Everyone is eagerly awaiting this season’s break, and for farmers in the broomrape quarantine area the date of the break has extra significance. Recording the date of the first major rainfall event for the season and measuring Growing Degree Days (GDD) has proven a useful tool for determining optimal spray timing to prevent branched broomrape emergence.

Research work in the past two years has looked at relating the concept of GDD to the lifecycle of branched broomrape. GDD is commonly used in other industries such as horticulture, to determine development rates such as flowering time of plants, and also determine best timing for spray applications.

For branched broomrape development GDD are calculated using daily min max soil temperatures, and begin accumulation from a specific starting point. The starting point used in the past two seasons has been the date of the opening rain as this prompts the germination of host weeds.

Research results have consistently found broomrape attachments on host plants after about 500 GDD, and the first emerged broomrape plants emerged at 1500 GDD. At approximately 1000 GDD there have been few new broomrape attachments on host plants, therefore strategically spraying at around 1000 GDD should increase the chances of getting better control of broomrape before it emerges. It also helps take a lot of the guesswork out of determining the optimum timing for spray applications.

The success of the GDD prediction rests in determining the correct starting point. In 2007, researchers found that broomrape emerged earlier than 1500 GDD after the ‘break of the season’. Upon reflection it was found that a less significant earlier rainfall event had led to an earlier germination of host weeds, which left uncontrolled led to the earlier emergence of broomrape. A second wave of broomrape emergence was observed at 1500 GDD from the proper break of the season and this has helped validate the theory. 

Dry conditions leading into this season have done little to build soil moisture reserves, however it will make it easier to predict the true starting point for measuring 1000 GDD for optimal spray timing of broomrape paddocks. Keeping an eye on the start of autumn weed emergence will also be important.

Growing Degree Days will be discussed in further detail at Branched Broomrape Farm Plan Workshops, held for landholders in the broomrape quarantine area. These workshops will begin early April and run until mid May.

For more information, please contact the Branched Broomrape Control Centre ph 1800 245 704.

AUTHOR: Tanja Morgan, Sustainable Agriculture Systems Consultant, Rural Solutions SA
CONTACT: Branched Broomrape Control Centre ph 1800 245 704
 
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