Pasture feed value is measured in several ways, the most important being metabolisable energy (ME) and digestibility. ME is the more useful unit for many applications. Feed value matters, because it drives animal intake and performance.
Metabolisable enery (ME)
ME is the energy in a feed available to the animal after allowing for faecal, urine and methane losses. It is measured in megajoules per kilogram of dry matter of the feed (MJ/kg DM).
ME is the energy available to the animal for maintenance, growth and production.
Typical ME in MJ/kg DM (these vary widely)
Dry matter digestibility
Digestibility is the percentage of energy in a feed available to the animal after faecal losses. This can be calculated as:
DM eaten - DM excreted x 100
Relating ME and digestibility
Digestibility and ME are related and can be approximately compared as follows:
Why is ME important?
ME is a key driver of animal performance in most NZ pasture-based systems. ME has a double effect, because as ME feed values increase, so do animal intakes. Conversely feeds with a low ME require extra time in the rumen to degrade. For example, it takes 45-55 hours for high fibre straw (ME 7-8) to pass through the rumen, compared with 18-24 hours for good quality grass (ME 11-12). When an animal’s rumen is full, it will stop eating, so feeds with low ME limit intake.
Water soluble carbohydrates (WSC)
WSC are simple sugars present in plant cells, and are an important energy source for animals. WSC content is highly correlated with ME. In most cases ME gives a better indication of animal performance than WSC, as production responses to WSC have been much smaller and less consistent than responses to ME. Increased levels of WSC have environmental benefits at times, by reducing the amount of nitrogen lost in the urine.
Crude protein (CP)
Animals need crude protein to maintain body tissue and produce milk/meat. Ideal CP levels for lactating animals are 17-18%. For live weight gain 12% CP is needed in the diet. NZ pastures are relatively high in CP, and it does not generally limit animal production, except in summer dry conditions.
Rather than increase animal production, pastures containing high concentrations of CP (≥24%) lead to increased levels of nitrogen in the urine and dung. This increases potential N loading in the urine and dung patches and subsequent risk of nitrate leaching from the soil.
Neutral detergent fibre (NDF)
NDF is the most common measure of fibre in feed analysis. NDF measures more than 90% of the structural components in plant cell walls, including lignin, hemicellulose and cellulose. In high quality pasture, the ideal NDF is 35-40%. As NDF moves above this, for example in a rank pasture, ME and digestibility fall.
Visually assessed feed quality
Nutritive value can be assessed visually, based on physical attributes. Always look at the whole paddock as quality can vary across a pasture.
- Legumes and herbs generally have higher feed value than grasses.
- Green leaf generally has high ME; dead matter, stem and seed head are significantly lower.
- High temperatures drop summer ME.
Importance of management
Good pasture management is the biggest driver to maximising feed value, and creating sustainable, efficient, profitable farm systems. Key strategies include:
- Managing pastures through late spring.
- Grazing to the correct residuals.
- Grazing at the right time.
- Maximising legume content.