Extension Expertise: Heat-Damaged Forages Lose TDN, Too

We have often been aware of protein loss from heat damage to hay baled too wet or haylage packed poorly. But seldom have we considered the TDN loss – something that is becoming more significant as grain prices rise.

In a worse-case scenario, the heated forage may burn. In a less-serious case, heated hay or haylage may become more palatable but have reduced protein availability and total digestible nutrients (TDN). Either case decreases the animal and economic value of the harvested forage.

The TDN equation below shows that each 1% loss of starches and sugars (NFC) is a 1% loss in TDN.

We examined samples collected at the Marshfield University of Wisconsin-USDA Dairy Forage Research Station.

Round bales were baled at varying moisture contents and had a range of heating. TDN loss is shown in the graph. Heating degree days (HDD) were calculated as bale temperature minus air temperature summed over days. For example, if bale temperature is 110°F and air temperature is 85°, then heating degree days for 10 days is 250 ((110-85) x 10).

Oftentimes, hay baled at greater than 20% moisture will heat to 130-140° and remain that way for two weeks or longer if bales are stacked. Similarly, poorly packed silage will get that hot and remain so for weeks.

The hotter the hay or haylage gets, and the longer it stays hot, the higher the heating damage. That relationship of TDN loss to heating is shown in the graph as largely a straight line.

We developed an NIR equation to predict TDN loss from heating and received scans of 3,612 spectra of farmer samples from commercial forage testing laboratories in the Midwest. They were collected in 2009 and early 2010.

The predicted TDN losses due to heating exceeded four TDN units in 50% of the producer samples, and exceeded eight TDN units in 16.2% of the samples. TDN is valued at around 14¢/lb with current grain prices, so a reduction of four units TDN represents an $11.20/ton loss. A decrease of eight units TDN is a loss of $22.40/ton.

Heat damage is a significant economic loss to many farmers. What can you do to minimize it? Bale hay at less than 16% moisture. If it must be baled wetter to avoid rain:

1. Consider using a preservative.
2. Consider wrapping bales in plastic. Eliminating oxygen stops respiration and heating.
3. Remember that bigger bales have a lower surface area-to-volume ratio and do not lose heat well. Therefore:

• Small square bales can be baled at up to 18% moisture and will dry if stacked loosely.
• Round or big square bales must be baled at 15-16% moisture.
• Larger bales, round or square, should not be stacked for seven to 10 days to allow heat loss from them.
• Make smaller big bales if hay must be baled wetter than 16% moisture – a 3’-diameter round bale rather than a 5’-diameter bale or a 4’-long medium square bale rather than one that’s 6’ long.
• Bales made in cooler fall weather lose heat more rapidly than bales made over summer at higher temperatures.

For haylage or baleage, pack the crop to 15 lbs dry matter per cubic foot to minimize oxygen content – and heating.