Why nutrients and contaminants must be equal parts of the “hay quality” evaluation.

By Meriel Moore-Colyer

There is no such thing as the “best” hay for horses. A hay that suits one horse may not be the best for another. What you feed your horse will be governed by his physical condition, work intensity and duration, the local climate and other feeds he is getting.

However, there is one truth, and that is bad hay is not good for any horse!

But what do we mean by “bad” hay? To fully understand this, we need to separate the nutritional value from the hygienic quality, although clearly when buying hay, it is essential that you fully consider both aspects.

Nutritional Quality
Some hays might be termed “bad” because they are low in nutrients and would not support the nutritional needs of some horses. However, this type of hay could actually be good for horses who are overweight, prone to laminitis and/or metabolic syndrome.

The reverse is also true with high-energy, nutrient-dense hay being potentially “bad” for the “fatties,” but ideal for those in hard work. So, it is all about determining what your horse needs and selecting the forage that meets the needs of your horse.

As a general guide, when hay has a high proportion of leaf to stem, is green in colour and well conserved (ie a DM of ≥ 85%), it will have a high digestible nutrient content. Lower nutritional value hays have a high proportion of stem to leaf and may also have lots of seed-heads present -- often referred to as “mature hay.” This hay should also be green and well conserved.

Certain grass species e.g. Lolium perenne (perennial rye grass) are nutrient-dense grasses so, if well conserved, can make highly nutritious fodder. Other grasses, like Festuca spp (red fescue) or the Poa spp (meadow grasses), are less “productive” and have a lower nutrient content. These tend to make the best hays for leisure horses and those prone to putting on weight.

Hygienic Quality
When discussing the hygienic quality of hay, we are generally referring to the bacteria, mould, yeasts and insect contents, although contamination by noxious plants should also be considered.

The hygienic quality of hay is somewhat independent of nutrient content, although certain aspects of the microbial profile can give you an indication of nutritional quality.

Bacteria are the most abundant microorganisms in the environment, and we can use their behaviour as a guide to how all other microbes (fungi, yeasts and microscopic insects) populate the grass and thus form a particular phyllosphere microbiome.

Bacteria, the most numerous colonists of plant leaves, often form large heterogeneous aggregates, constituting 30 to 80% of the total bacteria on the plant surface. Many of these aggregates, which can be as numerous as 108 cells/g of leaf 1, also harbour fungi 2,3,4 particularly spores from the filamentous fungi.

The fungi spores are more transient occupants than bacteria, while yeasts are also active and effective colonizers. The actual microbial profile on the leaf will be influenced by the leaf surface characteristics -- like glossiness and amount of crystalline wax, which is determined by grass species.

Perennial rye grass, for example, has a glossy underside to the leaf and this is a less effective host for epiphytic bacteria than the less shiny cuticles found on meadow and fescue grasses4. Furthermore, more mature grasses tend to have an increased microbial load than younger grasses5.

Diverse Microbiome
All of this adds up to a highly diverse microbiome, which is further influenced by external factors such as climate, daily weather patterns, soil conditions, nearby crops, trees, buildings and roads. The final profile that ends up on the conserved hay is also influenced by harvesting conditions, which determine whether certain microbes will die, survive or proliferate in storage.

Climate change has a role to play here and will likely pose an increasing challenge for hay producers. Warm-wet growing seasons, while producing good yields of leafy green growth also cause proliferation of microorganisms on the growing plant. Trying to conserve hay in these conditions is particularly difficult, as 5 consecutive dry warm days are needed to ensure proper drying before storage.

Barn-drying is an option, but it is expensive and uses fossil fuels so not an ideal answer.

All of this means that the hygienic quality of hay is hard to predict, because it is determined by a wide variety of continually changing factors. These factors will determine whether the microbiome contains high or low numbers of potential pathogens and whether these present challenges to the respiratory health of humans handling it and the animals consuming it.6

Factors that Reduce the Hygienic Quality of Hay
The main challenge to horses from poor hygienic quality hay, is that of air borne respirable dust (ARD), which causes allergic respiratory disorders in horses. Many of the fungi -- most notably Aspergillus, whose spores are known respiratory allergen -- are strongly implicated in severe equine asthma (sEA).

Aspergillus are ubiquitous fungi and populate the leaf in the field, but mainly proliferate during storage when hay has not reached the desired 85% dry matter before storing.

As well as Aspergillus spores, hay dust contains high levels of bacterial spores, plant fragments, pollen and mites -- the latter are an indication of high fungi and poor nutritional quality.

A high pollen count often indicates a mature, highly fibrous hay (and therefore less digestible) as most of the grass has matured into flowering heads.

Bacteria such as Thermoactinomycetes vulgaris, Sacchropolyspora rectivirgula and their cell walls i.e.lipopolysaccharides, along with beta glucans (components of cell walls) and numerous fungi including Aspergillus, Alternaria, Putrescentiae, Cladosporium, Geotrichum and many more, are all prevalent in hay.

More recent work has added tree and grass pollens and numerous arthropod species, such as mites, midges and cockroaches to this list.

