Moisture Content and the Hay Drying Curve

Many management decisions affect hay quality. After deciding when to cut, the next big decision is about the moisture content. Accurately timing how long to let the crop wilt when harvesting a forage crop will help ensure that it stores well. It either needs to have enough moisture to ferment correctly (silage/haylage or baleage systems), or low enough moisture content to preserve as dry hay.

There are three types of moisture to consider when wilting a forage crop: stem moisture, dew moisture, and air humidity. Stem moisture is the amount of water within the plant. Dew moisture is water outside the plant, whether it is from rain or condensation (dew). Air humidity affects how quickly the crop can dry. If the air is already holding a lot of water, it takes longer for a forage to dry down, even if the air temperature is hot.

The Wilting Process

A standing forage crop is typically between 70-85% moisture and will undergo three phases of drying. During the first phase of drying, the plants are losing moisture through open stomata in their leaves. Forage crops do not stop photosynthesis and cellular respiration when they are cut. These processes will continue until the plant’s moisture level drops below 60%. Stomata will stay open and enable crop drying so long as the leaves are exposed to light. Most of the wilting for a silage/haylage crop occurs in this way. Part of why hay at the bottom of a windrow dries slower is because the stomata are closed in the dark bottom of the pile. This is one reason why wide swaths reduce drying time.

In the second phase of drying, the stomata are closed (moisture content is <60%), and the crop is losing moisture from the stem rather than from inside the leaves. This wilting is slower than the first phase. Conditioning the crop can speed up this second phase by cracking the stem to allow more moisture out. Most of the dew moisture on the outside of the plant is lost during this second phase. Crops wilted for baleage will have some of their drying time in this phase.

The third drying phase is the slowest, as the remaining water (<30%) is held very tightly in the stems. However, it is critical for dry hay to get the moisture content below 18% for small square bales, and below 14% for large bales. Air humidity has a large effect on this drying phase, which makes it challenging in Ontario’s climate. Hay dryers can be used to speed up this phase and reduce the risk of the crop being rained on.

Hay Dryers

By minimizing the time it takes for hay to go through the third drying phase, hay dryers also reduce leaf loss – which preserves nutritional quality – and eliminates the danger of fire due to spontaneous combustion. The hay dryer is designed to dry tough or damp hay but not wet hay. Wilt the hay in the field to an average moisture content of 25% before baling. Bale the hay at normal baler tension and make slight adjustments, if necessary, for high moisture content. Remember that the bales will shrink slightly during the drying process.

Hay Preservatives

Hay preservatives are another tool to help make high-quality dry hay. Hay preservatives are additives that contain acid which enable dry hay to be baled at slightly wetter moisture content than ideal. The most common acid in hay preservatives is propionic. The goal of using a preservative is to prevent mould growth and spoilage in “almost dry” hay.

Bales treated with a preservative need to be stored with space around them so the acid and excess moisture can “sweat” out of the bale over time. It may take 4-6 months for the acid to fully dissipate. Do not store untreated bales next to treated ones, as the moisture will migrate from the wetter bales to the dry ones.

Caution! Hay preservatives should not be confused with silage inoculants, as these products work in opposite ways. Preservatives lower the pH of the hay to a point where mould and other microbe growth is limited, which prevents spoilage. Inoculants contain living bacteria that will ferment a forage crop in an airless environment like a silo; they will not prevent damp hay from spoiling.

High quality hay can always find a buyer. Understanding how forage crops wilt is fundamental to growing marketable hay. Tools, like dryers and preservatives, exist to speed up drying in the final phase and minimize spoilage risks.