How Rain Affects Hay Quality - Frequently Asked Questions

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 What will rain damage do to hay yield and how much of a decrease in yield can be expected?
Dry matter hay yields decrease due to respiration, leaf loss and leaching when hay gets rained on. Losses of water-soluble compounds from plant cells including carbohydrates, soluble nitrogen, minerals, vitamins, and lipids decrease yield.

The amount of yield loss is highly variable and difficult to predict. Losses can range from 5 per cent to almost 50 per cent. Several factors will determine the amount of dry matter loss experienced ranging from type of forage that is harvested to rainfall duration.

What happens to the quality of the hay with rain damage?
Rain and excessive moisture on cut forages can cause leaching, respiration and leaf loss which can decrease hay quality and yield.

Leaching is the movement of highly water-soluble cell contents out of a plant and is most often encouraged by one or more rain events. Most of these leached components are highly digestible by the animal, and include soluble carbohydrates, soluble nitrogen, minerals, vitamins and lipids. Almost half of dry matter leached by rain is soluble carbohydrate.

Wilted, dried forages are more susceptible to leaching than fresh-cut forage because the plant cells are not able to control passage of cell contents. Plant cells that lose water become flaccid and prolonged exposure to moisture can cause the cell walls to begin to break down. Moisture from rain transports water back into the cells and depending on how much the cell walls have degraded, this sudden reversal of osmosis can cause plant cells to rupture and leach out their contents.

Respiration is the breakdown of soluble carbohydrates by plant enzymes. It will always cause dry matter losses when making hay regardless if the forage has been damaged by rain or not. Respiration losses occur when forage moisture levels are above 30 per cent. Each time cut forages are wetted by rain, respiration is prolonged or restarts in cured forage that is already below 30 per cent moisture. This causes additional losses of sugars from the plants.

Leaf loss is a particularly significant problem in legume hays as compared to grasses. The amount of leaf loss experienced is highly variable. Rainfall may mean additional handling of the swaths prior to baling to speed up drying resulting in more leaf loss, particularly from alfalfa. Every time hay is raked, there may be a 5 per cent loss in tonnage due to leaf loss.

How else can rain decrease forage quality of hay?
Rain can encourage more microbial activity resulting in metabolizing of soluble carbohydrates, thereby reducing forage energy content. The risk for molds and mycotoxins development also increases as moisture increases.

Bleaching or degradation of chlorophyll by the sun is often associated with increased respiration of the plants after being cut. This happens regardless of whether hay has been exposed to rain or not. As respiration occurs and cell contents are leached out, the chlorophyll in the leaves reduce their ability to function. With rainfall encouraging the leaching of cell contents, especially on dry swaths, bleaching can occur much more readily.

Does rain intensity and duration make a difference in hay yield and quality?
Both rain intensity and duration impacts the amount of damage caused to the cut forage. Rain that occurs soon after cutting is less damaging than if rain occurs just before the cut forage is nearly dry and ready for baling. Recently cut forage is not going to soak in as much moisture from the rain, resulting in less nutrient loss and delayed baling. Rain that soaks into nearly dry forage will cause leaf shatter and nutrient leaching.

Generally the heavier the rainfall the greater the damage with more nutrients lost. Repeated rainfall events cause more damage than a single event. A violent storm that drops a lot of rain in a short period of time will cause less damage than a steady, light rain that lasts for a day or more. As a result, energy values decrease with leaching of nutrients, and fiber content of the forage increases.

Protein content in hay that has a high amount of alfalfa is generally higher, despite the losses in energy content. In grass-only hay, both energy and protein content will decrease and fiber will increase.

What can be done with rain-damaged hay?
To prevent further losses from heating (and risks of spontaneous combustion) and molding, it is best to allow the hay to dry down to less than 12 per cent moisture if baling into large square bales or 16 per cent moisture if baling into large round bales,. Differences in temperature between summer and late fall will determine the best moisture to bale hay for safe storage.

When hay is baled into round bales and left out in the field for two to three weeks during the sweat period, it is less likely to be affected by rain than if the hay is left out in the swath. Continuous exposure to rainy periods will reduce the quality of the baled hay over time. However, bales should be moved off the fields as soon as possible to reduce the damage to the plants under where the bales sit.

Store the bales in single rows, round side out, at a northwest-to-southeast angle on a well-drained location to allow the sun and wind to further dry the bales. There should be 8 to 12 inches of space in between each bale.

How do you determine forage quality from rain-damaged hay?
Collect a feed sample from the rain-damaged forage to send in for analysis. Use a core sampling tool, taking 15 to 20 random cores to mix in a pail and send away to a feed lab. It is recommended to work with a livestock nutritionist to help you develop a balanced ration for your animals when you receive the results to help allocate your forage inventory to your livestock according to their nutritional needs.

Rain-damaged, low quality hay should be used first when cow nutrient requirements are lowest, during mid to late pregnancy. Rain damaged hay can be mixed with higher quality hay to increase palatability and nutrient availability to the animals. It is advisable to save your best feed for cows in lactation or for young growing calves. Ideally, moldy hay should not be fed to young animals because of reduced feed intake and lower weight gains. Ran damaged hay should be tested for mycotoxins, especially before you consider feeding these bales to pregnant cows.
Sources and For More Information:
Rain Damage to Forage During Hay and Silage Making
Rainfall Effects on Wilting Forage

Submitted by: Karin Lindquist, AgInfo Centre, Alberta Agriculture and Forestry
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For more information about the content of this document, contact Karin Lindquist.
This document is maintained by Kathy Andersen.
This information published to the web on May 3, 2012.
Last Reviewed/Revised on December 21, 2017.