Compromised hygienic quality of hay often is a serious threat to horse health. The study examined the effect of different steaming conditions on molds, bacteria, and yeast as well as on the content of main macro- and micro-nutrients of either normal-quality or hygienically compromised hays. Hay samples were subjected to the following eight treatments each: unsteamed hays as control or hays steamed at 50 degrees C for 90 minutes, at 80 degrees C for 10, 20, or 30 minutes, and at 100 degrees C for 5,10, or 20 minutes. To evaluate a possible storage effect of the treated hays, subsamples were stored for 24 hours at room temperature. Data indicated increased bacterial counts, molds, and yeasts in hygienically compromised hay. Steaming did not affect total bacterial counts in this hay, but all steaming conditions were equally effective leading to a strong decline of molds and yeasts to a level comparable to the normal-quality hay. Interestingly, only steaming at 100 degrees C for at least 10 minutes reduced Bacillus spp. significantly. In the normal-quality hay, steaming lowered the already low bacterial counts further but caused leakage of sodium and copper. Hay steamed and left for 24 hours at room temperature retained its microbial and nutrient profiles. Overall, the study showed that steaming represents a useful strategy for reducing mold contamination in hygienically compromised hay, with no differences among steaming conditions. Most importantly, to reduce contamination of Bacillus spp. in hygienically compromised hays, steaming at 100 degrees C for at least 10 minutes is required. (C) 2018 Elsevier Inc. All rights reserved.