Rain- Damaged Hay

This may be the start of Global Raining instead of the Global Warming that Al “the Prophet” Gore has predicted. If you live in the Northwest you’ll have to deal with rain- damaged hay in one way or the other.

Rain can damage hay in many ways. Below is a list of several problems it causes.

1. Can cause moldy hay.

2. Causes hay fires when baling is done with too much moisture. I’ve heard of numerous haystack fires already and we’re nowhere near the end of hay season. An interesting note here is that these stacks were put up before all of the rain and stored outside. Apparently the bottom bales soaked up the huge amount of moisture in the ground. They began to heat and caught fire from internal combustion.

3. Nutrients are lost. Losses of over 3% protein and 10% TDN (energy) have been documented. NDF (or non-digestible fiber) increases thus lowering the energy value of the hay. A study in Kentucky found that 90-95% of the Vitamin A was lost when hay remained in the windrow for three weeks.

4. A dry matter loss of 5% per inch of rain that occurs while hay is in the windrow is possible.

5. If you put up your own hay, there is considerable expense to extra raking without any compensation.

6. Some of the hay appears to be baled brown sticks. It may not even look like hay and certainly has no more value than straw.

7. Hay that has gotten hot but not burned may have ‘heat damaged’ protein. This will show up as Crude Protein on a lab test, but will not be available to your animals. You can check for this by having the lab test for ADIN or ADI
protein. (acid detergent insoluble nitrogen or protein).

The amount of damage is related to how soon after windrowing the rain hits. Rain that soaks hay after it is nearly dry causes much more damage than a rain that hits immediately after the hay is cut. A light rain soon after windrowing may only cause a delay in baling, with little nutrient loss. However, a rain on hay that is nearly dry causes leaf shatter and leaching of nutrients out of the hay. The amount of rain that falls on the windrow also is a factor in damage. The heavier the rain is the more nutrients are lost.

One interesting point to note is that a short-violent storm that dumps a lot of rain has much less impact than a steady light rain that lasts for a day or more.

A report from the North Dakota Extension Service on dry matter losses from field to feeding shows why having to rake hay several times causes such a huge loss.

Treatment Range of Loss, Percent Average Loss, Percent
Mowing 1 – 6 3
Raking 5 – 20 10
Swathing w/ conditioner
1-10 5
Plant Respiration 2-16 5
Baling, % of windrow 1-15 5
Storing, % of stack
5-30 15
2-12 5
Hauling Hay 1-5 3
Feeding, % of stack or bale    
w/ feeder
1-10 5
w/o feeder
2-45 15
total, % of original
standing crop


This table also points out how important inside storage is. Outside storage can lead of a loss of 30% of the hay that was purchased or put into a stack. How it is fed also is another potential for savings or loss. Feeding on the ground caused
a loss as high as 45% while the loss suffered when using a hay feeder was only 10%. At high hay prices, some kind of feeder would seem to pay for itself in a relatively short time.

Occasionally a hay grower may spray preservative on hay so that it can be baled at a considerably higher moisture than normal. This treatment allows the capture of nutrients that would have been lost. However, if you buy it you need to know the moisture level as well as protein, fibers, TDN, and minerals. (Notice how I slipped in that sly reference to a forage test!)

Feed Moist Protein
...percent of dry matter...
Low moisture hay 12 21 60
Hi moisture hay 25 21 60

Pounds needed to feed 2 lb Protein and 5 lb TDN

Low moisture hay has 12% moisture and 88% dry matter. 10.8 lbs of this hay will provide those amounts of Protein and TDN.

High moisture hay has 25% moisture and 75% dry matter. 12.7 lbs of this hay provides the same amount of nutrients.

If the low moisture hay could be purchased for $100, then the high moisture hay would be worth $85.00 because it would take 15% more of the high moisture hay to feed the same pounds of protein and TDN. This is also true for any forages of similar type or nutrient content that have different levels of moisture.

Rain- damaged hay has a higher probability of catching fire, since it may have been baled at the upper end of a ‘safe’ moisture level in an attempt to get it up and off the field so that there is a chance for a future cutting.

There doesn’t appear to be any published information regarding the differences between large bales, small bales, and round bales when compared for susceptibility to rain damage. Small bales can be put up safely with more moisture than large bales. Small bales seem to accumulate mold when large bales won’t, given the same circumstances. It is important to recognize that almost 50% of the weight of a round bale is in the outer layers which certainly makes them more susceptible to a high percentage of loss.

Rain damage can lead to higher than normal bale moisture. It might be wise to keep watch of those stacks during the first two weeks to a month after baling. Bale temperature is the key to danger. This table serves as a good guideline:

Temperature Comment
130 F or below safe
130-140 F check every few hours
150 F Danger – move hay off by itself
175-190 F Call Fire Department
200 F or above Fire is present, even if not seen
Call Fire Department

Some rain-damaged hay can be quite palatable and a good buy for feeding cows and calves. However, if there ever were a situation where a feed test is warranted, this would be it. When feeding the hay, you don’t need to know what it was before it was damaged, but you certainly need to know what it is when you go to feed it. A simple test for protein may not be enough, if heating occurred and the protein has become unavailable.
Careful buying and testing offers the chance of a handsome return or saving when dealing with rain-damaged hay.

Thanks to Randy Mills, OSU Extension, Pendleton for his suggestions regarding this article.

Michael J. Mehren, Ph.D. is a livestock nutritionist who lives in Hermiston, Oregon and has been (b)rain damaged through the years. He may be contacted by Email @ mehrens@eotnet.net.

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