Making hay when the sun doesn't shine

Depending on the operation, hay production can be a high priority or an afterthought shoved out of the way by spring planting. From a feed value perspective, though, it is crucial to minimize risks and losses in the field. It is essential to maintain forage quality and to protect the forage from storage losses.

Harvests of hay are frequently complicated by poor drying and the threat of unexpected rainfall. You often are forced to choose between baling hay before adequate drying has occurred or subjecting wilting hay to rain damage. For hay that is not adequately dry, a potential problem is spontaneous heating. This process occurs when plant sugars are respired in CO2, water and heat up via microorganisms–principally fungi.

Traditionally, hay research has used small square bales to study how hay reacts to different conditions. However, due to cost and the limited labor available to handle conventional rectangular hay bales, many producers have been forced to consider larger bale sizes. Generally, larger bale sizes are more prone to heat spontaneously and require a reduced threshold moisture for acceptable storage. In addition, round bales show more measurable effects of spontaneous heating than are usually seen in a small rectangular hay bale. In other words, you’ll more often see brown spots in the middle of big bales.

Of course, brown spots aren’t good, but a hay fire is a total loss. A hay fire leaves you less worried about the nutritional value at stake and more concerned about the loss of a barn.
From personal experience, I can attest that it is really important to know the coverage and limits of an insurance product before a fire, rather than finding out the coverage after a fire. So, let’s concentrate on prevention.

It’s wet hay that causes fires. With adequate moisture to maintain the relative humidity of the air in the hay mass at 95% to 97% percent, heat generated by plant enzymatic activity and microbial growth may push temperatures to 160° F within a few days. This progression may take several weeks.

Above 160°F, oxidative chemical reactions are responsible for additional heat generation. This auto-oxidation greatly increases the potential for further rapid increase to combustion temperatures. In some research I’ve looked at, mixed hay at 44 percent moisture took a month to reach a temperature of 195° F, but then increased to 330° F in only three more days. Soluble carbohydrates were almost completely eliminated in the hay that reached 330° F.

Spontaneous combustion requires large quantities of oxygen. Thermal conductivity of dry hay is lower than moist hay, so heat transfer to the outside of the bale becomes progressively less effective as the hay dries. Thus, hay temperatures may increase rapidly after much of the moisture has been removed. Spontaneous combustion occurs near the outside of the bale because oxygen levels in the interior are reduced by microbial respiration.

To avoid hay fire damage

•    Adequate drying is the best solution; dry hay won’t produce heat-inducing fungi
•    Do not stack hay close to power lines, electric fences, trees or buildings
•    Do not park equipment close to hay stacks
•    Ensure adequate separation of stacks–use multiple stacks to reduce the chance of larger loss, especially with suspect hay
•    Exclusion of oxygen is an option: make silage
•    Use anti-mycotics (hay preservers) to allow more time for the hay to dry and knock down microbial growth

Fires are obvious. But bales that don’t get hot enough to burn can still deliver a non-visible loss, nutritionally speaking.

Hay that has gotten hot will test higher for crude protein than what the animal results indicate. It will look brown—and more brown if it was hot for a long time. You’ll also see this process in hay silage crops. They’ll come out looking and smelling like chewing tobacco. In corn silage, you’ll see heat-damaged kernels, which are usually associated with drier or poorly packed silages.

This is significant nutritionally because of what we call ADICP, the crude protein recovered within insoluble acid detergent fiber.

The browning reaction is important because ADICP has very low bioavailability to cattle. That’s what I mean by hay testing higher for crude protein than the livestock indicate.

If you are feeding heat-damaged hay or haylage, expect animals to digest the material at a higher rate (the rate of passage declines and digestibility increases). This often reduces energy intake on the animals, which is a big deal if you have cows that are on the feed to maintain their weight.
ADICP is commonly used as an indicator of heat damage; as hay heating increases, so does the ADICP. And while research is mixed, in general terms, you can count on crude protein in the hay plummeting if it has gotten hot.

Prevention is the best choice.