Mould growth in hay

Moistening of hay during storage is a well-known problem and often the hay is covered with straw to prevent mould growth. The water content in hay during storage should never exceed 15% to avoid the risk of microbial growth and even well dried hay may be damaged due to moistening during storage. The humidity in the atmospheric air can be high during long periods during autumn and winter which enables the water content in the surface layer of the hay to increase well enough for microorganisms to grow. In addition to the water content we have the water activity which describes how strongly the water is bound to the material and therefore also how available it is. Water activity is a better measurement of microorganisms’ growth condition and the water activity should be below 0.7 to thoroughly limit the possibility of mould growth.

How moisture in hay varies during winter storage and which types of mould then starts to grow were examined in this study. Field storage experiments were performed during two years on three farms where all had hay driers and the hay was handled in small bales.

The results of the study show that moisture content in the surface layer of hay exposed to the ambience increased rapidly during autumn. Even at 30 and 60 cm depth the moisture content increased but at considerably slower pace. A water activity of 0.7 was reached in the surface layer already in the beginning of October and then remained at 0.7 or higher until the end of March. At 30 and 60 cm depth the water activity was almost consistently below 0.7 the entire storage period.

In the beginning of August the fungi content was mainly low, usually below the detection limit. In the beginning of October no greater amounts were detected. However in the middle of November a considerable growth had started in the surface layer and Wallemia and Aspergillus were the dominating moulds. Contents of Wallemia and Aspergillus in the surface layer increased during storage up to one million colony forming units per gram or more. During winter there was also moderate growth of Penicillium and Eurotium. In the lower layers of the hay there were also mould growth but not at all as considerably as in the surface layer. At 60 cm depth the growth was even slightly less than at 30 cm depth.

In conclusion, the moisture content rapidly increase in the surface layer of hay exposed to the ambience during autumn and already in November the microbial growth has reached levels that are considered unacceptable. Even in lower hay layers there is some moistening and mould growth but not to unacceptable levels. To avoid health risks for both animals and humans all exposed surfaces of the hay should be covered with a protective layer of straw. This should be done as soon as possible before the humidity in the atmospheric air increases in the autumn.

Read more about dusty hay and respiratory disorders here and microbiological analysis of forage here.

Sara Muhonen, AgrD

Reference:
Sundberg M & Lundin G 2014. Mögeltillväxt i hö under vinterlagring. Vallkonferens 2014, Rapport nr 18, sid 155-158, Institutionen för växtproduktionsekologi, Sveriges lantbruksuniversitet, Uppsala, Sverige. (In Swedish)