Plant Structure

The defining characteristic of forages is that they contain a large portion of cell-wall material. The amount and type of plant cell-wall material determines the nutritional quality of forages.

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 cell. The secondary wall is thicker than the primary wall, giving cells tensile strength. The primary and secondary cell walls combined make up 40-80% of the forage dry matter. The main structural components of both primary and secondary walls are two complex carbohydrates called cellulose and hemicellulose. Cellulose is one of the most abundant organic materials on earth. Because higher animals cannot produce enzymes that digest cellulose, they make use of cultures of microorganisms residing in their digestive tracts. Ruminants have the most efficient system for digesting and utilizing cellulose.

Fig 1. Diagram of a plant cell showing cell-wall structure.

With advancing maturity, forage cells insert a non-carbohydrate material known as lignin into the primary and secondary walls. This complex compound is the main constituent of wood and gives the plants additional tensile strength and rigidity. Lignin 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 plant advances in maturity, more lignin is added to the complex of brick and blocks making them more difficult to break down and digest.

Microwave method to determine moisture content of forage

Supplies: Microwave oven; small scale capable of weighing up to 150 grams (10 oz.) in small increments (2-5 grams; 0.1 oz. or smaller); dry, dinner-size paper plate; glass of water.

Method: Select a representative sample of forage from all over the field. Samples should be taken from top, bottom and middle of swath. Weigh the empty paper plate and record the weight on the edge of the plate.

Weigh exactly 100 grams of forage onto the plate on the scale, allowing for the weight of the plate. (For example, if the plate weighs 30 grams, the total weight of the plate and forage is 130 grams.) For US measurements use exactly 10 oz. of sample plus weight of plate.

Spread the sample evenly over the plate and place it in the microwave with a half-filled glass of water in the back corner. Heat the sample for four minutes at full power.

Weigh and record the weight, then stir the forage and place the plate back in the microwave for another minute, taking care not to lose any of the sample.

Heat at full power but for only 30 seconds before weighing. Repeat the procedure until weight becomes constant. If the forage starts to char, shorten the drying intervals.

The final constant weight, minus the weight of the plate, is the dry matter content of the forage as a percentage. (For US measurements, multiply the final weight, minus plate, by 10 to get percentage dry matter.)