The quality of a pasture for sheep production is largely influenced by two main characteristics:
- the digestibility of pasture present
- the proportion of clover in the pasture sward.
Digestibility refers to the proportion of plant matter that is retained in the animal's body after eating. For example, a high quality pasture in its ‘green' state might have a digestibility of 80%. This would mean that 80% of the pasture eaten by the animal is utilised for body maintenance and growth, with the remaining 20% of pasture passed out as faeces. All digestibility measurements are expressed in terms of dry matter (DM).
Digestibility and metabolisable energy (ME) are directly related by a linear equation. Either measurement can be used to describe pasture quality and both measurements are usually reported by feed testing laboratories.
Impact of digestibility on feed intake
Digestibility of pasture not only determines the amount of nutrients that the animal can extract but also influences the speed at which the plant material is passed through the gut. Feed sources higher in digestibility are able to pass through the digestive system faster and this allows the animal to increase its daily intake. Conversely, low quality feed ‘clogs’ the system and slows the passage of feed through the animal. This explains why the availability of highly digestible feed is so important to animal production.
Digestibility of green feed versus dead feed
Green pasture will always be higher quality (60-85% digestibility) than dead herbage (35-60%) of the same species.
Pasture digestibility is also largely influenced by stage of growth. Pasture is of highest quality in the vegetative stage but declines rapidly after flowering.
Digestibility of different species
During the growing season, there are relatively small differences in digestibility between annual species, and as a general rule, green growing annual plants are highly digestible (70-80%).
Digestibility declines rapidly after senescence and plateaus at around 50%. The hotter the day-temperature the quicker the decline in quality, with many pastures only taking 30 days to reach 50%. In cooler areas, it may take up to 60 days to reach 50% digestibility.
Perennial species typically maintain higher quality into summer, depending on soil moisture and/or rain, with varying amounts of green plant matter present. Once dead however, the quality of the perennial sward can be as low as 35%.