No ‘ears’ (auricles) at the junction of the leaf blade and sheath
Seed head is a cylindrical ‘spike’, 2.5 – 10 cm long, with many densely packed spikelets. Each spikelet is surrounded by five to ten bristles, 5-8 mm long which are green initially but later turn golden-brown
Most of other species of Setaria have fewer bristles in their seed heads.
Biology
Germination typically starts in mid-October and peaks in mid-November depending on weather. Germination can begin at 16°C but optimal temperatures are between 20 and 30°C
Early seed heads appear in late December but mostly in January and February
Mature plants and empty seed heads will persist until the first frost. YBG reproduces only by seed
Seeds are dispersed by water (seeds are hard-coated and most float on water), soil movement, animals, and as contaminants of hay and maize. The barbed seeds stick to and are often carried in fur, feathers, or clothing
Seeds are usually dormant at maturity and require about 3 months of after-ripening before they can germinate
Most seeds survive only a few years under field conditions although, some deeply buried seed may survive for up to 10 years
Seedlings can emerge from soil depths up to 10 cm but optimal germination is from 1 – 2 cm depth
Counts have shown seed numbers in the soil up to 20,000/m2 but typically 5 – 10,000/m2 under light infestations.
Origin
There are eight species of Setaria in New Zealand, most of which are annuals. Overseas they are commonly called foxtails or millets
Yellow bristle grass (YBG) originally came from southern Asia where it is…“A relatively good natural grazing species, although leaf production is moderately low. It is a serious weed in some areas, especially since it only germinates late in the season once most control measures have already been applied. In some areas this grass plays an important role in stabilising bare soil to protect it from erosion” (- see references under ‘Further Information’)
YBG has now spread throughout Europe, Africa, North America and Australia
Although common on roadsides, YBG has ‘jumped the fence’ in the last 10 years and become a big problem in many farming regions throughout New Zealand.
Life-cycle
YBG occurs in areas with adequate summer rainfall, usually over 500 mm per year, although it can tolerate dry conditions once established
Grows in areas where the soil has been disturbed and especially where water collects, including cultivated areas, old pastures, along footpaths and the side of roads
Has a C4 photosynthetic pathway meaning that it grows best at higher temperatures and is frost tender
Fills a similar environmental niche as other C4 summer annual grasses such as crowfoot grass, summer grass and smooth witchgrass
However, due to the size and number of the seed produced it is more competitive than other species
Moderate to slow-growing, especially if the weather remains cool, and generally will not establish and compete in vigorous ryegrass/white clover swards
Not toxic to stock but they may avoid grazing the seed head. When grazed, YBG seed survives passage through the rumen and can be found in dung, where it establishes in any gaps
Readily invades run-out or damaged pastures that have been opened up by pugging, over-grazing or the death of flat weeds or winter annuals such as annual poa.
Impacts
Yellow bristle grass (YBG) does not provide good feed in late summer/autumn resulting in lower pasture quality. This can reduce milk production from YBG infested dairy pastures
Selective grazing can lead to lower pasture utilisation
YBG competition weakens ryegrass plants
YBG death opens pastures for establishment of winter weeds.
Control
Chemical control
Fenoxaprop (Puma S) is the first selective herbicide to be registered for the control of yellow bristle grass (YBG) in pastures
Fenoxaprop ) has a withholding period of 28 days, during which pastures must not be grazed
Fenoxaprop should be applied 7 days after grazing to ensure that there is sufficient YBG regrowth for the herbicide to be effective
Most effective on young seedlings – it will not kill YBG seeds once seed heads have matured.
Management on roadsides
Spray with glyphosate or fenoxaprop – timing is important, ideally spray before the 1st seed head emerges (late December to January depending on season and weather)
Try and create a thatch of dead plants to stop further YBG germination
Residual herbicides can prevent germination of YBG
Mowing won’t kill YBG but seed will be produced very low down and is therefore less likely to spread
Manual removal is an option for small amounts
Roadsides containing YBG should NOT be grazed between December and May
Don’t make infested grass into hay because the seeds can survive. Wrapped balage is suitable, provided it’s left for at least three months.
Management of small infestations
Use glyphosate or fenoxaprop to spray out the infestation but note that other weeds will quickly fill the empty space
Glyphosate can be used at all stages of growth but glyphosate or fenoxaprop will not kill the YBG seed if the bristles have already turned yellow. Fenoxaprop is best applied when the plants are at seedling stage
Manual removal of plants before seeding is a very good option if it is possible
Areas with YBG can be fenced off to stop seed being spread further afield
Don’t graze when the seed head is visible – this is too late – seeds can spread and germinate in dung
Be vigilant as YBG will germinate throughout the summer as conditions (rainfall) allow.
Management of large infestations
If using herbicide, note that more than one application may be required – most YBG germinates in late spring / early summer but some YBG will germinate much later in the season
Topping will improve utilisation and possibly reduce the quantity of YBG seed set
Avoid grazing a clean paddock after stock have been exposed to YBG seed
Use chain harrows to spread dung and avoid creating conditions which favour YBG establishment.
Management by pasture renewal
There must be two summers out of YBG seed production before new pasture is sown and during that time YBG must not be allowed to set seed!
Use crops in which YBG can be readily controlled e.g.
maize (use Latro as a post-emergence spray)
chicory (use Sequence as required)
turnips (use Sequence prior to crop canopy closure)
lucerne or other legume (use Sequence as required)
for annual summer crops plant annual ryegrass or winter cereal in between.
Look after new grass and keep all weeds out
Otherwise use normal pasture renewal principles (e.g. treated seed and appropriate endophyte, etc.) refer to Dairy NZ’s Pasture Renewal Guide.
Grazing management
Avoid overgrazing and opening of pastures to weeds
Overgrazing in summer can also expose the ryegrass crown to direct radiation and may kill the plants
Avoid moving stock from an infested to a clean paddock
In dry years supplementary feed might be needed early to prevent overgrazing
YBG will not be controlled by grazing as mature seed heads are produced within 21 days of grazing
Reduction of stocking rate may be required to achieve some of the above.
Further information
Champion P, James T, Popay I, Ford K 2012. An illustrated guide to common grasses, sedges and rushes of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 208 p.
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