Energy partition: gross to net energy
How feed energy divides into gross, digestible, metabolisable and net energy, where it is lost, energy systems, and a dairy cow energy budget.
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A kilogram of hay and a kilogram of maize grain contain almost the same amount of chemical energy, yet maize supports far more milk or growth. The difference lies in how much of that energy the animal can actually use. Nutritionists trace feed energy step by step through digestion and metabolism, subtracting the losses at each stage. This partition of energy is the basis of every energy system used to formulate livestock rations.
Energy partition and energy systems are standard topics in animal nutrition courses and exams.
The cascade
Gross energy (GE)
The total heat released when feed is completely burned, measured in a bomb calorimeter. Most feeds have about 18 to 19 MJ per kg of dry matter; fats have much more (about 39 MJ per kg) and minerals none. GE says little about feeding value, because it ignores digestibility.
Digestible energy (DE)
DE = GE − energy lost in faeces. Faecal losses are the largest single loss, typically 20 to 50 per cent of GE depending on digestibility: small for grains, large for mature forages.
Metabolisable energy (ME)
ME = DE − energy lost in urine and combustible gases, mainly methane from rumen fermentation. Methane typically takes about 6 to 7 per cent of GE in cattle (less in feedlot diets), a loss of energy as well as a greenhouse gas. For ruminant diets, ME is roughly 0.80 to 0.82 of DE.
Net energy (NE)
NE = ME − heat increment, the heat produced by digesting and metabolising the feed (fermentation heat, work of digestion and nutrient metabolism). The heat increment is larger for fibrous diets. NE is the energy actually available for:
- maintenance (NEm): keeping the animal alive at constant weight;
- production (NEp): growth (NEg), milk (NEl), pregnancy, wool, eggs and work.
The efficiency with which ME is converted to NE differs by purpose: highest for maintenance (often about 0.65 to 0.75), similar for lactation (about 0.6 to 0.65), and lower for growth and fattening (about 0.3 to 0.5), depending on diet quality.
How the values are measured
- Digestion trials: animals are fed a known amount of feed, and faeces are collected and analysed to find DE, often in metabolism crates that also collect urine.
- Respiration chambers and, more recently, the SF6 tracer technique and breath sampling units measure methane and heat production.
- Comparative slaughter experiments measure the energy actually retained in the body of growing animals.
- Prediction equations from routine feed analyses (such as NDF, crude protein and in vitro digestibility) are used in practice, because direct measurement is slow and costly.
Energy systems around the world
- Total digestible nutrients (TDN): an old US unit, the sum of digestible crude protein, digestible crude fibre, digestible NFE and 2.25 × digestible ether extract, as a percentage. Still widely used in practical feeding tables; it is roughly comparable to DE.
- ME systems: the UK system (Agricultural Research Council, 1980s) and Australia use ME with efficiency factors.
- NE systems: the US NRC/NASEM uses NEm, NEg and NEl for cattle; France's INRA uses UFL and UFV (feed units for lactation and meat); the Netherlands uses VEM.
- Pigs and poultry: DE, ME (corrected for nitrogen balance, AMEn in poultry) or NE systems.
Maintenance and metabolic body size
Maintenance energy scales not with body weight but approximately with metabolic body weight, W0.75, following Max Kleiber's work in the 1930s: a 600 kg cow needs much less than ten times the maintenance energy of a 60 kg sheep.
An energy budget for a dairy cow
A 600 kg cow eats 20 kg of dry matter a day of a mixed ration with 18.5 MJ GE per kg DM.
- GE = 20 × 18.5 = 370 MJ.
- Digestibility of energy 70 per cent: DE = 0.70 × 370 = 259 MJ (faeces 111 MJ).
- Urine 4 per cent of GE (15 MJ) and methane 6.5 per cent of GE (24 MJ): ME = 259 − 15 − 24 = 220 MJ.
- Heat increment about 35 per cent of ME (77 MJ): NE = 220 − 77 = 143 MJ.
Maintenance. Using the classic NRC value of 0.08 Mcal of NEl per kg of metabolic body weight: 6000.75 ≈ 121 kg, so NEm = 0.08 × 121 ≈ 9.7 Mcal, or about 40.6 MJ (1 Mcal = 4.184 MJ).
Milk. Milk with about 4 per cent fat needs about 3.1 MJ of NEl per kg. Energy left for milk = 143 − 40.6 = 102.4 MJ, enough for about 102.4 ÷ 3.1 ≈ 33 kg of milk a day, if protein and other nutrients are also adequate.
Of the 370 MJ eaten, only about 102 MJ, roughly 28 per cent, ends up in milk. Higher-digestibility forage or more concentrate would reduce faecal losses and raise the milk the same intake can support.
Key points
- GE − faecal energy = DE; DE − urine and methane = ME; ME − heat increment = NE.
- Faecal losses are largest; methane takes about 6–7 per cent of GE in cattle.
- NE is used for maintenance and production, with different efficiencies.
- Systems: TDN, ME (UK, Australia), NE (US NRC, INRA UFL/UFV, Dutch VEM); AMEn for poultry.
- Maintenance scales with metabolic body weight, W0.75 (Kleiber).
What the exams ask
- School-leaving and entrance papers: forms of feed energy and where energy is lost.
- Degree and research-fellowship entrances: the energy cascade, heat increment, efficiencies of ME use, TDN calculation, metabolic body weight and energy systems.
- Professional examinations: energy balance of dairy cows and growing animals, feed evaluation, and methane as an energy loss.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why are fats so energy-rich?
They contain more carbon and hydrogen and less oxygen than carbohydrates, so they release about 2.25 times as much energy when oxidised, and they are highly digestible.
Why is forage less efficient than grain?
Fibrous forages are less digestible, so more energy is lost in faeces, and their fermentation produces more methane and more heat increment.
Does reducing methane save feed energy?
Yes, a little. Methane is lost feed energy, so reducing it, for example through better diets or approved additives, can slightly improve efficiency as well as cut emissions.
Sources
- McDonald, P. et al. Animal Nutrition, 7th edition. Pearson, 2011. Chapters 11–12.
- National Academies of Sciences, Engineering, and Medicine. Nutrient Requirements of Dairy Cattle, 8th revised edition. National Academies Press, 2021.
- Kleiber, M. The Fire of Life: An Introduction to Animal Energetics. Wiley, 1961.
- Johnson, K. A. and Johnson, D. E. "Methane emissions from cattle." Journal of Animal Science 73, 2483–2492 (1995).
- Agricultural Research Council. The Nutrient Requirements of Ruminant Livestock. Commonwealth Agricultural Bureaux, 1980.
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