recommendations. Animal 2017,11 (6), 958-967. DOI: 10.1017/S1751731116002469
7. Zentek, J. (ed.). Supplemente zur Tierernährung und Diätetik: 37 Grünfutter, Heu und Silagen. 13th ed. Hannover: M.&H. Schaper; 2024.