The performance of livestock grazing grasses is directly related to the quantity and quality of grass on offer. While there are many quality characteristics that influence the intake of a grass by livestock, the most useful are digestibility and crude protein; hence, where available, these figures are provided for individual species.
Digestibility is a measure of the proportion of a grass (on a dry matter basis) that can be utilised by an animal. For example, if an animal eats 10kg of dry matter and 3kg is expelled as dung, then the feed is 70% digestible. Digestibility influences the speed with which a grass passes through the digestive system. Generally, grasses with higher digestibility will be processed more rapidly, allowing stock to eat more and so have higher production. Digestibility is also a useful measure of the quality of a grass as it is directly related to the energy content of a feed; energy being needed for body functions.
As a general guide: 70-80% digestibility is required for high livestock production; 60-70% digestibility is required for moderate production; and 55-60% digestibility is required to maintain dry stock. Below 55% digestibility, dry stock will lose weight. Digestibility is strongly influenced by the plants stage of growth. Grasses that are green, leafy and actively growing will have a higher digestibility than those that are in head or have hayed off.
Crude protein is an estimate of the total protein present in a grass. Protein forms the building blocks of muscle and its components are used in every system of the body. Crude protein is positively related to the digestibility of a grass and, in general, as crude protein increases, so does livestock performance (e.g. weight gain, milk production, etc).
The forage quality of grasses is very dependent on how the grass is managed. When maintained in the early to late vegetative stage, the quality of a grass is likely to be at the higher end of the digestibility and crude protein values listed for individual species and at the lower end during late flowering. However, maintaining a species at the vegetative stage only may be detrimental to its persistence.
As a further caution the quality figures listed are often based on limited measurements, different plant parts (e.g. some measurements have recorded green leaf, others entire plants) and are usually recorded under different growing conditions, all of which will affect quality data. Native grasses also tend to be very genetically variable due to their wide distribution, which subjects individual plants to different soils, climates and management history. This variability can be significant even at the paddock scale.