Rained-on hay can be a suitable forage, especially for horses prone to laminitis. Forage quality tends to be retained if:
The rain occurs soon after cutting when the forage has had little time to dry
The rainfall was a single, short event
The rainfall intensity was higher versus a longer, lower intensity event
The forage wasn’t re-wetted many times
The best way to check the quality of rained on hay is to have it tested.
Rainfall on cut hay laying in the field causes yield and quality losses. This lowers the value of the crop as an animal feed and a marketable commodity.
Rainfall reduces dry matter yield
Dry matter loss is most crucial to the hay producer. Dry matter loss results in decreased income since there’s less hay for baling, feeding and selling.
Dry matter loss appears to be greatest after the forage partly dries. Wisconsin researchers found that:
1 inch of rain on alfalfa after 1 day of drying caused 22 percent dry matter loss.
Similar hay dried without rain damage lost only 6.3 percent of the starting yield.
1.6 inches of rain on alfalfa over a few days caused 44 percent dry matter loss.
Michigan researchers looked at the effects of rainfall on field-cured alfalfa.
The first study found dry matter losses up to 34 percent
A second study kept rainfall intensity constant at about 0.7 inches spread over periods of 1 to 7 hours. Dry matter losses ranged from 4 to 13 percent. The highest losses occurred when the rain was spread over a longer time.
Overall, dry matter losses were much lower in these experiments despite the 2 inches of rain.
Grasses suffer the least amount of dry matter losses.
Yield losses of birdsfoot trefoil are less than alfalfa.
Red clover shows even less dry matter loss due to rain than alfalfa and birdsfoot trefoil.
How does rainfall reduce dry matter yield?
Leaching refers to the loss of water soluble compounds from forage when rain occurs. Unfortunately, these compounds are highly digestible to animals and include:
Readily available carbohydrates and soluble nitrogen
About one-half of the dry matter leached by rain is soluble carbohydrates.
Losses of soluble carbohydrate can be beneficial for some horses. Horses with laminitis need forage with low levels of carbohydrates. Laminitis is a painful and debilitating disease of the horse hoof. It usually occurs during periods of increased or rapid intake of water soluble and nonstructural carbohydrates.
To manage laminitic horses, owners soak their hay to lower the carbohydrate content. Soaking hay is cumbersome, messy and time consuming. Thus buying rained-on hay with naturally low levels of carbohydrates may be a good alternative.
Respiration is the breakdown of soluble carbohydrates by plant enzymes. This occurs at nearly 2 percent dry matter per hour in fresh forage.
Respiration and moisture decline almost proportional to each other until the plant reaches about 60 percent moisture. Once the forage reaches 60 percent moisture, each following rain will prolong or start respiration. This results in added dry matter loss.
In Wisconsin studies, 1 to 2.5 inches of rain caused 8 to over 20 percent leaf loss as a percent of the initial forage dry matter.
In Michigan studies, direct leaf loss was much lower (0.5 to 4.2 percent).
Perhaps the issue of leaf loss from rainfall is a moot point. Rain damaged alfalfa is more prone to leaf shatter after it dries. Rainfall also often means added raking and more leaf loss.
How does rainfall intensity and forage moisture affect losses?
Given the same amount of total rainfall, a low intensity rain will result in more leaching of soluble compounds than a high intensity rain.
As forage moisture declines, it’s more prone to dry matter loss from rain. In Wisconsin rainfall studies, the maximum loss in dry matter (54 percent) occurred when 2.5 inches of rain fell on nearly dry hay.
How does rainfall affect forage quality?
Most rainfall studies conclude that wetting of field dried alfalfa has little impact on protein concentration. Rained-on hay often has relatively high protein values compared to fiber content unless there’s great leaf loss. Leaves contain a lot of the plant’s protein.
As soluble carbohydrates leach, structural fibers make up a greater percent of forage dry matter. The digestibility of rained-on hay can decline 6 to 40 percent.
Microbe activity is thought to concentrate fiber content in rained-on forage. But added fiber isn’t made during the wetting process.
Reduced carbohydrate content results in an overall decrease in forage energy content.
Reviewed in 2021