Deciphering a Hay Test: TDN
By Brad Schick
Last time we looked at ADF and NDF and how it is used to measure a hay sample’s fiber content, which affects digestibility and forage intake — two factors that help predict animal performance. Today, we will look at Total Digestible Nutrients (TDN).
Often, the terms TDN and energy are used interchangeably when discussing forages and feeds, but realistically, TDN is one measurement of energy.
There are many different components that make up what we call TDN. TDN is a combination of digestible fiber, lipids and proteins. Acid detergent fiber, or ADF, is used to help calculate TDN from the fiber component. The lower the ADF, the higher energy the forage contains in most situations. When supplementation is being considered — especially on a diet that is primarily forage — TDN is one of the most important vales to know from our hay test. In many cases, TDN is overlooked. Diets may be lacking energy as much or more than crude protein because protein often receives more focus in diets.
TDN values will be affected by maturity of the forage, weather conditions in which the forage was grown and harvested, and fertilization or other agronomic practices. For example, Sandhills meadow hay TDN ranges between 50% and 65% on a dry matter basis.
Understanding energy is important for the health and nutrition of livestock. For cows, TDN is the value that typically needs to be used to calculate their needs which changes drastically with different stages of production.
Planning Summer Grazing
Last year’s drought conditions across the state left most pastures heavily used and short. It is known that the combination of drought and heavy grazing can lead to reduced pasture production this year, even if we get near normal rainfall. Now is the time to be planning spring and summer grazing.
While most areas have received some good snowfall this winter, there always some uncertainty as to how much spring and summer precipitation we will get. So, the key first step is to develop or adjust a good drought plan for your operation.
For native grass pastures, some reduction in stocking rate may be necessary. Slightly delaying turnout this spring will allow those grasses to accumulate some growth and help in the recovery process. Another management tip for pastures that were grazed first and into July in the 2022 season is to defer grazing of those pastures until later in the summer.
Seeded annual forages are another option that can be used to supplement any grazing needs. For spring or cool-season annual forages, the planting period is typically late March to early April. This would include things like oats, field peas or other spring cereal grains. With that planting date, these would provide grazing beginning in late May through early July.
For the summer annual forages, the planting period is typically late May and on into the summer months. This would include things like forage sorghum, sudangrass, sorghum-sudan hybrids, pearl millet and foxtail millet. Additionally, other species or legumes that are typical of some cover crop mixes could be part of the package as well. With a late May or early June planting of a summer annual, grazing should be available in July.
Planning ahead can help make your forage season run smoothly.
