Yellow bristle grass has now spread along roadsides throughout most of the North Island and has become a major problem on farms.
Yellow bristle grass is an upright annual growing 25-45 cm high
- although in open pasture its first leaves are typically parallel to the ground.
- The leaves are yellowgreen to green in colour and usually red or purple at the base.
- They are flat, hairless, soft and twisted.
- The leaf sheath is flattened.
- There are no ears (auricles) at the junction of the leaf blade and sheath.
- The ligule consists of a fringe of hairs 0.5-1.5 mm long.
- The seed head is a cylindrical 'spike', 2.5-10 cm long.
- It consists of many densely packed spikelets, with each spikelet bearing a single seed.
- At the base of each spikelet are five to ten bristles, 5-8 mm long.
- Initially the bristles are green, but soon change to a golden-brown.
- It is the colour of these bristles that give the grass its name.
- Most other Setaria species have fewer bristles in their seed heads.
- Incursions often occur from roadsides or around maize silage stacks
As a summer growing annual, yellow bristle grass reproduces only by seed. Seeds are dispersed by water, soil movement, animals, machinery, and as contaminants of crop seed and hay. The barbed seed heads are often carried in fur, feathers, or clothing. Seeds are hard-coated and most float on water.
Germination requirements are variable, depending on several factors, including environmental conditions. Germination can begin at 16°C, but optimal temperatures for germination are typically between 20 and 35°C. Germination typically starts in mid October and peaks from mid November to mid December depending on conditions. 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 seeds are large (about 4 × heavier than summer grass seeds) and seed heads normally contain about 90 seeds. A single plant can have up to 60 seed heads. Seeds are usually dormant at maturity and require about three 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 or more. Seedlings can emerge from soil depths of up to 10 cm, but optimal germination is at 1-2 cm depth. Counts have shown seed numbers up to 20,000/m2 but typically 5- 10,000/m2 under light infestations
YBG occurs in areas with adequate summer rainfall, usually where the annual rainfall exceeds 500 mm per annum, although it can tolerate dry conditions once established. It grows in areas where the soil has been disturbed, including cultivated areas, old pastures and along footpaths and the side of roads, especially where water collects. YBG has a C4 photosynthetic pathway so it grows best at higher temperatures and is frost tender. It is not toxic to stock but they may avoid grazing the seed head. YBG fills a similar environmental niche to 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 the other species. It is moderate to slow-growing, especially if the weather remains cool, and generally will not establish and compete in vigorous ryegrass/white clover swards. However, YBG seed can survive passage through the rumen and establishes in any gaps, especially those caused by dung. It also readily invades run-out or damaged pastures that have been opened up due to pugging, overgrazing or the death of flat weeds or winter annuals such as annual poa.