Valuing forages on a megajoule of metabolisable energy (MJME) basis is a handy and straightforward concept. For all stock classes, we can balance an animal’s requirements:
• Demand for energy (maintenance, live weight gain, pregnancy and lactation) with
• Supply of energy (MJME/kilogram of dry matter (DM) multiplied by kgDM eaten per day)
By defining animal production targets (e.g. kilograms of live weight gain or milksolids), the MJME rating of a feed and the kgDM available, we can develop a feeding plan that allows animal’s to meet these performance targets. However, MJME is simply a calculated measure of forage quality; it doesn’t take into account other factors that drive the production performance of forage-fed sheep, cattle and deer. Other nutritional factors of forage such as crude protein (CP), neutral detergent fibre (NDF), water soluble carbohydrates (WSC), fat and minerals can affect animal productivity too. Looking at MJME in isolation will not necessarily predict animal performance.
Crude Protein (CP)
High concentrations of rapidly rumen degradable crude protein (CP) often accompany high MJME feed test results. Leafy, lush high MJME spring or autumn pasture may contain CP levels in excess of 30% CP. Dairy cows at peak lactation require no more than 20% CP. Lambs growing at 300 grams of live weight per day require no more than 15% CP.
If CP levels exceed the requirements of stock, feed conversion efficiency may fall. Animals can lose surplus protein as urinary urea which is a waste of the animals energy and can potentially reduce the efficiency of your feed.
Neutral Detergent Fibre (NDF)
Inadequate levels of physically effective NDF (peNDF) in high MJME forages may fail to support appropriate rumen function. For a lactating dairy cow, the first autumn grazing of a lush high quality Italian ryegrass testing at
13.0 MJME ‘on paper’ appears to be a perfect complete feed. However, a low concentration of NDF (e.g. 28 to 32%
NDF/kgDM) increases the risk of ruminal acidosis because forage-fed dairy cows require 35% NDF for optimum ruminal function.
The risk of animal health issues for stock grazing forage can’t be evaluated on the basis of MJME alone. High MJME lush, leafy pasture often contains high concentrations of potassium and CP, and low levels of magnesium and calcium which increases the risk of metabolic challenges (hypomagnesaemia and hypocalcaemia) for pregnant and lactating cattle. This can occur particularly when the passage of high MJME forage through the rumen is rapid.
Measuring your pasture quality based on only MJME means you could be missing out on identifying potential mineral deficiencies.
In addition to the nutritional factors, the type of forage and the environment can also affect the intake of high
• The presence of standard endophyte alkaloids, rust, effluent applications to pasture and high concentrations of minerals or compounds including nitrates and sulphur may limit the intake of high MJME forages
• Tetraploid ryegrasses are often consumed more readily than diploid ryegrasses, even when the MJME value for each ryegrass is similar
• Lower than ideal pre-grazing pasture mass (kgDM/ha) can reduce MJME intake. For cattle particularly, reduced bite sizes on short pasture will reduce intake of otherwise high MJME pasture, particularly if grazing time is limited. Wet muddy conditions further reduce utilisation of high MJME forage
The evaluation of feeds based on MJME is better than kgDM alone; however, it doesn’t take into account other aspects of forage nutrition. When working to improve the nutrition of forage-fed stock, we need to understand the importance of all nutrients, not just MJME.
The PGG Wrightson Seeds team consider all aspects of forage quality during the breeding and development of pasture species, giving you forages that will meet the needs of your sheep, cattle and deer.