Written by Becky James, BSc, MSc

While hay can be stored outside, it’s important to protect hay from the elements by storing it under cover in a building located on an elevated well-drained site, that doesn't leak and has good ventilation. This should ideally be a separate building from where the horses are stabled. There are two reasons for this, hay is a potential fire and health risk. 

Hay stacked on bare ground or concrete wicks moisture up through the bottom tiers, ruining the hay. Wooden pallets are an ideal base or telephone poles can be used to help insulate hay from wet soil or concrete and provide some ventilation.

If storing small square bales, stack the bottom layer on their sides with strings facing sideways instead of up. The uneven surface allows better air circulation. Stack the second layer with the strings facing up perpendicular to the first layer. Stack the third layer perpendicular to the second and so on to lock the stack in place and make it more stable. 

Storing hay outside is not ideal, but if necessary cover the fully-cured hay with a tarpaulin to keep out weather and light. This should also be done if stored in a roof-only shed. Sunlight bleaches hay, causing it to lose nutritional value, especially protein and vitamin A. When your hay resources become low it would be a great time to move your pallets around and sweep out any bits of hay that have (or could) become a mouse and rodent housing.

Dry matter losses during storage result from plant respiration (the continuation of normal plant processes), microbial activity, and weather deterioration. 

Even at low moisture levels (20% or less) there is some loss due to respiration and low numbers of microorganisms, but this is constant across hay types and essentially unavoidable. At higher moisture levels (above 20%) where mould growth is likely to be visibly detectable, dry matter losses are greater, and significant levels of heating (which can also lower forage quality) occur due to microbial activity. Although numerous bacteria are present in hay, fungi account for most of the microbial growth. Heating of hay is related to moisture content. Peak temperature is often reached within a week after baling, but with higher moisture hay and conditions which limit heat escape, it may take as much as three weeks. At safe moisture levels (less than: 20% for small rectangular bales; 18% for round rectangular bales; and 16% for large rectangular bales) inside storage losses are typically around 5% of dry matter, but losses several times higher have been reported for extremely moist hay. 

Microbial activity associated with heating uses soluble carbohydrates, which reduces digestibility and increases fibre levels.

Problems with conserved forage

Even the best quality hays contain high levels of respirable particles (RP). RP’s are less than 5 µm in size and have a 50% chance of being inhaled deep into the lungs, and can cause airway inflammation and ultimately respiratory disorders such as Inflammatory Airway Disease (IAD) and the more severe Recurrent Airway Obstruction (RAO) also known as “Heaves”, “Broken Wind” or COPD”.  A recent study by Erck-Westergren and Dauvillier showed that steaming hay with a Haygain hay steamer reduced the occurrence of IAD by 65%.

Airway inflammation can cause poor performance without the horse showing obvious clinical signs and so often goes undetected.

Several studies have shown that when horses are referred to the vet for poor performance, inflammatory airway disease is often the cause.

Dr David Marlin explains how even low grade sub-clinical respiratory disease (disease present but without any clinical signs such as cough, nasal discharge, etc) can have quite a significant effect on health and performance.

Allen et al 2006 found a 70% prevalence of IAD in national hunt race horses referred for poor athletic performance.  Nolen-Walston et al 2009 reported cytologic evidence of IAD in 81% of the 98 horses examined.

These support earlier work by Gerber et al 2003 who examined asymptomatic well performing sport horses and found subclinical inflammatory airway disease in all 26 horses.

RAO is characterised by airway bronchoconstriction, neutrophilic inflammation and excessive mucus production which results in reduced dynamic lung compliance, increased pulmonary resistance and reduced performance. (Jackson et al 2000). It is a disease affecting older horses (predominantly 8 years plus) and has been estimated to affect 10-17% of the total horse population in the UK (Hotchkiss et al 2007).

This may mean that in horses over 8 years old, the prevalence is nearer to 1 in 2 or 1 in 3 horses with RAO. In most cases, it is caused by mould spores commonly found in the stable environment, either from the forage, the bedding or both.

Research shows that Haygain hay steamers kill mould, bacteria, fungal spores and mites, and eliminate virtually all respirable dust particles helping you to manage and prevent respiratory conditions. Click here to view all the latest research on steaming hay.