A comparison of different steaming techniques was done in this experiment which showed that steaming in a specifically designed hay steamer (HG 600) was significantly more effective at reducing microbes in hay compared with home-made steamers and soaking.
Proceedings of the European Equine Health and Nutrition Congress. Ghent, Belgium March 2013 Taylor, J. and Moore-Colyer, M.J.S2013 The effect of five different wetting treatments on the microbial concentration in hay for horses. https://cdn.shopify.com/s/files/1/1144/4268/files/Experiment_11.pdf?2800679123729222980