Cursing the raindrops

By Mike Rankin

A week ago, I emailed the manager at our company’s Hoard’s Dairyman Farm and asked him if they had contemplated cutting alfalfa yet.

His response: “We’re going to cut some today, before the rain tonight, ted it tomorrow, then chop on Thursday, before the rain on Friday.”

Such is the life for many haymakers in 2019. The onslaught of rain events this spring has not only severely delayed planting of corn but has already wreaked havoc on forage quality in the West and threatens to do the same as alfalfa and grasses reach or pass optimum quality in the North.

Those putting up high-moisture forage have an uphill battle. If you’re in the dry hay business, it’s a Mount Everest situation.

The ever-present question in the forage business is: “Do I cut now and risk getting it rained on, or do I watch the forage quality decline daily in the field as I wait out a harvest window?”

It’s a question that really has no right answer for all situations. Further, some producers will avoid rained-on hay at all costs; others fall into the “riverboat gambler” category.

The science of rained-on hay is an interesting one and there have been numerous studies to document what happens when a field of beautiful, cut forage gets power washed by Mother Nature. The carnage that results for both yield and quality is highly dependent on when the rain hits (or how dry the forage is when the drops fall), the amount of rain, and the rain event duration.

Wilted forage that is rained-on is subject to the following impacts:

• Enhanced and prolonged plant respiration that reduces soluble carbohydrates and the overall energy content of the forage. Dry matter loss also occurs.

• Leaching of soluble carbohydrates and certain minerals out of the forage. The result is lower quality, but it also may affect forage fermentation qualities.

• Leaf shattering in the case of legumes. If not directly from the rain, often from the additional mechanical manipulation (raking and tedding) that is needed to reboot the drying process.

• Additional microbial activity that metabolizes soluble carbohydrates. In addition to lower energy content, this also raises the risk for mold and mycotoxin development.

• Color bleaching, which may lower the product’s market value.

Yield takes a hit

Though not always as noticeable as forage quality, rainfall during the wilting process can cut potential yields. There are three primary mechanisms at play.

First, rain may prolong plant respiration, which chews up valuable carbohydrates (sugars). If the forage is wilted below about 50 percent, which is when respiration normally all but ceases, research has shown that rewetting can cause plant respiration to reactivate, furthering the loss of nonstructural carbohydrates.

Rain can also simply cause the leaching of carbohydrates out of plant tissues. The same occurs with some minerals such as potassium. Finally, dry matter losses occur simply from the loss of leaves caused by additional windrow or swath manipulation.

In some early Wisconsin research, dry matter losses from rained-on hay totaled 22 percent when alfalfa was exposed to a 1-inch rain after one day of wilting.

In one Michigan trial, maximum losses were reported at 34 percent. For a second trial, the rainfall amount was kept constant at 0.7 inch but spread over periods from one to seven hours. The dry matter yield losses ranged from 6 to 12 percent with the greatest loss occurring when the rain was spread over the longer duration.

Grass hay is also subject to yield loss, though leaf shatter is less of an issue and the greatest losses occur when the forage is nearing baling moisture.

Forage quality washed away

With the loss of soluble carbohydrates through respiration and leaching, the percentage of structural carbohydrates (fiber) becomes greater. This means a higher percentage of neutral detergent fiber (NDF), lower fiber digestibility (NDFD), more ash, and a lower relative forage quality (RFQ).

In Wisconsin research, NDF bumped up about 4 or 5 percentage units for alfalfa that received a 1.1-inch rainfall and 12 to 13 percentage units for a more severe 1.9 inch-wetting over an extended eight-day time period of wet weather.

Often, when rainfall amounts are moderate, it is not unusual to see a rise in crude protein (CP) percent because its components are not lost to the same extent as nonstructural carbohydrates. In the case of alfalfa, the extent of leaf loss will play a role in the final CP concentration.

Fermentation challenged

When forage is chopped for high-moisture feed, rainfall that occurs during the wilting process may negate its potential fermentation qualities. This is especially true for alfalfa that has been exposed to multiple rain events over an extended period; in such cases, there are greater amounts of sugars and starches lost that would normally act as substrate for lactic acid-producing bacteria during fermentation.

In these situations, a bacterial inoculant is highly recommended. Finally, rain-damaged forages are more prone to clostridial fermentations that yield ammonia and butyric acid as end products.

The impact that rain has on forage quality and yield has proven easy to document. The decision of when to cut is often much more difficult.

Fortunately, the internet-based weather prediction models of today are a step ahead of relying on your uncle’s bum knee. Even so, they can only predict rather than stop the rain, which right now is what we need.