Dry Matter Estimation of Forage Crops by Near-Infrared Technology

October 21, 2020 at 9:11 pm | Updated October 21, 2020 at 9:11 pm | 7 min read

The importance of dry matter (DM) estimation is well established in animal nutrition. Dry matter estimation of forage crops is so important that it is conducted several times between farm and trough. It is, therefore, surprising that the methods used for dry matter estimation are usually traditional, and the use of near-infrared (NIR) based tools has just begun. Here, we explain the different stages in forage preparation, which need dry matter estimation, and how the NIR\ technique can help farmers reduce their work burden and simultaneously improve profits by using a single device.

The Need for Dry Matter Estimation in Forage Crops

The important part of the forage is the nutrients. When feed is given based on the weight of the forage, an accurate estimation of the nutrients is not possible, since moisture content can vary in various feed materials and at different times of production and storage.

Changes in moisture content affect the weight of the feed. An increase in moisture content and the resulting increase in weight of feed decrease the proportion of nutrients. Less moisture content reduces the weight but increases the nutrient percentage of the feed.

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Animal growers need to maximise the nutrients they supply to the animals. Otherwise, the nutrients missing in hay will have to be complemented by using extra concentrates, which places an additional economic burden on them.

In these circumstances, giving feed on a weight basis is not the best means of ensuring ideal animal nutrition.

Dry Matter as an Indicator of Animal Nutrition

The dry matter content of the feed is a better indicator of the nutrients being supplied to the livestock.

Dry matter (DM) is the sum total of all the solid components in a plant without the water content. So, dry matter estimates nutrients vital for animals, such as cellulose, starch, sugars, structural compounds, proteins, fats, pigments, and minerals.

By using dry matter, it is possible to calculate the optimum amounts of different feed and, therefore, the specific amounts of nutrients. If the feed amounts are not adjusted according to the DM content and the same weight of feed is given to the animals, they will either get too much or too little of proteins and energy.

A deficiency of nutrients will affect milk production. Providing an excess of nutrients is a waste of feed and money; moreover, the animal wastes result in heightened nutrient pollution.

In forage crops, it is not just the DM content that is important, but also moisture content. The relationship between the two parameters is simple, as seen in the equations below.

DM = Total weight – moisture content


Moisture content = Total weight – DM content

Therefore, DM estimation is also helpful in determining moisture content.

Stages in Forage Production

It is necessary to know the DM and moisture content of the crop at various stages to fix harvest time, monitor quality, and prevent forage spoilage and disasters on the farm.

Harvest time

Dry matter needs to be estimated to fix harvest time so that the crop has optimum amounts of nutrients and is palatable for the animals. Dry matter increases with maturity; however, if crops are harvested too late, they become fibrous and are unpalatable. Moreover, there is a loss in nitrogen levels, which is an important nutrient.

The method of harvest and later storage will also determine the ideal DM that crops should have at harvest. DM estimation in the pastures is necessary to make sure grass/crops are ready for ensiling. The grass also has to be monitored, so that they are cut when they have the correct DM percentage for baling. The ideal DM content will differ based on the type of crops and the season in which they are harvested; Table 1 outlines the details of optimum DM percentage needed.

Table 1: “Dry matter content for various silage crops and type of harvesting.”
(Image credits: https://www.afia.org.au/index.php /resources/silage-fact-sheets/making-quality-silage/142-guide-to-the-dry-matter-testing-of-silage)

Treatment in Silos

The correct moisture content in crops needs to be ensured for proper fermentation in silos. Too much moisture will only seep nutrients out and extend the time of fermentation in silages, affecting the final quality of the feed.

While it is stored in the silos, there can be weekly changes in DM due to the fermenting process, crops used, or weather, which can add moisture to the silage. The DM content of the final silage will also depend on the method of storage.

Hay Storage in Bales

Grass can be stored as hay without being fermented. Here, too, moisture content can be crucial. Hay that has too much moisture will encourage mold growth, which affects the quality of hay by reducing nutrients and palatability.

Moreover, too much moisture can cause the hay to heat up and lead to spontaneous combustion. In many cases, it has led to disasters on the farm (See Figure 1).

Besides the crop type, the ideal moisture and DM content that hay should have will depend on the bale size and shape in which it is stored. According to Penn State University,

  • Small and square bales should have 18-20 percent of moisture content and 80-82 percent of DM.
  • Large and round bales should have 15-15 percent of moisture content and 82-85 percent of DM.
  • Large and square bales should have 12-15 percent moisture content and 85-88 percent of DM.

According to Penn State Extension, DM should be tested twice a week, while the forage is stored during feeding.

