Bristly Dairy Farm Dilemma
Yellow bristle grass is an increasing problem on dairy farms. Image: K.Tozer
Dairy farms around the North Island are struggling to keep pastures clean as another weed, yellow bristle brass (Setaria pumila), is making its presence felt. Yellow bristle grass (YBG) is one of eight Setaria species found in New Zealand, and while not all of them are considered weeds, S. pumila has become a big problem, particularly in Taranaki, Waikato, South Auckland and the Bay of Plenty. Recent data suggests a possible hybrid origin for many plants commonly referred to as YBG, and it appears that this form, which may have recently arisen in New Zealand, is extremely weedy.
Originally from southern Asia, YBG has spread throughout Europe, Africa, North America and Australia. It was most likely brought accidentally to New Zealand as a contaminant in grass seed. As the name suggests, YBG has a bristly seed head which attaches easily to the hair of animals and can be moved between farms in feed such as hay. The loss of grass production on dairy farms due to the presence of YBG has been estimated at around 20%, which lowers farm productivity considerably. The cost of buying in supplementary feed to keep milk production at an acceptable level has become a big burden for some dairy farmers, and in some cases has resulted in more YBG seed arriving on the property.
Although palatable to stock during the spring, cattle won’t graze YBG once it starts forming a seed head (January – May). Outside of this period heavy grazing can reduce the prevalence of the plant, but the flip side of this is that pugging of the ground can enhance YBG seed germination. Instead, farmers are mostly relying on costly chemical control, which has the added burden of a withholding period of 28 days where stock cannot graze the pasture. Manual removal is used for small infestations, but careful pasture management is required, such as topping paddocks prior to seed set and harrowing paddocks to reduce seed germination from dung pats. Despite best efforts, the grass is continuing to spread.
So what are the biocontrol options for this plant? “A collaborative effort between AgResearch and Landcare Research has recently produced a feasibility study to try to answer this question,” said Trevor James from AgResearch, who led the study. One goal was to determine the genetic variation in populations of YBG in New Zealand. To achieve this, we collected YBG samples from 20 different sites (mainly from the North Island and the top of the South Island) and knot root bristle grass (S. gracilis) from one site. Molecular biologist Gary Houliston was surprised with the results from the initial molecular work. “We sequenced phylogenetic regions from both the nucleus and chloroplast and were surprised at what we found. While the nuclear region was a very good match for S. pumila, the chloroplast result appeared more like S. sphacelata,” said Gary. S. sphacelata was available for comparison from an agricultural trial of the species near Kaikohe. This species of grass was also planted on Whatawhata Research Station in the western Waikato region, and the seed was sourced from Australia, Kenya and South Africa. Suspiciously, the invasive type of YBG is common in the regions where S. sphacelata has been trialled, adding weight to the hypothesis that the reason for YBG suddenly becoming problematic after many years of being present in New Zealand is the formation of an aggressive hybrid. However, further studies would be needed to be sure that the invasive type is a local hybrid rather than an introduced genotype.
“We also found that morphological features that have been used to identify YBG and related species do not line up well with genetic variability detected and therefore cannot be reliably used for identification purposes,” warned Gary. “We also don’t know how widespread the weedy form of YBG is yet,” added Trevor. Further molecular work is needed to compare material found in New Zealand with material from overseas in order to attempt to narrow down where the plants originated from, and indicate where best to search for potential biocontrol agents.
A literature search has suggested that it might be better to go down the track of looking for pathogens to control this weed rather than insects. Few insects are known to feed on YBG, none appear to be suitable for biocontrol purposes, and pathogens appear to offer more options. However, because of the taxonomic uncertainties around Setaria and the issues identified in using morphological features to assign names in this group, it is possible that literature referring to YBG, or related species, may be inaccurate, including the host range of pathogens reported. If similar genetic forms can be found overseas it would enable those populations to be surveyed for possible biocontrol agents. If they cannot be found, the next best option would be to survey the parent plants or the most similar material that can be identified. “Any future work will also need to carefully consider which Setaria species should be targeted for biocontrol in New Zealand, because there is a danger that focusing on just the weedy hybrid YBG form could result in it just being replaced by other Setaria entities,” concluded Trevor.
This study was funded by the Ministry for Primary Industries Sustainable Farming Fund.