Plant characteristics

The main difference between diploid and tetraploid ryegrass is the number of chromosomes per cell. Diploid plants have two sets of chromosomes per cell whilst tetraploids have four. Tetraploids have an increased cell size due to this and have a higher ratio of cell contents (soluble carbohydrates) to cell wall (fibre), indicating that they have a higher water content per cell. Diploid plants have more tillers per plant and due to the lower water content per cell have a higher dry matter per kg of feed and also more energy than tetraploid plants. Diploid and tetraploid plants have similar protein levels.

Animal performance benefits

There are several benefits in terms of animal performance, to the higher ratio of cell contents to cell wall in the tetraploid plant. One is that it is more palatable to animals promoting improve intake and increased animal production. In addition tetraploid cell contents consist of rapidly available nutrients, such as sugars and starches necessary for more efficient rumen function. However it is important to remember that tetraploids have a higher water content in the cells so when livestock are full from grazing the actual dry matter intake could be lower compared to that of diploids plants. A mix of tetraploid and diploid ryegrass will achieve a more balanced pasture retaining a good level of dry matter production, an improved balance of quality feed and if used for silage will be more easily cured than a pure tetraploid pasture.

Grazing management

It is important to monitor grazing pressures on tetraploid ryegrasses as they are sensitive to overgrazing because of their excellent palatability. If grazing occurs to a lower residual than with diploid ryegrasses it may compromise persistence. Similar to diploid ryegrasses, tetraploid ryegrass require nitrogen applications to a similar level to ensure good growth after grazing. Tetraploids have been artificially created by plant breeders using the "colchicine' treatment.

Farm management benefits

Tetraploid ryegrasses have higher establishment costs than diploids as the recommended sowing rate for tetraploids is much higher due to the larger size of the seed (2-3 times heavier) (Table 1) (Reed, 2000). If planting tetraploid ryegrass in a mix, the plants will allow about 10% more clover in the pasture due to the reduced tiller density promoting an increased pasture quality.

Table 1: Establishment cost for Diploid and Tetraploid ryegrasses.

	Diploid	Tetraploid
Seed per kg	500,000	255,000
Recommended sowing rate	15 - 25 kg/ha	25 - 35 kg/ha
Number of seeds per ha sown at 25 kg/ha, based on 70% effective establishment	8,750,000 seeds / ha	4,462,500 seeds per ha
Cost	$60 - $100 / ha	$87 - $122 / ha

Source: Irwin Hunter