Bacteria in hay

July 26, 2017

Hay remains the preferred forage to feed stabled horses both in the UK (King 2012) and globally (Virkajärvi et al. 2012). Despite this horse owners often know very little about the hay which they are feeding to their own horses. The bacterial content of hay is a potential risk to equine health and any subsequent wetting treatments have the potential to reduce or increase this risk.

The quality of hay as regards to its bacterial content is influenced by a number of factors but will primarily come down to the conditions under which it is grown, harvested, conserved and stored (Longland et al. 2011).

The bacterial community in the soil in which a hay is grown has the potential to effect the microbiome (bacterial community) of the hay once cut due to the transference of bacteria from soil to plant through the root system. The weather can effect bacterial content. For example if the weather at the time of harvest is wet then the rows of hay would likely create an warm moist environment in which bacteria would proliferate these conditions could also then create an anaerobic environment supporting many of the acid producing bacterial species which could be detrimental to equine health.

Storage conditions of hay should also be optimised in order to reduce moisture content within a hay crop as an abundance of moisture can provide the ideal environment for bacteria to proliferate (Hlödversson 1985). It has been found that the ideal moisture content within a hay post harvest to limit nutrient loss and prevent an excess of bacterial growth is between 18 and 20% (Wilcke et al. 1999).

Managing the bacterial content of hay should be a priority to both the forage producers as well as the horse owner as any bacteria that enter the Gastro Intestinal (GI) tract could disrupt digestion. The GI tract is very susceptible to disturbance (Dougal et al. 2013). The introduction of bacteria in large numbers from unhygienic hay could cause changes to fermentation levels and ultimately cause metabolic disorders. It is for this same reason that any changes in diet should be introduced to a horse gradually to allow for the bacteria of the GI tract to adapt.

Any wetting treatment a horse owner subjects hay to has the propensity to effect the bacterial profile of said hay. Soaking hay remains a common practice across the equine industry. Hay is soaked primarily for one of two reasons; 1. To reduce airborne respirable particles as these have been implicated in equine asthma (Dixon et al. 1995) and 2. To reduce Water Soluble Carbohydrate (WSC) levels (Cottrell et al. 2005), as high WSC levels have been linked with a number of diseases most notably laminitis (Elliott 2007).

Although the reduction in WSC and dust content has well documented benefits with regards to certain diseases it does not fully outweigh the potential dangers of soaking hay. The practice of submerging hay in water has the potential to cause extensive bacterial proliferation in a short time frame (Moore-Colyer et al. 2014).Soaking for 9 hours has been found to increase bacterial population fivefold (Moore-Colyer, et al. 2014) and even 1.5x in only ten minutes (Moore-Colyer and Fillery 2012).           

With the bacterial content of hay in mind a different approach to wetting treatments must be adopted.

High temperature steaming has been shown to reduce the bacterial profile of hay by up to 99% (Autoren, 2016) (Moore-Colyer et al. 2016)The high temperature denatures the proteins and nucleic acids within the bacteria making them non-functional. This therefore vastly reduced the risk of an allergenic or pathogenic response within the horse as a direct result of unhygienic forage. Soaking does reduce WSC content with greater efficiency than steaming however the increase in bacterial content should then be addressed by a post soak steaming treatment to reduce the bacterial load in the now low WSC content hay.

Although research has shown the benefits of steaming with regards to reduction in bacterial numbers little is known about the bacteria present in hay and the potential pathogenicity and so new research is being undertaken to determine what bacteria are present in hay, what risks these bacteria may pose and how they are effected by a number of different wetting treatments.



Cottrell, E., Watts, K. and Ralston, S. (2005) Soluble sugar content and glucose/insulin responses can be reduced by soaking chopped hay in water. Proc. Equine Sci. Soc. 293-298

 Dixon, P., Railton, D. and McGorum, B. (1995) Equine pulmonary disease: a case control study of 300 referred cases. Part 1: Examination techniques, diagnostic criteria and diagnoses. Equine veterinary journal. 27(6): 416-421

 Dougal, K., de la Fuente, G., Harris, P. A., Girdwood, S. E., Pinloche, E. and Newbold, C. J. (2013) Identification of a Core Bacterial Community within the Large Intestine of the Horse. PLoS ONE. 8(10): e77660

 Elliott, J. The first Waltham–Royal Veterinary College Laminitis Conference. Ed. Harris PA, Hill Sn

 Hlödversson, R. (1985) Methods for estimating and preventing storage losses in moist hay. Sveriges Lantbruksuniv.

 King, L. (2012) A survey of forage feeding practices in UK. BSc thesis. Royal Agricultural College, Cirencester Glos. UK

 Longland, A., Barfoot, C. and Harris, P. (2011) Effects of soaking on the water-soluble carbohydrate and crude protein content of hay. Veterinary Record: Journal of the British Veterinary Association. 168(23)

 Moore-Colyer, M. and Fillery, B. (2012) The effect of three different treatments on the respirable particle content, total viable count and mould concentrations in hay for horses. InForages and grazing in horse nutrition. Springer: 101-106

 Moore-Colyer, M. J. S., Lumbis, K., Longland, A. and Harris, P. (2014) The Effect of Five Different Wetting Treatments on the Nutrient Content and Microbial Concentration in Hay for Horses. PLoS ONE. 9(11): e114079

 Moore-Colyer, M. J. S., Taylor, J. L. E. and James, R. (2016) The Effect of Steaming and Soaking on the Respirable Particle, Bacteria, Mould, and Nutrient Content in Hay for Horses. Journal of Equine Veterinary Science. 39: 62-68

 Virkajärvi, P., Saarijärvi, K., Rinne, M. and Saastamoinen, M. (2012) Grass physiology and its relation to nutritive value in feeding horses. InForages and grazing in horse nutrition. Springer: 17-43

 Wilcke, B., Cuomo, G. and Fox, C. (1999) Preserving the value of dry stored hay.

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