By Ben Beckman
Hay put up too wet can lead to a number of issues, most notably mold and heat. Moisture keeps otherwise dormant microbes and fungi active, decreasing forage quality and creating heat. Too much heat can create risk of combustion.
However, even heat that doesn't reach the level of combustion can cause issues with hay. Since hay is not protected from oxygen like most anaerobic fermented feedstuffs (silage, haylage, etc.), high temperatures, moisture and oxygen allow aerobic bacteria to grow, using plant protein and sugars for growth and producing carbon dioxide, water and heat. Too much of this and temperatures can rise high enough to kick off a process called the Maillard reaction.
Even if you've never heard of the Maillard reaction, you probably are familiar with it. First described in 1912 by the French chemist Louis-Camille Maillard, the Maillard reaction is the browning process that happens between amino acids (proteins) and sugars under heat.
We use it all the time when cooking. From searing a steak to baking bread, the Maillard reaction is what produces that flavorful browned or golden outer portion of most cooked foods.
What does this have to do with hay? While it's typical to have bales generate heat after harvest because of the curing process, too much moisture has the dual issue of, first, helping trap heat already created instead of letting it dissipate quickly, and second, acting as a catalyst for the Maillard reaction once it kicks off at about 170 degrees F.
The Maillard reaction takes normal proteins and sugars and changes them through a series of chemical reactions into something called the Maillard polymer. The resulting hay is sweet-tobacco smelling and golden-caramel in color, and cows love it.
The problem is that the resulting Maillard polymer, while tasty to animals, isn't great nutritionally. The chemical reactions actually have tied up proteins and lowered the forage's true crude protein content.
A standard crude protein feed analysis won't be able to pick up this difference, however, and will report the number like nothing is wrong, resulting in overestimating the available protein in forage and underfeeding animals.
Luckily, the solution is fairly simple. If you suspect you have bales that have had some caramelization occur, add on a test for heat-damaged proteins (HDP) or acid detergent insoluble crude protein (ADICP).
While adding a bit more to the cost of the forage analysis, this additional test will show how much unavailable protein content there is because of the Maillard reaction, and rations can be adjusted accordingly. At some labs, this test also will be shown with the adjusted crude protein content after damage has been factored in.
While we are on the lookout for hay that could be heat damaged, it's also a great time to look bales over for excessive mold growth. The same wet conditions that allow the Maillard reaction to occur also can be great for mold.
While not always toxic, mold can reduce hay quality and palatability, so allowing animals the freedom to pick through heavily molded bales is a great option. For more information on considerations when working with moldy hay, check out the November BeefWatch article "What to Do with Wet Hay."
After last winter's cold weather, and a wet summer, using every last bit of hay available to its fullest potential is going to be important. Testing forage quality of your hay, whether it's your own or purchased, is a critical first step to optimizing hay use, but make sure to take a closer look when taking those samples.
Keep an eye out for signs that a Maillard reaction could have occurred, and if significant, running the additional HDP test will be essential. With a true idea of available crude protein, making the right decisions on how much hay to feed this winter and when to start supplementing will be that much easier.
Beckman is a Nebraska Extension educator.