Management of aggressive weed informed by updated guide
The second edition of the Ute Guide to help farmers manage infestations of yellow bristle grass was published in June 2011.
The aggressive spread of the weed is a significant threat to dairy pastures. While it has been present in New Zealand for over 100 years, it is only over the last 10 years that the grass has started to rapidly infest clean dairy pasture.
“This is still an emerging problem,” says AgResearch weed scientist Dr Trevor James. “There is a real danger that if it is not controlled it could emerge as one of the major problems for dairy farmers within the next 20 years.”
Yellow bristle grass was identified as a significant problem on farms in the wet, warm climate around Mt Pirongia in the Waikato some 10 years ago. It spread quickly from there and is now found across the Waikato and South Auckland and is rapidly invading the dairy farms of Taranaki.
“The problem with yellow bristle grass is its low nutritional quality when it is in seed during summer and autumn and its aggressive spread,” says Dr James. “Cows learn to avoid it when it’s in seed in summer and autumn; this results in a massive seed set. Seeds are spread by water and soil movement, stock, summer crops including chopped maize for silage, and contractors mowing around roadside marker posts.”
Germination normally starts in mid-October and peaks from mid-November to mid-December depending on conditions. Seeds can number up to 20,000 per square metre but more typically are 5-10,000 per square metre under light infestations. Due to the size and number of the seeds produced, yellow bristle grass is more competitive than many other weed species.
Yellow bristle grass can infest up to 10-12% of pasture which means a potential loss of 20% of pasture yield when the zone around the plant, which cows will not eat, is included.
The plant starts seeding in late December and it continues to seed through until the first frost in late autumn.
Dr James has been working with the Yellow Bristle Grass Action Group since its formation seven years ago and published the first edition of Yellow Bristle Grass – the Ute Guide in June 2007.
“Our focus was to make farmers aware of the threat and to provide them with management guidance to prevent it becoming established. These management practices are still the front line of defence.”
Dr James says the reason for the recent aggressive spread of the weed is unknown but droughts and the resulting overgrazing of pastures may have contributed.
In the summer of 2012 AgResearch conducted trials with a proprietary chemical currently used to control grass weeds in cereal crops in the South Island.
“The chemical does not affect pasture and the trials were positive,” says Dr James. “However chemical controls are best used as a strategic tool rather than a total solution.”
He says that a great deal of support is being provided by DairyNZ and its consulting officers to help farmers tackle the problem.
“We've been holding about six on-farm field days a year with anything from 50 to 150 farmers attending each one,” said Dr James. “The consulting officers have been really proactive in helping us to get the message across.”
He is also working with the Foundation for Arable Research and large-scale maize growers to help develop a set of guidelines to reduce the spread of yellow bristle grass seeds in maize and maize silage.
In the coming year research will continue into chemical controls and also to identify how the seeds are spreading.
According to the Yellow Bristle Grass Action Group “with this information [in the Ute Guide] farmers now feel confident that they can manage this problematic weed”, the cost of which is estimated to exceed $4.2 million per year in lost dairy production, just in the central Waikato triangle between Hamilton, Pirongia and Te Awamutu. The New Zealand-wide cost of yellow bristle grass is likely to be much higher.
The total project investment from 2007 to 2011 was $423,555 by the Sustainable Farming Fund, DairyNZ and other investors.