WEEDS love predictable farming systems and if farmers do the same thing at about the same time each year, the weeds learn to adapt.
Many graingrowers have said weeds such as barley grass, brome grass and wild oats once germinated evenly with the opening rains, and now germinate later, avoiding the knockdown and early season herbicides.
That has now been confirmed by some recent Australian Herbicide Resistance Initiative (AHRI) research led by Aniruddha Maity from Texas A&M University, United States, as well as Roberto Lujan-Rocha and others from AHRI.
Part of the cause of the extra dormancy is that more dormant weeds have bigger seeds.
According to the research, while that was true for barley grass and brome grass, there was a weaker correlation for wild oats.
AHRI content writer Peter Newman said there was also a correlation between herbicide resistance levels and seed dormancy.
"It's unlikely the resistance mechanism caused the dormancy and more likely that the longer a field has been in crop, the higher the resistance levels, and the dormancy has resulted from the farming season," Mr Newman said.
The remaining question left for AHRI to answer was whether predictable farming has played into the hands of the weeds and if growers can exploit the adaptation to their benefit.
In 2018, 108 populations of weeds were sampled just prior to harvest for the study, with their seeds germinated on agar periodically in the nine months following seed collection to test for dormancy levels.
Those weed populations were taken from either continuous cereal rotation (wheat and barley), diverse crop rotations (wheat, barley, canola and legumes) or pasture and crop rotation (multi-year pasture in rotation with cereal, canola and legumes).
These field populations were always compared to a ruderal population from an adjacent roadside, bushland or fenceline.
Mr Newman said the barley grass from six populations sampled from a continuous cereal rotation didn't germinate at all in the seven months following harvest, compared to the ruderal populations sampled from nearby roadsides.
"This was the biggest dormancy adaptation result seen in this study," he said.
"This continuous cereal rotation is more common in the lower rainfall regions of Western Australia and this result will come as no surprise to graingrowers in this region.
"Barley grass from the pasture and crop rotation were more dormant than the ruderal populations from nearby."
The diverse crop rotation had more dormancy than the pasture and crop rotation and both field populations had more dormancy than the nearby ruderal populations.
Wild oats from crop fields did have more dormancy than their ruderal counterparts, although this difference was only minor at seven months after sampling for the continuous crop and the pasture and crop rotation.
Wild oats from the diverse crop rotation were the most dormant.
"Barley grass from the pasture and crop rotation had 70 per cent greater seed size than ruderal populations and brome grass from crop fields had 26pc larger seed," Mr Newman said.
"There was a strong correlation between seed size and dormancy levels for these weeds where larger seeds were more dormant.
"Wild oats also showed a correlation but to a lesser extent and this phenomenon has also been observed for other weed species in international literature."
According to the study, weeds from the most intensive cropping systems were not only the most dormant, but they also had the highest levels of herbicide resistance.
No resistance was detected from the ruderal populations collected from fencelines and roadsides.
However, that doesn't mean herbicide resistance causes dormancy.
Mr Newman said it was likely high levels of resistance were a result of high cropping intensity.
"It is this same predictable cropping system that leads to weeds adapting to increase dormancy to delay weed emergence to avoid exposure to herbicides," he said.
"However, the paper did allude to studies where the resistance mechanism may cause changes to plant hormones, such as abscisic acid, which can have a direct effect on dormancy levels, but this is likely rare and not well understood at this time."
It has long been said that diversity is the answer to herbicide resistance and the study has shown growers may also need to use the same thinking to help manage other adaptations such as seed dormancy.
"Varying the timing of seeding for a given field will keep the weeds guessing," Mr Newman said.
"Perhaps practices at the end of the season such as swathing, crop topping and harvest weed seed control can exploit the fact that the weeds have germinated later and their seeds are more susceptible to the timing of control."
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