Understanding Corn Silage Quality

Plant Structure*

(*Based on C Holland and W Kezar, The Pioneer Forage Manual — Nutritional Guide. Pioneer Hi-Bred International Inc. 1999.)

Stover

The defining characteristic of plant tissue, such as that comprising the stems and leaves of corn stover, is that it contains a large portion of cell-wall material. The amount and type of plant cell wall material determines the nutritional quality of the stover.

A young plant cell has a single outer layer referred to as the primary cell wall (Fig. 1). As the plant matures, a second cell wall is laid down on the inside of the first cell wall. This secondary wall is thicker than the primary wall, giving plant cells tensile strength. The primary and secondary cell walls combined make up 40-80% of the dry matter content of corn stover. The main structural components of both primary and secondary cell walls are two complex carbohydrates called cellulose and hemicellulose. Cellulose is one of the most abundant organic materials on earth. Animals cannot produce enzymes to digest cellulose or hemicellulose. However, the micro- organisms residing in the digestive tract of ruminants (primarily in the rumen) produce enzymes which efficiently digest and utilize cellulose.

Figure 1. Diagram of a plant cell showing cell-wall structure
(from Advanced Forage Management, 1999).

With advancing maturity, forage cells insert a complex non-carbohydrate material known as lignin between the primary and secondary walls. Lignin gives the plants additional tensile strength and rigidity and can be thought of as the primary skeleton of the plant cell. Lignin is important from a nutritional perspective because it is totally indigestible and its presence reduces the availability of the cellulose and hemicellulose portions of the forage. The primary cell wall is like a layer of bricks, the secondary wall like a layer of cinder blocks laid inside the bricks and lignin is like mortar added later between the bricks and cinder blocks. As the corn plant advances in maturity, more lignin is added making the cell walls more difficult to digest.

Grain

A corn kernel is largely comprised of three parts: the pericarp (outer coating or hull), the endosperm, and the germ (or embyro) as shown in Fig. 2.

Figure 2. Parts of a corn kernel

The pericarp or hull of the corn kernel is a thin outer covering made up of two layers. Removal of this part of the corn kernel results in corn bran, commonly used for cooking. The endosperm, which comprises up to 82% of the kernel’s dry weight, is the source of energy for the germinating seed. In all field corn, the endosperm is comprised of two types of starch: vitreous and floury (Fig. 3). The proportion of these starches is controlled genetically. The floury endosperm is the softer starch, and as the kernel matures, this type of starch dries down to create the “dent” in the top of the kernel of dent varieties. Vitreous starch, more abundant in flint corns, tends to be harder for cows to digest because the starch granules are embedded in a dense protein matrix. Therefore, as the amount of vitreous starch present in corn kernels increases, total tract starch digestibility decreases, unless the silage is ‘processed’ (see Corn Silage Processing section).

Figure 3. Types and placement of starch within the corn kernel
(from Dairy Herd Management Vol. 35 No.11, Nov. 1998).

The germ of the kernel contains all the genetic information, some nutrients, and oil needed to enable germination and early growth. The germ is comprised of 25% fat (corn oil) which is high in linoleic acid (see Fats in Corn Silage section). The tip cap is not covered by the pericarp as it is the attachment point between the kernel and the cob.

Assessment of corn silage quality*

(*Based on C Holland and W Kezar, The Pioneer Forage Manual — Nutritional Guide. Pioneer Hi-Bred International Inc. 1999.)

What is ‘Detergent’ Fibre?

The ‘detergent-fibre’ method for assessing quality of forages was introduced about 1970. The earlier crude fibre system failed to generate accurate estimates of digestible nutrients over a wide range of forages; it tended to underestimate good quality forages and overestimate poor quality forages. The detergent system of forage analysis is now the most common way to assess forage quality. Fig. 4 shows a schematic of the detergent system of forage analysis. Detergent analyses are performed on dried and finely ground samples.

Figure 4. The detergent (Van Soest) procedure to
partition forage fractions.

Neutral Detergent Fibre (NDF)

For determining NDF, samples are boiled in a special detergent at a neutral pH of 7.0, then filtered. The soluble portion that passes through the filter contains highly digestible nutrients which were contained within the cells (see Table 1). The insoluble portion of the forage that does not pass through the filter is called the ‘neutral detergent fibre’. This fraction contains the cell wall material including cellulose, hemicellulose, lignin and silica (Table 1). The proportion of NDF in corn stover increases with the advancing maturity of corn. In recent years, seed companies have been developing hybrids for silage production where increasing maturity does not necessarily result in decreasing fibre digestibility.

On a simple level, neutral detergent fibre is sometimes used as a (negative) indicator of feed intake. As the NDF increases, animals tend to consume less forage. The relationship between NDF and intake is:

Feed intake (dry matter) as percent of body weight = 120/NDF(%)

Example: corn silage with an NDF value of 40% will be consumed at 120/40=3% of body weight.

The paradox of NDF is that while it can be a negative indicator of quality it is also required by ruminant animals.

Acid Detergent Fibre (ADF)

Acid detergent fibre is the portion of the forage that remains on the filter after a finely ground forage sample is treated with a detergent and strong acid. It includes the largely digestible cellulose, indigestible lignin and inorganic silica. Acid detergent fibre is important because it is negatively correlated with digestibility of forages. As the ADF increases, the forage becomes less digestible, primarily because the amount of indigestible lignin is increasing. Total digestible nutrients (TDN) values are calculated directly from ADF values as follows:

  • Corn silage: TDN = 87.8 - (0.70 x ADF%)
  • Legumes and grasses: TDN = 88.9 - (0.79 x ADF%)

Note that TDN declines less rapidly with increasing ADF in corn silage than in grass and legumes.

Lignin and silica

Lignin is the wood-like, non-carbohydrate component that cannot be digested by ruminants. Further, lignin decreases availability of cellulose and hemicellulose. The lignin fraction can be determined by further treatment of the ADF fraction with a very strong acid. All plants have the capacity to accumulate significant quantities of silica (or sand) although corn does not accumulate as much silica as many grasses. The silica fraction is left as ash after a forage sample is ignited in a special furnace. Mud or dust picked up on the corn will add to the silica content. Silica content can be reduced by raising the cutting bar (see Effect of Cutting Height).