Figure 1: Spontaneous hay combustion because the moisture content in the bales was high. (Image credits: https://www.horseandman.com/handy-tips/why-hay-spontaneously-combusts-ways-prevent-it/06/21/2012/)

Traditional methods of DM Estimation

Most of the traditional methods involve taking a sample, around fifty to hundred grams of the feed. The fresh/initial weight of the feed is taken and then it is dried. After the sample is completely dry, the final weight is taken and used to calculate DM. The formula for calculating DM is as follows:

Final Dry Weight (gms) X 100 = ……….. % Dry Matter
Initial Wet Weight (gms)

Different methods can be used to dry the sample. The following are the ones usually used on farms:

  • Microwave is very popular as it is not expensive, and samples will dry in five to ten minutes. However, the samples have to be monitored constantly to ensure they don’t burn.
  • Koster is an electrical appliance, which blows warm air on the sample to dry it in twenty-five to fifty minutes. There can be some errors due to loss of sample.
  • Vortex Dryer is easy to use and is not expensive. It gives accurate measurements within twenty-five to fifty minutes.
  • Forced Air Oven has a drying time of twenty-four to forty-eight hours, and the ovens are expensive.
  • Food Dehydrator can be used for multiple samples, needs very little monitoring, and has a drying time of two to eight hours.

Disadvantages of Traditional Methods

For months, DM and moisture content has to be monitored constantly during feeding operations, before harvest, and during harvest. This makes traditional methods of DM tedious because of several disadvantages, such as,

  • The need for equipment and space. The problem with the traditional method is that it requires at least a simple laboratory/room.
  • Length of time involved. The method is laborious, as it involves many steps. Even if a fast drying method like the microwave is used, the whole process is still time-consuming and needs careful planning and organising. The repeated measurements of DM by traditional methods require a significant time investment.
  • Destructive nature of tests. The drying methods destroy the feed used as samples. Since they are repeated often, a lot of feed is wasted for this process.

Non-Destructive Modern Methods

There are many modern devices available, which can help in quick and non-destructive measurements of moisture content and DM. These do not have the disadvantages of the traditional methods. Some of them are moisture meters and NIR-based devices.

Moisture Meters

These are hand-held devices that estimate the level of moisture in hay by measuring the electrical resistance of hay. Moisture conducts electricity, so hay with more moisture content will allow more electricity flow.

The problem with moisture meters is that they come in varying qualities, and only high-quality moisture meters are accurate. Moreover, most of them are suitable only for hay and not silage.

NIR-based Tools

There has been enough research to show that Near Infra-red (NIR) spectroscopy accurately estimates DM in whole plants and in forages. As these studies point out, the method requires no sample preparation, nor use of any chemicals.

The F-750 Produce Quality Meter is one such NIR-based small hand-held device. A ray of NIR light is directed towards the plant, hay, or silage. The different compounds in the target absorb and reflect different parts of the light, based on their composition. The principle of spectroscopy is a sophisticated technique that gives precise, rapid measurements within a few seconds.

The NIR tool has many advantages, including the following:

  • It can be used anywhere: in the pastures, farms, silos, or barns.
  • As it gives rapid results, a farmer can test several bales of hay or parts of a crop per day.
  • Since there is no destruction of materials, it can be used as many times as necessary.
  • NIR can be used with all the crops that are used as forage, such as grass, clover, and alfalfa.
  • It is suitable for determining harvest time and quality during storage.
  • It is useful for silages and hay.

Better Application of NIR Technology

NIR spectroscopy is a well-established technique that has long been used to test dry matter content in fruits and vegetables to control quality and harvest time. By optimising feed production and animal nutrition, small NIR based tools can help to reduce the quantities of forage crops needed while simultaneously increasing animal production.

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Featured blog image courtesy of Bernard Spragg. NZ


Australian Fodder Industry Association. Guide to Dry Matter Testing of Silage. Retrieved from https://www.afia.org.au/index.php/resources/silage-fact-sheets/making-quality-silage/142-guide-to-the-dry-matter-testing-of-silage

Batten, G.D. (1998).Plant analysis using near infrared reflectance spectroscopy: the potential and the limitations. Aus J Exp Agric, 38: 697– 706. DOI: 10.1071/EA97146

Cozzolino, D., & Labandera, M. (2001). Determination of dry matter and crude protein contents of undried forage by near infrared reflectance spectroscopy. J Sci Food Agric, 82: 380– 384. DOI: https://doi.org/10.1002/jsfa.1050

Dairexnet. (2019, August, 16). Dry Matter Determination. Retrieved from https://dairy-cattle.extension.org/2019/08/dry-matter-determination/

Guide to Dry Matter Testing of Silage. Retrieved from https://www.afia.org.au/index.php/resources/silage-fact-sheets/making-quality-silage/142-guide-to-the-dry-matter-testing-of-silage

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Measuring the Dry Matter Content of Forages, Silage Note 7. Retrieved from https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0007/229291/silage-note-7-measuring-the-dry-matter-content-of-forages.pdf

Morón, A., García, A., Sawchik, J., & Cozzolino, D. (2007). Preliminary study on the use of near‐infrared reflectance spectroscopy to assess nitrogen content of undried wheat plants. Journal of the Science of Food and Agriculture, 87:147-152. DOI: https://doi.org/10.1002/jsfa.2691

Vough, L.R. Evaluating Hay Quality. Retrieved from https://www.extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/4-H/resources/animal_science/FS644.pdf

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