The productivity of sheep is largely governed by the amount of pasture that can be eaten and it's nutritional value.  Both pasture quality and quantity will influence these factors and need to be estimated when assessing pastures to plan for animal performance. In this section find information on;

Pasture Quantity
Pasture Quality
Digestibility
Digestibility's relationship with animal intake and changes with pasture composition

Pasture Quantity -  How is it measured?

The quantity of pasture in a paddock is estimated in kilograms of dry matter per hectare, expressed as kg DM/ha.

Feed On Offer (FOO) or Herbage Mass(HM) is a method to assess amount of pasture available to the animal and is assessed by estimating the dry weight of all above ground plant material. Some reference materials may use a method which relies on leaving several centimetres of plant material behind, so it is worth checking which method is being used. 

The lifetimewool guidelines use the method that includes all above ground plant material unless otherwise stated. NSW DPI and Grazfeed® use herbage mass (further information). 

Measuring pasture quantity on-farm

To make pasture quantity assessment more efficient on commercial grazing properties, the lifetimewool project is developing a ‘Photo Gallery' that can be used in the paddock to quickly estimate the amount of green pasture by comparing what is seen in the paddock with a range of photos of known quantity.  

Another commonly used method involves using a "pasture stick" or ruler, to estimate an average height of pasture across the paddock, and then factoring in pasture density to make an estimate of pasture quantity. This method is most accurate when used on grassy perennial pastures of even composition.  They are not suitable for clover based annual pastures or pastures that have variable compositions.

Pasture Quality - How do we determine it?

The quality of a pasture for sheep production is largely influenced by two main characteristics.  They are:

1. The digestibility of pasture present.
2. The proportion of clover in the pasture sward.

Figure 1. Digestibility relationship with dry matter

Digestibility - What is it, and how is it measured?

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 (See figure 1).  All digestibility measurements are expressed in terms of dry matter.

Digestibility and Metabolisable Energy (ME) measurements of feed are directly related. Either measurement can be used to describe pasture quality. Digestibility and ME can be calculated by a feed testing house. The relationship between digestibility and ME are shown in Graph 1 below.

Graph 1. Relationship between ME and digestibility in forages.
Source: FEEDTEST 2006.

Impact of digestibility on animal 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 allow the animal to increase its daily intake.  This is an important concept to understand as it explains why high by digestible feeds have a much greater potential for animal production in areas such as growth, wool production and lactation.

Graph 2. Energy intake for a 50 kg dry ewe grazing a 1000 kg FOO pasture. This prediction was formulated using Grazfeed®.

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.  The exceptions are Persian and Arrowleaf clovers (both annual species) which are typically 5-10 units higher than sub clover in January.

Stage of growth also has a large influence on pasture digestibility.  Pasture is of highest quality early in the vegetative stage and digestibility gradually declines as the pasture progresses through the vegetative and reproductive phases, as shown in Figure 2. 

Digestibility differences between species

During the growing season, there are relatively small differences between the digestibility of annual and perennial grasses, as shown in Figure 2.  However, towards the end of the growing season, the quality of annual grasses declines quickly as they produce seed heads and die.  In contrast, perennial grass species maintain higher quality for longer, with varying amounts of green plant matter present.  Once dead the quality of the sward can be as low as 35%.

Annual pastures, particularly in a Mediterranean climate, decline rapidly after senesence and then stabilize at around 50% digestibility (Figure 3).  The hotter the day temperature the quicker the decline, with many pastures only taking 30 days to reach 50%.  In cooler climes, decline may take up to 60 days to reach ~50%. Further work on the conditions affecting decline is underway.

Figure 3: Digestibility decline of annual grasses and clovers in a Mediterranean climate
source: M. Hyder, DAFWA

Clover Content 

Sheep will gain weight faster, grow more wool and produce more milk when grazing legume pastures compared with grass pastures of the same pasture quantity.  Although some legume species such as lucerne, White Clover and Persian Clover tend to have slightly higher digestibility than even the best perennial grasses, the main reason for the increased performance is due to an increase in animal intake.  Sheep prefer eating legumes to grass species and as a result will tend to eat higher quantities of feed in pastures with greater legume content. 

Figure 3. The relationship between increased clover content in a pasture and weight gain in 3 month old Merino and 2nd X prime lamb wether weaners, grazing 1000 kg FOO at 75% digestibility. This prediction was formulated using Grazfeed®.

Tools:

Pasture Photo Gallery
Herbage Mass compared to FOO