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Australian Herbicide Resistance Initiative (AHRI)

Pollen flow of resistance genes

Grass

“It wasn’t me, it was him!”

Many grain growers blame their neighbours for developing herbicide resistance. Growers often feel helpless! Regardless of how well you manage your weeds, unfortunately it is inevitable that you will inherit the resistance problem from your neighbour.

It is true that resistance genes can flow a long way in pollen. AHRI researcher Dr Roberto Busi demonstrated that ryegrass pollen can travel up to three kilometres. More research in theUShas shown that gene flow via pollen varies between species, depending on the level of out crossing and pollen characteristics. It is no surprise that the resistance world champion weeds, annual ryegrass and Palmer amaranth (pigweed), have the ability to spread their genes far and wide in pollen.

In many cases, it is true that it is almost impossible to prevent resistance evolution due to this easy transfer of resistance genes. For this reason, grain growers should focus on managing the seed bank of resistant weeds and spend less time worrying about where the resistance came from.

Annual ryegrass

In this study conducted by AHRI’s Dr Roberto Busi and others (September 2005), 80 large VLR1 plants were placed into the undisturbed bushland established to be free of Lolium Rigidum (annual ryegrass). Individual susceptible plants were placed at varying distances (0–3,000 m) from the wheat and pasture fields known to be infested with herbicide resistant annual ryegrass (Table 1).

Table showing results of study conducted by AHRI’s Dr Roberto Busi and others (September 2005)

Table 1. Total number of plants recovered/introduced, seeds recovered, seeds tested for herbicide resistance (R) to sulfometuron (ALS herbicide), sulfometuron R seedlings and R frequency to sulfometuron (R seedlings/seeds tested) at each distance established in the field experiment.

The bad news is that resistant ryegrass genes travelled up to three kilometres. This pollen was able to produce viable ryegrass seed that was resistant to ALS herbicides.

The good news is that the further away the ryegrass plants were from the pollinating plant, the fewer seeds it produced. This study was done in a situation where there were low levels of pollen competition. Therefore, in a normal field situation, there is likely to be lower levels of resistance gene transfer from long distances as the small number of pollen grains that travel a long distance are likely to be in high competition with pollen from nearby plants in the same field.

Species variation

University of Arkansas researchers noted that if the weed is a highly self-pollinating species (such as barnyard grass), the level of resistance spread would be much lower compared to a highly out crossing species (such as pigweed). For example, in research conducted in Fayetteville, Arkansas, approximately 3% out crossing was observed in barnyard grass plants present within a few feet and was very rare (<0.01%) at a distance of 150 feet (Bagavathiannan et al. 2012). Furthermore, the University of Adelaide has reported out crossing of up to 6% in glyphosate resistant barnyard grass from Queensland. In contrast, a study conducted in Georgia quantified as much as 20% to 40% outcrossing among pigweed plants that were up to about 1,000 feet apart from each other (Sosnoskie et al. 2012).

The distance to which outcrossing can occur is influenced by pollen grain size, pollen load (availability), the means of pollen movement (through wind, insects, etc.) and the period of pollen viability, which is affected by inherent pollen characteristics and environmental conditions (air temperature and humidity). For a wind pollinated species, frequent high velocity winds can move pollen to fairly long distances in the direction of the wind, whereas pollen mediated transfer of herbicide resistance can occur even at farther distances in weed species that are pollinated by insects such as bees. Wind dispersal of pollen grains will also be greater in a weed that grows taller than crop canopy. The level of out crossing can be spatially variable, but growers should realise that all that is needed for resistance build up is a single resistant seed that can survive herbicide applications and reproduce.

Summary

Resistance genes can travel a long way in pollen. Therefore, it is true that a grower who is using responsible weed management to minimise resistance evolution is at risk of inheriting un-wanted resistance genes from neighbours with a less rigorous system. For this reason, it is best to simply focus on good weed management, regardless of your resistance status. With herbicide resistance now so widespread in many countries, it is time to accept the fact that resistance will spread from farm to farm, and get on with the business of running down the seed bank of the resistant weeds.

After all, resistance is not a problem if you don’t have any weeds!

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