How rain affects hay quality and yield

What causes weather-related losses in hay quality?

Rained-on hay can lose quality due to:

  • Leaching of soluble carbohydrates, proteins and certain minerals as rainwater falls on and moves through the cut forage.
  • Increased and prolonged plant respiration due to the rewetting of hay above 30-40 per cent moisture. This leads to losses in yield, soluble carbohydrates and overall energy content.
  • Leaf shattering and loss due to increased raking to dry the windrow after a rain. Frequent wetting-drying cycles also increase leaf loss.
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  • Microbial activity continues in wet hay, resulting in the breakdown of the plants and consumption of their nutrients. Mould growth can also occur.
  • Colour bleaching. This effect, though, is more an esthetic issue than a quality issue.

How much can rain reduce hay yield?

There have been many studies done looking at the effects of rain on hay. One study conducted in Wisconsin recorded dry matter losses as high as 22 per cent when alfalfa hay was exposed to one inch of rain one day after being cut. Similar alfalfa hay never exposed to rain only lost six per cent of its potential dry matter yield. Alfalfa hay exposed to 1.6 inches of rain over a few days lost 44 per cent of its potential dry matter yield.

In another study in Michigan, the effects of one- to seven-hour rains were examined on alfalfa hay. Here, dry matter losses ranged from four to 34 per cent, and were greater the longer the rain lasted.

Grass hay often will not experience the same degree of loss as alfalfa hay. The majority of yield loss in alfalfa hay is due to leaf loss. Grass leaves are not as easily lost.

How does rain reduce my hay yield?

Rain reduces dry matter hay yields through its effects on: leaching, plant respiration and leaf loss. Leaching is when water-soluble components of cells move out of them. Most of these compounds are easily digested by livestock and include: carbohydrates, soluble nitrogen, minerals and lipids. They are leached out and lost when cut forage is rained on.

Plant respiration is the breakdown of soluble carbohydrates by enzymes. It occurs even when plants don’t receive rain. Respiration losses occur from the time the plant is cut until the forage reaches 30 to 40 per cent moisture.

Each time cut forage is rained on, respiration either continues or starts again if the forage was already less than 40 per cent moisture. In both cases, dry matter is lost, although the exact amount is difficult to estimate.

Studies differ when it comes to the impact of rain on leaf loss. In the Wisconsin studies, leaf loss ranged from eight to 20 per cent.

In Michigan, leaf losses where only 0.5 to four per cent. Based on experience, most producers know that rain-damaged hay is more at risk to leaf shatter after it dries. Extra raking or tedding is often needed to speed up drying, which also increases leaf loss.

Does rain intensity and forage moisture make a difference?

The studies tend to agree on this issue. Given the same amount of rain, a low-intensity rainfall causes more leaching of soluble carbohydrates than a high-intensity rainfall.

Plus, as the moisture content of the cut forage decreases, the more dry matter will be lost when rained on. Hay that is almost dry enough to be baled will lose more dry matter when rained on than hay that has just been cut.

In the Wisconsin studies, up to 54 per cent of the potential dry matter yield was lost when almost-cured hay received 2.5 inches of rain.

How does rain affect the quality of my hay?

Most of the studies have shown that there is little impact on crude protein levels when field-cured hay is rained on. It is common to see high crude protein values compared to fibre levels. However, due to the leaching of soluble carbohydrates, acid detergent fibre and neutral detergent fibre levels will increase. This lowers the digestibility of the hay and its overall forage quality. It is always a good idea to get a forage analysis done on your hay before feeding it to your livestock.

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