Wednesday, July 15, 2015

Guildford Grass control in pasture.

Onion or Guildford Grass (Romulea rosea).
A question we get from time to time is how to control Onion or Guildford Grass (Romulea rosea) in pasture:

Note: this is not the same as Onion weed or Nut Grass (rarely seen in pasture, mainly undervine).  

Onion grass is a perennial herb with distinctive growth behavior. It looks like a grass, grows like an annual, and can spread rapidly . The plant not only produces abundant seed, but also corms underground to survive the hot and dry summers in Mediterranean environments of temperate Australia.

Originated from South Africa.Generally found in areas with low competition from other perennial plants due to low fertility, overgrazing, fire or poor seasonal conditions. 

Causes:

The two main causes of onion grass infestation are autumn bare ground and lack of competition from desirable pasture species. Prevention involves managing pastures to maintain above 70 per cent ground cover and maximising growth during autumn and winter. 

Make sure you have a high proportion of desirable perennial species (grasses, legumes, herbs) in the pasture, maintain good soil fertility and use rotational grazing to maximise growth and persistence of perennials.


Onion grass does not respond to fertiliser as many other plants do. A glasshouse experiment revealed that the herbage yield of onion grass did not differ significantly when various rates of fertiliser were applied.
 

Non –herbicide control methods:


Onion grass is highly sensitive to close defoliation.  Cutting to one centimetre above ground at three to five week intervals reduced onion grass corm mass by 70 per cent, seed pod density by 100 per cent and plant density by 60 per cent compared with the non-defoliated control.

Cutting to five centimetres above ground also reduced onion grass corm mass by 58 per cent, seed pod density by 94 per cent and plant density by 35 per cent. Cutting at flowering only considerably reduced seed pod numbers (90 per cent), and corm mass to a lesser degree (27 per cent), but did not affect onion grass plant density.

These results suggest that more intensive grazing in the field has the potential to reduce the corms and seed produced by onion grass, and consequently the density of onion grass. Fertiliser application may not decrease onion grass density directly but could have an impact on onion grass through competition of other species that respond favourably to fertiliser in autumn and winter.
 

Herbicide control methods:


Onion grass can be controlled in established pastures using an appropriately registered metsulfuron-methyl herbicide(Aim, Ally , Lynx, Brushoff).

It is important that spraying is conducted at the point that the old corm is exhausted and the new corm is developing approximately six to eight weeks after onion grass has emerged. This will permit enough chemical to be absorbed by the new corm to kill it. 

Onion grass is very susceptible  to fungus spot and rust on the leaves in July and August and this is therefore not a good time to apply foliar herbicide to it.

Spraying at flowering can get rid of flowers and seeds but not corms.

Risk to clover: Note that products containing metsulfuron-methyl are likely to kill clover species for the remainder of the season after spraying and may effect the clover population in subsequent years. Use of a wick-wiper or rotary wiper for application of this chemical to onion grass can help to protect pasture in some situations. 

Risk to native vegetation: Note that if applied inappropriately, herbicides, particularly metsulfuron-methyl may affect beneficial native plants other than native grasses.
 

Management in crops and new pastures:


Sow a crop or annual grass pasture the year prior to establishing a perennial.

Spray with an appropriately registered knockdown herbicide to kill all pasture species in autumn.
Sow crop or annual grass with required fertiliser.

If more onion grass emerges after sowing, spray six weeks after the onion grass germinates with an appropriately registered metsulfuron-methyl herbicide.

Note that controlling onion grass prior to spring sowing is often difficult. Products containing metsulfuron-methyl may have an applicable plant-back period usually ruling out spring pasture establishment after spraying in winter. Spraying with a knockdown herbicide in spring to remove the onion grass prior to sowing may not prevent the onion grass from germinating the following year.

For more information please contact Ben South.