Developing an early flowering ARGT-resistant ryegrass

Summary

To control annual ryegrass toxicity (ARGT), livestock producers have generally had to reduce the ryegrass content in their pastures to a low level. Guard, the first ARGT nematode resistant ryegrass, developed from Wimmera ryegrass, has given livestock producers in the medium to high rainfall cropping regions the option to maintain rye grass in the pastures. Most outbreaks of ARGT now occur in the low rainfall regions, where Guard is not adapted. An early flowering ARGT nematode resistant cultivar would provide stockowners in districts with short growing seasons, the option to retain ryegrass as a significant component in their pastures.

A high frequency of the ARGT nematode resistance genes in ryegrass pastures can provide effective biological control of the nematode Anguina funesta, thus preventing the toxin producing bacterium Clavibacter toxicus reaching hazardous levels in the pasture. To improve the agronomic performance of the new cultivar, Guard, the first ARGT resistant cultivar released in 1994, was crossed with Progrow a Westerwold ryegrass. Selected plants from this cross were then crossed with an early flowering ecotype from Western Australia (W A) to shorten the time to maturity. Later flowering lines were kept as possible replacements for Guard. To minimise adverse affects on cereals grown in rotation with the pasture, the new cultivar was screened to ensure it was resistant to cereal cyst nematode and take-all, and susceptible to grass selective herbicides. utcome A new early flowering ARGT nematode resistant cultivar has been developed.

Early observations indicate it has excellent seed yield, which reduces the cost of production per unit yield. It also appears to have vigorous, leafy growth, characteristics inherited from Progrow. Confirmation of its agronomic performance will have to wait till there is sufficient seed for comparative studies. Three other lines are being evaluated, two may be combined as a potential replacement for Guard if they have better early vigour and / or higher dry matter yields, the other line may be useful in high rainfall pastoral areas. All potential cultivars have been screened to confirm they are resistant to CCN and take-all and susceptible to grass selective herbicides. Resistance to the nematode has potential to achieve long term control of ARGT.

A field trial established at Turretfield SA, to study persistence of nematode resistant plants in different mixtures of Guard and the local ryegrass found the proportion remained unchanged for four years. A mixture of 3: 1 (Guard to local ryegrass) produced a ryegrass population that reduced nematode multiplication by 90% relative to the local ecotype in each of the 4 years the trial was established. Commercialisation Valley Seeds is our commercial partner for the ARGT nematode resistant ryegrass cultivars. By 2000, they should have sufficient seed for a limited commercial release of the new early flowering cultivar. Seed multiplication of possible later flowering cultivars is less advanced, so commercial release of these is not expected until at least 2001.

Potential Beneficiaries Graziers in districts with short to medium growing seasons who want a productive grass in their pastures to increase stocking rates and minimise the risk of ARGT will benefit from the new early flowering cultivar. Graziers in the medium to high rainfall regions will have the option to use a later maturing ARGT nematode resistant cultivar. Hay producers may benefit by using the cultivars to reduce reservoirs of the nematode and bacterium in non arable areas. Infection in these areas can spread to infest hay crops making it potentially unsaleable. Use of these cultivars will potentially provide long term suppression of the nematode and bacteria associated with ARGT.

Reducing reservoirs of infection will benefit all fanners operating in the district as this will reduce the rate of spread into uninfected paddocks. Benefits It is difficult to quantify the benefits of the new cultivar(s) until they have been evaluated in the field to determine dry matter yields in a range of regions. Use of ARGT nematode resistant cultivars could provide long term, non chemical control of ARGT. This could be important with herbicide resistance becoming more common. Unlike classic predator / prey biological control systems, the level of resistance in the ryegrass population is not dependent on maintaining a significant population of the target organism.

The main factor determining the proportion of nematode resistant plants is the amount of background ryegrass present in the paddock when the new cultivar is sown. To be sure the resistance to the nematode persists the new cultivar will need to be managed to flower at the same time as the locally adapted ryegrass.