The single most important measure of pasture nutritive value is digestibility, the proportion of the consumed pasture actually used by the animal. This usually ranges from >80% down to <40% as the season progresses and pasture plants pass from the young, vegetative stage to maturity and seed set (Fig. 2). Intake declines more rapidly at digestibilities below about 65 70%, because fiber digestion in the rumen is slower and restricts the rate at which material leaves the rumen. This means that on a pasture of low digestibility, sheep will reach their intake limit before their nutrient requirements are satisfied. Conversely, at higher pasture digestibilities, plant material passes through the rumen more rapidly, and the animal can eat more. Moreover, when herbage of higher digestibility is consumed, there is the added benefit that the metabolizable energy obtained from it by the animal is used with higher efficiency for maintenance and growth. In general, high levels of production can only be supported by pastures of 70% digestibility or above (Table 1). Pastures of 60 65% digestibility will support moderate levels of animal production, but below 55 60% digestibility, pastures will only maintain dry stock. Below 50% digestibility, weight loss is likely, regardless of the amount of pasture on offer.
In most cases, the protein content of green pasture is in the range 15 25% and probably provides soluble protein in substantial excess relative to the requirements of the rumen. By contrast, the very low protein contents (3 6% DM) in low-quality, dead pastures can fail to meet the needs of the rumen microbial population for soluble nitrogen. The rate of fiber digestion in the rumen will then be reduced and intake will fall. In this case, there is benefit in supplementing the animals with a source of soluble protein (see the following section) to increase the digestion rate and, thus, the intake of low-quality forage.
Fig. 1 Effect of amount of pasture (75% digestibility) on the intake of a 50 kg sheep. [Data derived using GrazFeed decision support tool. (From Ref. 4.)]
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