The Challenges of Making Quality Hay for Dairy Cows

It is never too early to prepare for making high-quality feed. Making quality dry hay can be challenging, especially when conditions are cool and wet.

To produce high-quality, dry hay, you need to harvest it to preserve the forage's nutritive value. Moisture level at bailing is critically important to ensure quality. If you bale hay when moisture is too high, it could result in molds, dust and bad odours. Also, you could lose a substantial amount of dry matter and nutrients due to spontaneous heating.

When moisture levels are greater than optimal, plant cells and microorganisms generate heat when they consume forage's carbohydrates. This heat can get trapped in the hay mass. Even hay baled at 15 to 20 per cent moisture may have some heating a few days after harvest that could last for seven to 10 days. Usually, temperatures will not be excessive, so damage is minimal.

Several factors can affect the impact of post-harvest heating:

  • bale size and type
  • compaction level
  • storage.

The larger the bale, the harder it will be for heat to escape, which can increase nutrient loss. As bale size and compaction levels increase, moisture remains trapped in the hay mass for a longer period. This can increase heat damage. Likewise, if freshly harvested hay is stored in a tight pile, excess moisture and heat will not readily escape and overheating might occur for several weeks.

A Dairy Forage Research Center team in Wisconsin studied the relationship between moisture content at baling, bale sizes and forage quality, using a Heating Degree Day (HDD) system. The HDDs are calculated by subtracting 30 degrees Celsius from the maximum internal bale temperature measured in Celsius for each storage day. The difference is totalled each day until the internal bale temperature drops below 30 degrees Celsius.

The team's findings show small square bales, harvested at a moisture level of 20 per cent or less accumulated a low level of HDD-93 or fewer. As bale size and diameter increased, baling at 20 per cent moisture content resulted in more HDDs and a higher risk of spontaneous heating that could result in significant nutrient losses. As HDD exceeds 150, nutrient losses increase substantially.

In the past, heat damage was mainly attributed to reduced crude protein digestibility from chemical changes between carbohydrates and protein, known as Maillard reaction. Recent research indicates protein digestibility is reduced and energy losses are significant. Energy concentration in forage decreases as non-fiber carbohydrates (NFC) are depleted, while fiber concentration increases. Since cows digest almost all NFCs, eliminating NFCs markedly impacts hay's energy content. As HDDs exceed 315, the NFC losses can jump as high as 25 per cent from original levels.

The HDD system can be readily used on the farm to monitor a new crop of dry hay. The only equipment required is a bimetallic stemmed thermometer with a long stem or a moisture tester that has a long probe with a built-in thermometer. You'll need to take bale temperatures daily until the internal temperature goes down to 30C. You can then calculate the HDD value and assess potential heat damage.

Another tool to assess your forage crop is the Acid Detergent Fiber-Crude Protein (ADF-CP). This laboratory test measures the amount of heat-damaged protein in a forage sample, reported as a percentage of the crude protein (CP). It measures the portion of crude protein bound to the ADF fraction of the cell wall, or the portion not available to the cow. A certain amount of binding between fibre and protein occurs naturally even though no heating took place. This protein is often subtracted from the feed's total crude protein to estimate available crude. ADF-CP values higher than 10 per cent on the analysis report indicate some of the forage has overheated. If the bound protein is too high-less than 12 per cent of CP-then the crude protein value should be adjusted. The analysis will often provide an adjusted CP value. You should use this value when formulating ration.

Although dry hay was the research team's main focus, silage and haylage may be affected when exposed to spontaneous heating, resulting in similar nutrient losses. Forage moisture level at harvesting, particle size, packing, and excluding oxygen, are key factors if you want to minimize nutrient losses from heat damage.

This article was originally published in the February 2014 edition of the Milk Producer Magazine.


  • Effects of spontaneous heating on forage protein fractions and in situ disappearance kinetics of crude protein for alfalfa-orchardgrass hays packaged in large round bales. W. K. Coblentz , P. C. Hoffman, and N. P. Martin J. Dairy Sci. 93 :1148-1169.
  • Effects of spontaneous heating on fiber composition, fiber digestibility and in situ disappearance kinetics of NDF for alfalfa-orchardgrass hays. W. K. Coblentz and P. C. Hoffman J. Dairy Sci. 92 :2875-2895.
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Moisture levels at baling (without acid) to minimize heating losses

  • Small square bales: 15 to 18 per cent moisture
  • Large round bales (soft core): 13 to 16 per cent moisture
  • Large rectangular and large round hard core bales: 12 to 15 per cent moisture

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