A plant cell consists of a cell content and a cell wall. The cell contents contain substances such as starch, sugars and proteins that are important for milk production. The cell wall consists of structural carbohydrates, which give structure and strength to the plant. A cell wall is made up of a middle lamella (containing mainly pectin), a primary cell wall (containing mainly cellulose and hemicellulose) and possibly a secondary cell wall (containing mainly lignin (wood dust) and possibly cutin (cork dust)). The layers have different compositions in different crops. The main cell wall constituents are cellulose, hemicellulose and lignin, together also expressed as NDF (Neutral Detergent Fiber).
Animal cells, unlike plant cells, only have a cell membrane and therefore do not contain structural carbohydrates.
In order for the entire cell content to be utilized, the cell wall must first be broken down by rumen microbes. During this breakdown, volatile fatty acids (acetic acid, propionic acid and butyric acid) are formed, which provide 60-70% of the energy needed by a dairy cow.
In addition to energy, structural carbohydrates also contribute to good rumen function because of their structural value. Thus, the ferment ability and degradation rate of cell wall components, in addition to their content, is an important parameter in determining the nutritional value of feed material. In addition, because lignin is virtually non-degradable in the rumen, it may hinder the digestibility of cell content and other cell wall components.
Roughage contains high levels of cell wall components: straw and hay up to 60-70%, brewer grains, potato press fibers, pressed beet pulp and grass silage around 45-55%, and maize silage around 40%. The longer a crop is in the field, the more it will lignify (giving a higher lignin percentage) and thus has a relatively higher cell wall (and NDF) content, a higher structure value, and a higher saturation value. Intensive fertilization, on the other hand, usually leads to a lower cell wall content.
The determination of cell wall constituents or structural carbohydrates used to be expressed in terms of crude fiber, which, however, does not give a good picture of digestibility. Nowadays, the more accurate Van Soest method is used. The Van Soest method consists of the following steps:
- Determination of the NDF (Neutral Detergent Fiber) content by boiling a sample in a neutral solution, whereby the fraction hemicellulose + cellulose + lignin (+cutin) is determined. All cell wall constituents (excluding the readily degradable pectin) are thus included.
- Determination of the ADF content (Acid Detergent Fiber) by placing the sample in a weakly acidic solution. The fraction of cellulose + lignin is determined in this process.
- Determination of the ADL (Acid Detergent Lignin) content by putting the sample into a sulfuric acid solution, whereby the lignin content is determined.
- Hemicellulose (relatively easily fermentable) can be calculated as: NDF - ADF
- Cellulose can be calculated as ADF - ADL. The crude fiber content is the closest to this.