Herbage production depends on the genetic potential of the pasture plants in that environment, competition between plants, rainfall amount and distribution, temperature, soil type, soil fertility, disease and the grazing intensity.
In contrast to annual pasture systems, some of the biomass produced by perennial plants is not available to livestock. For instance, some may be inedible, such as woody stems (stems >2-5 mm in diameter are usually considered inedible for sheep), while some may be out of reach of animals without cutting.
Voluntary feed intake:
Variation in voluntary feed intake accounts for at least 50% of the variation that is observed in feeding value of forages. Voluntary feed intake is not easy to predict. Characteristics that influence voluntary feed intake include: gut fill; clearance rate of digesta from the rumen (influenced by digestibility); energy; protein content; palatability; feeding behaviour and species; and class and physiological state of the animal.
Other related factors include the amount of time the animal grazes and ease of grazing reflected by bite mass and bite frequency. Plant morphology also plays a role, for example, browsing from sparse shrubs is more time consuming than eating from a dense sward.
Physical and metabolic control mechanisms:
A conceptual model for the regulation of pasture intake by the grazing animal based on a combination of physical and metabolic signals has been developed. Digestibility of herbage has a large influence on voluntary feed intake as it determines the rate that plant material can be cleared through the rumen.
Put simply, animals can only eat as fast as the rumen clearance rate will allow. Low clearance rates are associated with increases in the generation of satiety signals from the rumen to the central nervous system. This limitation represents a physical constraint to intake that is particularly important for feed sources with a low digestibility.
There are also metabolic signals, for example, an energy deficit generates hunger signals, relative to the size of the deficit. Conversely, high levels of energy within the animal causes the generation of satiety signals.
These metabolic signals are more significant for animals consuming highly digestible feeds. Intake by the animal is regulated through the central nervous system based on a combination of hunger and satiety signals generated by physical and metabolic responses to the feed.
With very low quality feeds, grazing animals can reach a stage where the rate of digestion is so slow that they are simply unable to consume enough feed to grow. Under these conditions, the physical signals override the hunger signals generated through low energy levels within the animal.
The predicted relationship between digestibility and voluntary feed intake for a three year old, 50 kg dry Merino ewe is illustrated in Figure 2, using the ruminant nutrition model GrazFeed. Assuming that all diets contained 15% crude protein, an average ewe of this size, condition and age requires the digestibility of the feed to be above 60% for growth. Generally crude protein is lower in feeds of low digestibility, in which case the line in Figure 2 would be steeper.
Palatability:
Palatability (or feed preference) depends on any characteristic of the feed that increases or inhibits intake of forage whether the forage is offered alone or as a mixed sward. If an animal rejects forage then clearly that forage will be of reduced feeding value even if its nutritive value is high.
Palatability is also likely to be regulated by the central nervous system through a combination of pre- and post-ingestive feedback signals. The physical and metabolic signals described above fit within this framework, but so do others such as taste, odour, texture, where the learned behaviour has often been developed in response to either nutrients or toxins.
Evidence for nutrient signals has been reported in experiments under a range of environmental conditions. Under normal circumstances, ruminants prefer foods with high digestibility and avoid those with low digestibility and high fibre. Growing lambs will select from a range of different crude protein levels to obtain a mixture of feeds that meet their crude protein requirement, but will avoid the excessive intake of degradable protein.
However, there is also clear evidence that these nutritional signals may be overridden by others related to the presence of toxins and other antinutritional factors. For example, compounds such as tannins, oxalates, coumarins and nitrates, which are all commonly found in grazed plants can depress feed intake and alter dietary selection.
Many perennial plants generate a range of compounds as a deterrent to grazing and to improve persistence in their environment. Annual plants tend to focus resources on the production of seed as a persistence mechanism.
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