(Not So) Dry Hay: Handle With Care!

Making quality dry hay can be a challenge, especially when conditions are cool and wet, as we experienced last summer in many parts of the province. Hopefully, this year will be different.

In order to produce quality dry hay, it must be harvested to preserve the nutritive value of the forage. Moisture level at bailing is of critical importance to insure quality. If baling hay at too high a moisture content; molds, dust and funny smells are expected. There is more; substantial losses of dry matter and nutrients can occur due to spontaneous heating. When moisture levels are greater than optimal, plant cells and microorganisms consuming forage¡¦s carbohydrates and biological activity generate heat that gets trapped in the hay mass. Even hay baled at 15 to 20% moisture may undergo some degree of heating a few days after harvest that may extend for 7 to 10 days. Usually, temperatures will not be excessive so quality damage is minimal.

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

  • bale size and type
  • level of compaction
  • storage

The larger the bale, the less easy it is for heat to escape thus increasing the extent of nutrient loss. As bale size and compaction levels increase, moisture remains trapped in the hay mass for a longer period of time thus increasing heat damage. Along the same line, if freshly harvested hay is stored in a tight pile, excess moisture and heat will not escape readily and overheating might occur for several weeks.

A team of investigators at the Dairy Forage Research Center in Wisconsin studied the relationship between moisture content at baling, size of bales and forage quality using a Heating Degree Day system. The Heating Degree Days (HDD) are calculated by subtracting 30 from the maximum internal bale temperature measured in .C, for each day of storage. The difference is summed each day until internal bale temperature drops below 30 .C. Their findings show that small square bales, harvested at a moisture level 20% or less accumulated a low level (93 or fewer) of HDD. As bale size and diameter increased, baling at a moisture content of 20% resulted in more HDD accumulated and a higher risk of spontaneous heating that could lead to significant nutrient losses. As HDD exceed 150, nutrient losses begin to increase substantially.

In the past, the effect of heat damage was believed to be mainly a reduction in the digestibility of crude protein as a result of chemical changes taking place between carbohydrates and protein known as Maillard reaction. Recent research findings indicate that not only is protein digestibility reduced but energy losses are at least as significant. Energy concentration in the forage decreases as non fiber carbohydrates (NFC) are depleted, while fiber concentration increases. Since NFCs are almost all digested by the cow, the disappearance of NFCs markedly impacts the energy content of the hay. As HDD exceed 315 the NFC losses can be as high as 25% of their original levels.

This 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 equipped with a long probe with built in thermometer. Internal bale temperatures are taken daily until the internal temperature gets down to 30.. The HDD value can then be calculated and an assessment of potential heat damage can be made.

Another tool to assess potential heat damage to a forage crop is ADF-CP. It is a laboratory test used to measure 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 that is bound to the acid detergent fibre fraction of the cell wall, or the portion that is unavailable to the cow. Some degree 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% (as % CP) on the analysis report indicates that some overheating of the forage has occurred. If the bound protein becomes too high (>12% of CP) then, the crude protein value should be adjusted. The analysis report will often provide an adjusted CP value. This value should be used in ration formulation.

Although dry hay was the main focus of attention of the research team, it is fair to assume that silage and haylage behave likewise when exposed to spontaneous heating and similar nutrient losses can be expected.

In order to minimize nutrient losses due to heat damage, forage moisture level at harvesting, particle size, packing and oxygen exclusion are key factors to manage.

Moisture levels at baling (without acid) to minimized Heating losses

  • Small square bales: 15 - 18% moisture
  • Large round bales (soft core): 13 - 16% moisture
  • Large Rectangular and large round hard core bales: 12 ¡V 15% moisture


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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