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Most pastures are made-up of legumes and grasses. The most frequently grown legumes are clovers and lucerne. Legumes are important as they have higher levels of digestible protein and greater concentrations of calcium compared to grasses and therefore significantly increase the nutritional value of pasture.
Unlike grasses, clovers convert nitrogen from the air to a form that plants can use. Legumes are only able to provide nitrogen in the presence of special soil “bugs” (micro-organisms) called rhizobia that live in the roots. Compared with legumes, grasses have greater year-round production, provide more feed in late autumn, winter and early spring and have a greater tolerance to grazing. Pasture plants can be perennial, biennial or annual.
Perennials
Perennial plants live for more than two years. They have a greater potential for year-round growth and their deep root system binds the soil to reduce dust, soil structure decline and erosion. Perennials can reduce the rate of environmental degradation (soil salinisation and acidification). Perennial species require moisture in summer to survive. The severity of summer determines if and what perennial species can be grown in a region. Phalaris and white clover are an example of a perennial grass and legume respectively.
Annuals
Annual plants survive for only one year. They must set seed in spring and germinate from seed the following autumn. Annual plants survive summer as dormant seeds and this enables them to grow in environments with harsher summers than perennials. In autumn, germinating annuals are frequently at a competitive disadvantage as perennials already have an established root system to use the available moisture and nutrients.
In contrast, the annual must first grow roots before it can access moisture and nutrients. This is why perennials are frequently more productive than annuals in autumn and winter. Subterranean clover and annual ryegrass are examples of an annual legume and grass respectively. The ideal pasture for livestock is one that:
- Supplies high quality feed on a year-round basis
- Meets most nutritional needs of livestock
- Maintains ground cover to reduce dust, erosion and soil compaction
- Causes no livestock health problems
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The target
Pastures should contain a minimum of 20 to 30% legume on a dry weight basis. Lower levels of legume in the pasture will reduce nutritional value and pasture growth. Wherever possible pasture should contain perennial species. The higher the portion of perennials in the pasture the better.
How pasture grows
To successfully manage pasture a basic understanding of how pasture grows is required. This understanding will assist in increasing both pasture productivity and persistence. The species present, soil moisture and nutrients, temperature and grazing management governs pasture growth. Pasture has three phases of growth. It is important to appreciate these stages as they can have a significant impact on pasture quantity and quality.
Phase I:
Pasture in this phase is less than five centimetres in height. Growth is slow as there is insufficient leaf area to trap available sunlight and covert it to energy for plant growth. Extended grazing in Phase I will kill plants and has the potential to cause erosion, runoff and weed invasion.
Phase II:
Is the stage of greatest plant growth and pasture should be kept in this phase for as long as possible. Pasture in this phase is 5 to 15 cm in height. They tend to have greater nutritive values, persistence, and resistance to weed invasion and soil structure decline.
Phase III:
Pastures in this phase are greater than 15 cm in height and are mature and of poorer quality. Spring pastures that have been poorly managed often fall into this category. In some situations animals grazing Phase III pastures can not physically consume enough pasture to meet their nutritional requirements.
Pasture management is difficult. Pasture growth rates can vary between 10 kg DM/ha/day in winter to 80 kg DM/ha/day in spring. In some environments, 80% of the pasture grows over only 25% of the year.
The target
Pasture should be kept 5 to 15 cm in height therefore, in Phase II. Unfortunately this is difficult to achieve, especially in winter and spring. Winter is often the time of greatest feed shortage as the pasture, due to low temperature, fails to grow at a reasonable rate. It is likely that the pasture may fall back into Phase I during this time. The challenge is to ensure that pasture remains close to the Phase I/Phase II border so that when favourable conditions occur the pasture can quickly move into full production.
The amount and quality of pasture
The quantity of pasture in a paddock, that is, all the herbage above the ground, is referred to as herbage mass and is measured in kilograms of dry matter per hectare (DM kg/ha). Pasture varies in its moisture content and needs to be converted to dry matter values for comparisons. It is also important to know what proportion of the herbage is green and dead as this has an important influence on the nutritional value of pasture.
There is no single way to determine pasture quality. Factors used to estimate pasture quality range from paddock assessment of clover and percentage green of the pasture to laboratory testing that determines protein, energy, digestibility and fibre. Digestibility is the amount of pasture eaten, which is retained by the animal. If green pasture has a digestibility of 75%, 75% of what the animal eats will be used, while the remaining 25% will pass out as waste. Feed of a higher digestibility will be digested more rapidly allowing most animals to eat more feed. There are many factors that can assist in making a judgement as to the quality of pasture.
Species in the pasture
Pasture species vary in their digestibility (Table below). The differences in the digestibility between annual and perennial grasses appear towards the end of the season when annual grasses quickly loose quality as they become reproductive. Perennial grasses maintain some quality feed into summer especially with summer rainfall and when the flowering stage is delayed.
Table 1. Digestibility classes of pastures
| Digestibility group | Pasture species |
| Very high digestibility (75-85%) | White clover, Persian clover. |
| High digestibility (70-89%) | Perennial ryegrass, phalaris, tall fescue, lucerne, capeweed, sub and balansa clover. |
| Moderate digestibility (60-75%) | Cocksfoot, soft brome and barley grass, wallaby grass and fog grass. |
| Low digestibility (55-65%) | Onion grass, sweet vernal, spear, weeping, kangaroo and silver grass. |
Amount of dead herbage in the sward:
Green pasture is always more digestible (55-85% digestibility) than dead herbage (35-65% digestibility) of the same species. Pasture quality declines with most species as they move into the reproduction phase in spring. Pasture is the highest quality early in the vegetative stage and gradually declines as the pasture ages.
Pasture growth stage
Large quantities of dead herbage mixed with green can significantly reduce the nutritive value of a pasture. Animals are selective grazers, but are forced to eat dead grass while foraging out green grass. As a consequence the overall quality of their diet is less. This frequently occurs during autumn when pasture from the previous season has not been grazed. When assessing and sampling pasture for analysis, the green and dead components need to be separated.
Parts of the plant
In most plants leaves are of higher quality than stems. The difference is largely dependent upon “woodiness” of the stem. The digestibility difference between the stem and the leaf of lucerne is between 10 and 15%. The stem and the leaf of sub clover are similar in quality.
The previous version of this note was published in September 2003.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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