While the factors listed above will determine what is prevalent in any particular hay in a given season, all of the above have been identified in young-nutrient dense hay and mature fibrous hays from USA, Canada, France and UK.

So, hygienic quality must be considered separately from nutrient content when purchasing hay.

Bacterial Profiles & Digestive Health?
Ericsson et al.7 reported the equid gastric microbiome is composed of a core of multiple small bacteria groups which may be easily influenced by feed. Although the four major phyla which represent 96% of those present, namley Proteobacteria, Cyanobacteria, Actinobacteria and Firmicutes, are also present in the gastrointestinal tract, they are in different proportions and that could pose a challenge to digestive function.

In a recent study8 where the bacteria profile of 3 different hays, i.e., 2 x meadow hays and a perennial rye grass hay were mapped, the profiles were 81-87% similar in the genera present but proportions were different.

The meadow hays showed more similar profiles to each other compared with the perennial rye grass hay. This may be due to the plant factors mentioned above (glossy leaf and cuticle wax content) which favour some bacteria over others. However, overall the total bacteria numbers were similar and ranged from 6.66.to 8.06 log10 CFU/g.

Some researchers have reported quite high levels, 4.9 log10 CFU/g, of Enterobacteriaceae in PRG hay and this could be concerning as this family of bacteria contain known pathogens such as E.coli, Salmonella and Klebsiella, which are undesirable in large quantities in feed.

Although these bacteria are present, they are not normally a problem in well-conserved hay. However, when hay is stored at too high a moisture content, the warm damp conditions favour bacteria growth, and this causes a rapid proliferation of these potentially pathogenic bacteria.

Although the low pH in the pyloric region of the stomach may neutralise many of the bacteria in feed, the fact that animals commonly suffer gastrointestinal upset after eating contaminated foodstuffs suggest that bacteria load does have a major impact on gut health.

Ensuring Hygienic Hay
The best way to ensure that you choose the most appropriate hay for your horse, is to:
1. Ensure it is 85% dry matter
2. Check for poisonous weeds
3. Open a couple of bales and check that no mould is present
4. Check the colour (should be greenish) and the smell from a distance, which should be sweet. (Please don’t put your nose in it and sniff as you can also become dust sensitive!)
5. Ideally send a sample off to a lab and ask them to measure the ARD, total bacteria and mould contents


When feeding any hay to a horse in a confined space, I suggest you pre-treat by steaming.


Prevention is always better than cure and even nutritious, well conserved hay can contain significant amounts of airborne respirable dust. Avoid setting up an allergic respiratory disorder by separating the horse - dust interaction.


Work9 has shown that steaming is a much better method for dealing with pathogens in hay than soaking. Furthermore, filling hay nets with dusty hay before soaking exposes you to significant levels of potentially allergenic airborne respirable dust, whereas steaming a bale removes the dust from your breathing zone and ensures you are feeding clean hay to your horse.

About the author - Meriel Moore-Colyer is a Professor of Equine Science in the UK and runs her own company: Equine Nutrition Research and Consultancy. She can be reached at merielenrc@gmail.com.


Cited References
1. Beattie GA, Lindow SE (1995) The secret life of foliar bacterial pathogens on leaves. Annu. Rev. Phytopathol. 33, 145–172 https://doi.org/10.1146/annurev.py.33.090195.001045 PMID: 18294082
2. Lindow S E and Brandl MT (2003) Microbiology of the Phyllosphere. Applied and Environmental Microbiology. 69, 1875–1883. https://doi.org/10.1128/AEM.69.4.1875-1883.2003 PMID: 12676659
3. Morris C E, Monier J-M, Jacques M-A (1997) Methods for observing microbial biofilms directly on leaf surfaces and recovering them for isolation of culturable microorganisms. Appl. Environ. Microbiol. 63, 1570–1576. PMID: 16535579
4. Beattie GA (2002) Leaf surface waxes and the process of leaf colonization by microorganisms, p. 3–26. In Lindow S. E., Hecht-Poinar E. I., and Elliott V. (ed.), Phyllosphere Microbiology. APS Press, St. Paul, Minn.
5. Muller CE. (2009) Influence of harvest date of primary growth on microbial flora of grass herbages and haylage and on fermentation and aerobic stability of haylage conserved in laboratory silos. Grass and Forage Science 64, 328–338.
6. Couetil LL, Cardwell JM, Gerber V. Lavoie J.-P. Le´guillette R. Richard E.A. (2016) Inflammatory airway disease of horses revised consensus statement. J. Vet. Intern.Med. 30, 503–515. https://doi.org/10. 1111/jvim.13824 PMID: 26806374
7.Ericsson AC, Philip J, Johnson M, (2016) A Microbiological Map of the Healthy Equine PLoS One Nov. 2016
8. Moore-Colyer M, Annette Longland, Patricia Harris, Leo Zeef , Susan Crosthwaite. (2020). Mapping the bacterial ecology on the phyllosphere of dry and post soaked grass hay for horses. Plos One. Jan 27, 2020 https://doi.org/10.1371/journal.pone.0227151