Ash can trash your forage value

Written by Dr. Jim White on .

When reviewing the numbers on a forage analysis, most producers pay close attention to the crude protein and fiber levels of hay they’re feeding to their livestock. But are you checking the ash?

The ash content in harvested forages can have a significant role in animal performance. In the lab, the amount of ash in a forage sample is determined by burning off the organic material and weighing the residue that is left. Much of the ash will be made up of minerals, such as phosphorus, potassium, calcium, magnesium and others. These minerals are essential for both plants and livestock.

However, a potentially large fraction of ash in the feed can be from soil contamination. This shows up principally as silica, which is not digested by the animals. Silica isn’t necessarily detrimental, but it does dilute the nutrient content and contributes nothing to feed value. Ash takes the place of nutrients on approximately a 1-to-1 basis. Further, added ash can bring greater mold and yeast counts, which are soil-borne organisms.

High forage ash content is consistent with drought, dry weather and dusty conditions as well as flooding, wet soils and mud. Heavy rains have the potential to splash soil particles onto forage. Rodents, gravel roads and aerobic deterioration have also been associated with higher ash values.

Looking at large numbers of samples from the lab, we see the ash content range from 5 to 18 percent. Values trend in the lower end of that range for grasses and haylage. The objective is to keep external soil contamination below 3 percent of the mass, which roughly means the forage should test under 11 percent ash. Values above that represent external sources and are negatively associated with forage quality and animal performance.

The way forage is harvested can increase the external component of forage ash content. A June 2017 article in Hay and Forage Grower offers these management practices that can minimize ash percentage:

  1. Consider different cutting height. Though a low cutting height offers more yield potential, it also results in more soil being incorporated into the forage. A cutting height of 3 inches for alfalfa and 4 inches for grass is a good compromise.
  2. Make wide swaths. This will not only speed the drying rate but also keep the wilting forage on top of the stubble and off the ground.
  3. Use flat knives on disc mowers. These create less suction and introduce less soil into the forage than angled knives.
  4. A mower’s cutterbar setting affects the amount of dirt pulled into the cut crop. Be sure to keep the angle of the cutterbar as flat as possible. If the cutterbar is tipped too far forward, it will force the knives to scalp the soil surface.
  5. Make sure rakes and tedders are properly adjusted and maintained. The goal is to move forage, not the soil. This is easiest when the tines are adjusted so they barely touch the ground, where they will hit a sheet of paper, but not scrape dirt.
  6. Rake as little as possible. Often hay has to be raked, but use strategies that minimize hay movement over the ground. Mergers are much more effective in this regard as the hay is actually picked up before being moved. The forage mat should be flipped, with the intent of allowing sunlight to hit earlier shaded material.
  7. Widening the swath or windrow can reduce the ash level. Packing all of the cut crop into a narrow, heavy windrow usually causes it to sink through the stubble and sit directly on the ground. A wider swath will spread the weight of the cut crop across a larger surface area, helping it sit on top of the stubble.
  8. Control rodents. In addition to the damage they do to fields and plants, their dirt mounds are easily incorporated into forage windrows.
  9. Keep storage areas clean. Dirt can be added at the storage and loading site as easily as in the field. Keep silage bags and silo piles on well-drained, solid surfaces.

It is important to note that the sum of minerals on most forage reports is not total ash content because mineral analysis usually does not include silica, which is the largest proportion of ash. So look for an “ash” value on the report. The greater attention to ash content has now led many forage laboratories to offer a neutral detergent fiber (NDF) analysis that is ash-free. This is designated as NDFom or aNDFom on the forage analysis report. For samples with low ash, the NDF and the NDFom will be similar.