Watch out for the Maillard gang this summer. Across the country, these guys sneak into barns and burn them down. Official police documents may list the cause as “suspicious” or “spontaneous combustion,” but the real culprit is this gang. There are four members: Amadori, Schiff, Strecker and their leader, Maillard. They’re involved in all sorts of shady deals – but they like to specialize in hay. Our hay.

Lane woody
Lane Livestock Services / Roseburg, Oregon
Woody Lane, Ph.D., is a certified forage and grassland professional with AFGC and teaches forage/...

I’m talking, of course, about wet hay and barn fires. There’s no such thing as “spontaneous combustion.” The cause is very definite and predictable – it’s the notorious Maillard reaction.

Here’s the story: The Maillard reaction is a complex chain of reactions that can occur between soluble sugars (carbohydrates) and proteins in environments like moist hay. The Maillard reaction produces a brown, gooey substance that looks a lot like caramel. In fact, it is caramel.

Two other names for the Maillard reaction are “non-enzymatic browning” and “caramelization” – the same caramelization that, under the controlled cooking conditions on your stove, creates those chewy, glossy tan confections that bring back pleasant memories of summer beaches and county fairs.

Here’s where the four guys come in. The Maillard reaction begins when soluble sugars in moist hay combine with amino acids in that hay to form something called a glycosylamine. This compound is rather unstable, and its atoms quickly do-si-do into an Amadori rearrangement, which then releases one water molecule to form a Schiff base.


This molecule then transforms itself in a reaction known as the Strecker degradation by combining with another amino acid and losing a molecule of carbon dioxide. The resulting compounds then polymerize with each other – a process similar to the making of nylon – to form something that looks like an amorphous, brown, yucky mess. This is the final product of the Maillard reaction: the Maillard polymer.

To understand how the Maillard reaction affects hay, we need to keep the following details in mind: Firstly, the Maillard reaction gives off heat (an exothermic reaction). Secondly, the reaction goes faster in high temperatures. A rise of 10ºF will more than double its reaction speed.

Thirdly, water acts as a catalyst to the Maillard reaction, which means the presence of water will increase the reaction rate. And finally, the Maillard polymer, which contains 11 percent nitrogen, is completely indigestible by dairy cattle. Which means that all its nitrogen and carbohydrates are nutritionally lost to the animals.

Moist hay is prime territory for a runaway Maillard reaction.

Let’s stack some damp hay bales in a barn and see what happens (damp = 22 to 24 percent moisture or higher, even lower for large round bales). Initially, some fungi and bacteria will grow on the moist leaves and stems. This is quite normal. They break down some of the carbohydrates and give off a little heat.

Everyone knows that even good hay will heat slightly when it’s first put into a barn (it “sweats”). But here’s a kicker: Water holds heat. The extra water in damp hay acts as a heat trap, preventing heat from escaping easily into the surrounding air. Because this damp hay contains so much moisture, heat begins to build up in the bales.

Within a couple of weeks, as the internal temperature rises above 130ºF to 140ºF, the regular fungi and bacteria die off, and some heat-resistant fungi begin to flourish. These fungi use up more carbohydrates and add to the heat load in the hay.

As the temperature rises to more than 170ºF, other carbohydrates and proteins in the hay begin to combine on their own without assistance from any living organisms or enzymes. This is the start of the non-enzymatic browning reaction – the Maillard reaction – which begins to generate its own heat.

The bale temperature soon climbs past 190ºF. At that point, even the heat-resistant fungi die off. Nothing can live in those bales. The Maillard reaction takes over, and events now spiral out of control. The brown Maillard polymer begins appearing throughout the hay.

Moisture accelerates the reaction, which begins to go faster and faster, generating even more heat. Temperatures climb rapidly – 300ºF, 400ºF. At 450 to 525ºF, the flash point of the hay is reached. The surrounding oxygen reacts with the hay. Fire. Explosive fire. Spontaneous combustion. Catastrophe.

If we can stop this process before it gets out of control – like dragging the warm bales out of the barn – we’ll still have nutritional problems with the hay, but we’ll also still have a barn. The nitrogen locked up in that Maillard polymer is nutritionally unavailable to livestock. Which means that the hay’s available protein is lower than its standard crude protein analysis.

For example, if a heat-affected hay analyzes at 16 percent crude protein, and 25 percent of its nitrogen is tied up in Maillard polymer, the available protein value of that hay is only 12 percent. The hay will have a slightly yellow-brown tinge to it (hence the name “browning reaction”).

A forage report on this hay would list the unavailable portion of the nitrogen as acid detergent insoluble nitrogen or acid detergent insoluble crude protein. And for the purposes of balancing rations, we would consider this hay at 12 percent protein.

So now what? Let’s step back for a moment. The best way of dealing with the Maillard reaction is to prevent it, so here are some basic rules of thumb: Fungi and bacteria don’t survive when moisture is less than 15 percent.

Less than 15 percent moisture in any hay is the best of all worlds. (Well, we can hope for good hay-making weather, can’t we?) In practice, however, to be safe, square bales should always contain less than 22 percent moisture. Less is better. Large round bales should always contain less than 18 percent moisture when stored in barns because large bales naturally retain internal heat longer than square bales.

And if you’re in the market for buying someone else’s hay, always look at the color. If you see a yellowing or browning, compared to normal hay, or in the worst case a blackening, steer clear. Observe the hay under natural light because the bluish fluorescent light tends to mask subtle differences in color. Open up the bales. Dustiness means mold spores, which means that hay must have been wet enough to allow mold growth at some time after it was cut.

Also look at the barn. Are there any holes in the roof that could have dripped water? And finally, is the barn still standing? If not, it’s a sure sign that the Maillard gang was there. PD

Woody Lane is a livestock nutritionist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available by contacting the author by email.

PHOTO: “If we can stop this process [heat acceleration] before it gets out of control – like dragging the warm bales out of the barn – we’ll still have nutritional problems with the hay, but we’ll also still have a barn.” Photo by Lynn Jaynes.

Woody Lane