When to Feed New Hay: Should I wait six weeks before feeding?

Every summer, as the first bales are stacked in barns across the country, the same question starts doing the rounds on yards and social media: when can I feed new hay. Answers vary but include:

“Leave it six weeks.”

“Never feed freshly baled hay.”

“It needs to mature first.”

It's advice that's been handed down through generations of horse owners, but where did it come from? More importantly, does modern research actually support waiting before feeding new hay? The evidence suggests the answer is no.

Provided hay has been properly made, cured and stored, there is no scientific reason why newly baled hay cannot be fed straight away. The important factor isn't how old the hay is but how well it was made. Read on to find out what actually determines whether new hay is safe to feed.

Where Did the Six-Week Hay Rule Come From? Is it Backed by Science?

Like many pieces of traditional horse management advice, the recommendation to leave hay for six weeks after baling has its roots in a very different era of farming.

Historically, hay wasn't stored in the small or large square or round bales we're familiar with today. Instead, it was stacked into enormous hay ricks, often many metres high, where thousands of kilograms of forage were packed tightly together. If the hay had been baled before it had dried sufficiently, naturally occurring bacteria and fungi continued to respire within the stack, generating heat. In severe cases, temperatures could climb high enough to cause spontaneous combustion.

Waiting several weeks allowed those stacks to cool and stabilise before they were handled or moved. The advice wasn't originally developed because fresh hay was thought to upset horses' digestive systems (though this is an important consideration, more on this below); it was largely a practical measure to reduce the risk of fire and spoilage.

Today's hay production is very different. Improvements in mowing, conditioning, baling technology and moisture monitoring mean that well-made hay reaches a stable state much more quickly than traditional haystacks ever did.

Moisture, Not Age, Determines Whether Hay Is Safe to Feed

Scientific studies examining forage preservation consistently show that moisture content, not the age of the hay, is the critical factor determining its stability.

Research by Rotz and Muck (1994) demonstrated that excessive moisture at baling allows continued microbial activity, leading to heating, mould development and losses in nutritional quality. Collins and Moore (1995) similarly described how the majority of post-harvest changes occur when forage is baled before adequate drying has taken place.

Properly cured hay should typically contain around 85% dry matter, equivalent to a moisture content below approximately 12–15%. At this point, microbial activity falls dramatically, meaning the forage is no longer fermenting or producing significant heat.

Problems arise when hay is baled too wet. Moisture levels above around 18–20% increase the likelihood of microbial respiration, heating, mould growth and nutrient losses. If you've ever opened a bale that feels warm, smells caramelised or has visible mould, you're seeing the effects of excessive moisture, not the effects of “new” hay.

In other words, a poorly made bale stored for three months may be far less suitable to feed than a well-made bale stored for three days.

Does New Hay Upset Gut Health?

Although the age of the hay isn't usually the issue, changing forage can still affect some horses. This is because no two cuts of hay are exactly alike, and your horse's gut is sensitive to change.

Weather conditions, stage of maturity at cutting, grass species and soil type all influence the nutritional composition of forage. An early-cut hay is often more digestible and energy dense than a late-cut crop, while sugar, protein and fibre levels may also differ considerably between fields.

Research into equine hindgut fermentation has shown that changes in forage composition alter the microbial populations responsible for digesting fibre (Pagan et al., 2012). This helps explain why some horses develop mild digestive disturbances, softer droppings or increased gas when transitioning onto a different batch of hay or new pasture. It isn't simply because the hay is freshly baled and fed “too soon” or the grass is necessarily “too rich”, it's because the microbial ecosystem within the hindgut is adapting to a different source of fibre.

For that reason, gradual changes between forage batches (and when introducing any new feed or supplement) remain sensible wherever practical, particularly for horses with sensitive digestive systems.

What to Check Before Feeding New Hay

Rather than asking how old the hay is, it's far more useful to assess its quality.

Good hay should feel dry throughout the bale, smell fresh and grassy, and show no evidence of mould or damp patches. It should never feel noticeably warm inside, as this may indicate ongoing microbial activity.

If hay is cool, dry, well-cured and has been stored correctly, there is no evidence-based reason to delay feeding simply because it was baled recently. That said, it's still best to introduce it gradually by mixing it with your old forage.

Supporting Gut Health When Feeding New Hay

At Equell, we often talk about supporting the hindgut microbiome because forage is far more than a source of calories, it's the foundation of digestive health.

Every batch of hay contains its own unique blend of fibres, and different fibre types nourish different microbial populations within the hindgut. As forage changes throughout the year, so too does the nutritional landscape presented to those microbes.

Wild horses naturally consume dozens of different plant species each day, gradually exposing their microbiome to an extraordinary diversity of fibres. Modern horses, by comparison, often have access to far fewer fibre sources, making seasonal forage changes a more significant event for the hindgut ecosystem.

Gut health is just one reason why some horses transition onto new hay without any noticeable changes, while others may experience softer droppings or mild digestive upset as the microbial community adapts. Supporting that transition isn't about avoiding new hay altogether, it's about helping the microbiome adapt to dietary change.

This is what inspired the development of our Equell Gut Food. Rather than focusing on adding microbes (probiotics), Gut Food was formulated with a Diverse Prebiotic Fibre Matrix™ and fermentable plant fibres, providing multiple substrates for the beneficial hindgut microbes already living in your horse's gut. Because different fibres support different microbial populations, diversity matters - the gut doesn't always need more bacteria, it needs more prebiotics for the bacteria already there to feed on and thrive.

The aim isn't to replace good forage; nothing can, as horses are fibre fermenters and their diet should always be forage first. Instead, Gut Food complements modern diets by reintroducing some of the fibre diversity horses naturally evolved to consume.

The Bottom Line: When Is New Hay Safe to Feed?

The long-standing advice to wait six weeks before feeding newly baled hay has persisted for generations, but modern evidence suggests it's the quality of the hay, not its age, that matters.

If this year's hay has been properly cured, contains an appropriate moisture level, has cooled after baling and shows no signs of mould or spoilage, there is no scientific reason to leave it sitting in the barn before introducing it into your horse's diet.

As with any dietary change, introducing a new batch gradually remains sensible wherever possible, not because fresh hay is inherently unsafe, but because every forage change presents a new blend of fibres for the hindgut microbiome to adapt to. Ideally, introduce your new supply of hay gradually by mixing it with your current supply before it runs out.

And while no supplement can ever replace the value of good-quality forage, supporting microbial diversity through a varied, fibre- and prebiotic-rich diet may help some horses navigate these seasonal feeding changes and transitions more comfortably.

References:

Collins, M., & Moore, K. J. (1995). Post-harvest changes in hay. Agronomy Journal.

National Research Council. (2007). Nutrient Requirements of Horses.

Pagan, J. D., et al. (2012). Forage type and hindgut fermentation in horses. Journal of Equine Veterinary Science.

Rotz, C. A., & Muck, R. E. (1994). Changes in forage quality during harvest and storage. Journal of Dairy Science.

Wilkinson, J. M., & Fenlon, J. S. (2014). Silage fermentation and stability. Grass and Forage Science.

 

The Equell Team, 10.7.26