Nutritive value refers to the responses in animal production per unit of voluntary feed intake and is a function of the digestibility of nutrients and the efficiency with which the nutrients are used for maintenance or growth. It is influenced by plant maturity, genetic variation, environment and management.
Metabolisable energy:
Metabolisable energy (ME) is the amount of energy available for absorption by the animal after digestion and fermentation of the feed consumed. The energy value of feeds can be characterised as the megajoules (MJ) of ME/kg of DM at the maintenance level of feeding (M/D).
The dry matter digestibility of a feed (DMD) is often used to predict the ME. DMD is simply the difference between the quantity of feed eaten and the quantity that is excreted as faeces, expressed as a percentage of total DM eaten.
Insoluble minerals (ash) and acid detergent fibre (ADF) represent the bulk of the indigestible material excreted.
Minerals that are soluble and absorbed by the animal have no energy value, however, they may contribute to the DMD. If feeds have a high ash content (e.g. saltbush), digestible organic matter in the dry matter (DOMD) is the most appropriate method for comparison with other feeds.
Figure 3 illustrates the relationship between DMD, DOMD and metabolisable energy for feeds within the normal ash range (90-120 g ash/kg DM).
Measurements of in vivo DMD are time consuming and expensive as they involve studies with penned animals. A number of laboratory (in vitro) techniques have been developed to predict DMD by determining modified ADF or by measuring DM disappearance in a rumen fluid or pepsin/cellulase digestion system.
More recently, techniques have been developed to predict in vitro and in vivo DMD using Near Infrared Reflectance Spectroscopy (NIRS). It is important to note that laboratory-based predictions have been developed for a narrow range of ‘typical’ forage species. Predictions of DMD or ME for novel forages may be unreliable until there has been more extensive animal house in vivo calibration.
Figure 4 illustrates how the digestibility or energy value of herbage impacts on live weight changes. The data were generated using the ruminant nutrition model GrazFeed by ‘offering’ ad lib roughage with 45-85% DMD and 15% crude protein. The classes of stock were:
- three-year old, dry Merino ewe weighing 50 kg (same as Figure 2)
- 50 kg pregnant ewe carrying twin lambs, 100 days after mating
- 50 kg lactating ewe with twin lambs, 25 days after lambing
- three-year old, dry Merino ewe weighing 50 kg subject to cold temperatures (0-15°C) and 5 mm of rainfall.
Figure 4 illustrates that the feed quality required for maintenance of liveweight for a 50 kg ewe depends on the ewe’s physiological state and on the climate. A dry ewe is predicted to maintain bodyweight on hay of 60% DMD (8.6 MJ/kg DM), however a lactating ewe with twin lambs is unable to maintain bodyweight even with a diet of 85% DMD (12.7 MJ/kg DM) because she cannot eat enough to meet her energy requirements.
When these requirements are compared to the DMD of subterranean clover, it becomes apparent that annual pasture residues in summer and autumn contain insufficient energy for maintenance of liveweight for any of the ewes.
Sheep will only maintain weight through selection of the highest energy components of the feed or through supplementation. Table 1 presents DMD values of a range of annual and perennial pasture species in spring and autumn.
Most, but not all of the pastures would provide enough energy for the dry and pregnant ewes in spring, however few species provide enough energy in the autumn. Table 2 is an estimate of the quantity of dry matter in autumn required to provide the same energy as 1 tonne of lupins.
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