Sampling sward to determine key value of grass - digestibility

HIGH production performance from grazing livestock has a major influence on farm profitability.

This is achieved by ensuring high intakes of high quality grass. Grazed grass when correctly managed is of high nutritive value.

Grass quality, as indicated by organic matter digestibility (OMD), can be maintained at a high level throughout the grazing season under good management practices.

Grass quality is indicated by grass digestibility which governs dry matter intake and production performance of cattle.

High digestibility grass has high intake potential and high energy content.

High digestibility grass is characterised by high leaf content, low true stem content, high protein concentration, medium fibre concentration and short-med regrowth interval.

Digestibility is the major determinant of the energy content of grass.

The digestibility of grass exhibits a characteristic pattern of change during the year.

The highest values are obtained in spring (80 - 85%), lowest in mid-summer (78-80%) and somewhat higher again in autumn (79 - 81%).

When the quantity of grass is not limiting, the primary factor influencing intake by grazing animals relates to the digestibility of the grass available.

The changes in digestibility are associated with changes in the amount of green leaf, mature stem and dead material.

As the grass plant matures, the proportion of leaf to stem decreases and this is associated with a decrease in digestibility.

Green leaf is highly digestible and animals will select green leaf over stem.

Grass will be low in digestibility when it has a lot of stem, flower heads and dead material.

The problem with this type of grass is that the intake will be low because animals find it less palatable.

They find it difficult to graze and they digest it less i.e. they extract fewer nutrients from it.

A quick assessment of grass being offered to animals can be made by cutting a small representative sample of grass.

This can then be visually assessed for sward morphology:

•What colour is the sample? Bright green swards are an indicator of high digestibility, while pale green/ yellow swards are less digestible.

•What is the texture of the sample?

Green leaf has a soft texture which is easier for animals to consume and digest, while stem has a coarse texture which is more difficult for animals to consume and digest.

•What is the leaf to stem ratio? A manual separation of the sward morphological components can be used to give an indicator of grass quality with a well-managed grazed sward containing between 70-80 per cent green leaf, 15-20 per cent green pseudostem and less than 5 per cent mature stem and 5 per cent dead material.

Every 5 per cent increase in sward leaf content results in a 1 per cent increase in digestibility.

Well managed grazing swards which are grazed at the 2.5 to 3 leaf stage of growth.

This means that the third leaf on a tiller is emerging or fully emerged.

Grazing at this stage of growth is optimal as it maximizes green leaf content in the sward and avoids leaf senescence which would occur if sward is left to grow.

Grass quality can be maintained by grazing swards to a post grazing sward height of approx. 4cm throughout the summer period.