The nutritive value of pastures can be assessed in several different ways.
Energy content of the herbage is the most important value. Livestock obtain energy from the pasture when rumen microbes break down the digestible proportion of the dry matter. The more digestible a pasture is the more of it is utilised and the more energy is obtained from each kilogram of dry matter.
Metabolisable Energy (ME) content of the pasture is expressed as Megajoules of Metabolisable Energy per kg of dry matter (MJ ME/kg DM).
Digestibility is the proportion of the pasture eaten that is retained by the animal to gain weight, produce milk, wool or grow a foetus. If 70% of the pasture eaten is retained and used by the animal and 30% passes out as faeces, the pasture has a digestibility of 70%. The digestibility of plants declines as they mature and enter their reproductive phase. This is caused by thickening of cell walls, increased stem:leaf ratio and movement of sugars from the leaves into the seeds. This means that ruminants obtain less energy per kilogram of pasture as plants send up a seed head and dry off.
Relationships between ME and Digestibility are shown below based on the equation ME = 0.203 X DOMD – 3.01
|Converting Digestibility to Metabolisable Energy (MJ/kg DM)|
|Digestible Organic Matter (%)||Metabolisable Energy (MJ/kg DM)|
It is important to differentiate between the energy content of the pasture or supplement expressed as MJ/kg DM and the energy intake (MJ/day) that a particular animal might obtain from the pasture. This is discussed under the “Feed and ME intake” tab.
Protein Content (%) is also important. Generally, the higher the energy content, the higher the protein content. However, legume based pastures and legume grains have a higher protein content for any given energy value than grass based pastures or cereal grains.
Energy content is normally the limiting factor with pastures, but in dry pastures some native species, low protein content may limit animal production especially in young animals. Therefore, it’s important to know both the protein and energy content of supplements and pastures.
Neutral Detergent Fibre (NDF) and Acid Detergent Fibre (ADF) are measured in some laboratories. In general, as the concentration of fibrous material increases the rate of digestion and digestibility decreases. For most pasture based systems, energy content and protein % are sufficient to determine feeding value of the pasture and supplementary feed requirements.
Nutritive value testing is undertaken by a number of laboratories across Australia. Search on line for Feed testing laboratories in your region.