What is energy?
Energy is not an actual physical nutrient that we can see and measure when we look at a feed. There are three main components that contribute to a feeds energy value and these are carbohydrates, fat and protein and the breaking down of these molecules are what gives the animal energy to fuel its body processes.
Basically, energy holds molecules together. When molecules are split by chemical reactions that are undertaken in the body (all body processes require energy), they release this energy in the form of heat. This energy is then available to the animal but some will be lost as urine, faeces, gases (e.g. methane) and heat.
The molecule in which animals store energy is called adenosine triphosphate (ATP). Heat is released when a phosphate is split from this molecule. Animals renew the supply of ATP by breaking down glucose which supplies sufficient heat to drive the chemical reaction to tack the phosphate back on.
So how do animals build up supplies of glucose? The principle source is in the form of carbohydrates in the diet. These carbohydrates include sugars and structural materials, particularly cellulose. Proteins and fats also supply energy. They are digested by the rumen bugs to form small acids. These in turn are absorbed into the blood which transports them to the liver where they are converted into glucose.
Energy value of feeds
Energy is measured as megajoules (MJ), with the total amount of energy stored in a feed being the gross energy (GE). However, not all of this is available to the animal. Gross energy minus energy in faeces = digestible energy (DE). When urine and gases are then taken from this, it leaves metabolisable energy (ME). If losses such as heat are then taken out, we are left with net energy (NE), which is what the animal may then use for maintenance and production.
Cereal grains and molasses are high-energy feeds. They have large amounts of stored energy in soluble sugars which are easily digested and absorbed by the body to form glucose.
The energy content of the pasture diet eaten by cattle can be determined using Near Infrared Reflectance Spectroscopy (NIRS) technology. For more information see Assessing pasture diet quality (NIRS).
End of dry season pasture usually has low energy value. It generally has considerable amounts of stored energy (easily demonstrated with a match!), but the molecules which store it are very difficult to digest (many cannot be digested!).
The energy value of cattle feed is therefore related to:
- how much energy is stored in it, and
- how digestible it is.
Hayed-off grasses, more typical of the early dry season, may have quite reasonable energy value. They may not be high in soluble sugars but their structural components are still digested readily enough.
Table 1. Approximate dry matter digestibility (DMD), metabolisable energy (ME) and protein content of some feeds
|Feed||DM (%)||DMD (%)||ME (MJ/kg DM)||CP (%)||Tropical grass plus legume|
|Phase 1: Early, rapid growth||Low (<30)||70||10.0||10–16||12–16|
|Phase 2: Beginning to grow stem, mostly green||60||8.5||8–10||10–12|
|Phase 3: Flowering and seed set, growth slows, 10–30% green||Medium-High (50–70)||55||7.5||6–8||7–10|
|Phase 4: Senescence, no growth, no green||High (>80)||50||6.5||3–6||7|
|Quality lucerne hay||80–90||70||9.5||22||NA|
*NA = not applicable
Temperate pastures – Figures for temperate pastures can be estimated by adding the following to the figures above in Table 1:
- Digestibility +10
- ME +1.5
- Protein +2–4
Tropical grass plus legume – These figures would apply only where there was sufficient legume in the pasture to increase animal liveweight gain.
Energy requirements of cattle
The amount of energy cattle need daily is variable and depends mainly on animal weight, growth rate and reproductive state (see Table 2).
Table 2. Typical daily energy (MJ of ME) requirements of Brahman-cross cattle
|Animal type||Maintenance*||Extra energy needed for:|
|growth rate of 0.5kg/day||late pregnancy||milking 5kg/day|
|400kg cow (store)||68.5||35.5||12.0||22.5|
*Maintenance = growth nil, not pregnant, not lactating **NA = not applicable
Table 3 gives two working examples which put daily energy utilization into perspective:
- 400kg lactating cow during the wet season
- 200kg weaner during the dry season.
Table 3. Examples of daily energy utilisation by beef cattle in the dry tropics
|Time of year||Wet season
Pasture 50% digestible
Pasture 40% digestible
|Animal (Brahman-cross)||400kg cow in store condition producing 5 litres milk daily||200kg weaner in store condition|
|ME needed to maintain weight||91MJ||42MJ|
|Outcome||Maintains weight||Rapid weight loss|
|Pasture plus urea + sulphur dry lick|
|Outcome||Rapid weight loss|
|Pasture plus M8U|
The example shown above for a lactating cow is based on a 400kg cow in store condition. A typical Brahman-cross mature cow will weigh 500kg (Fordyce pers comm.) in store condition (body condition score 3). In the late dry season in northern Australia the metabolisable energy (ME) values of the pastures are unlikely to provide enough energy for a typical 500kg lactating cow. In the wet season pastures should provide enough energy for a lactating cow, hence it is recommended to seasonally mate so that cows are lactating during the wet season.
Further information on energy requirements of beef cattle and how to correct energy shortfalls can be obtained from attending a Nutrition EDGE workshop.
Geoffry Fordyce, Queensland Alliance for Agriculture and Food Innovation, and Kiri Broad, Department of Agriculture and Fisheries.