Have you ever wondered how the spread of blackgrass has been so dramatic on this island? Some things are known for certain, others are possible or probable causes and others still are speculative but plausible.

This article looks at the possible causes in the hope that we can learn from this to help prevent some future spread of problem weeds.

To begin with, there must be some chance that we had an indigenous blackgrass population but we do not know this for sure, yet.

Research currently under way at Oak Park hopes to provide answers to this and other questions, including the origin of many of the populations currently found to be resistant to herbicides and possibly of British origin.

That said, many of the early problems seen in this country in the late 1970s and 1980s were associated with fields where seed had been imported from Britain at the time.

Many fields infested around this time continued to show infestation through the years despite continuous efforts to eliminate the problem.

In the early 1990s, one of my former colleagues from UCD took me through parts of Dublin county to show me blackgrass growing in roadway hedges. This was either indigenous or the seeds had blown from trucks carrying grain or straw.

Expansion of infestation

So, we had some level of blackgrass contamination in parts of the country for some time, but why have we seen the considerable expansion in infestation severity over the past decade?

In my many conversations with Stephen Moss from Rothamsted (the expert of the day) he always maintained that the greatest single driver of blackgrass was early autumn planting.

The plant prefers to germinate early and so the earlier we sow the more of it that grows.

This fact can also be used against it in terms of integrated pest management (IPM) and stale seedbeds but we did not act in time. As well as earlier sowing, there was also a bigger percentage of the overall crop area sown in winter in the past few decades.

We have also seen more minimum tillage establishment and now we also have herbicide resistance, which makes some populations very difficult to control.

Spread pathways

There are several infestation pathways that have received a lot of attention in recent years. These include machinery, compost, manures, birds, wild animals and seed.

Machinery spread comes directly from other infested fields, mainly through combines and balers, but seeds can also be carried on tractors and sprayers, as well as in soil and on people’s clothes.

Some of these transmission mechanisms carry greater potential relevance than others. But most are still relatively local mechanisms and seem unlikely to be the main reason for the spread of blackgrass across much bigger areas of the country over the past decade or two. In looking at the expanding footprint of the infested, area one must look more closely at seeds.

Could seed be the major cause?

There are many specific standards for certified seed and we are primarily bound by EU legislation.

However, the increase in problems with grass weeds in recent years has led to the introduction of higher voluntary standards and zero tolerance for the presence of a range of grasses including wild oats, sterile brome and blackgrass.

Blackgrass is a real scourge on tillage farms where it is the equivalent of TB for livestock producers.

Perhaps we can do little more at seed multiplication level for the cereals we multiply here and most traders are now unwilling to import cereal seeds that are only certified according to minimum EU standards.

But what about other seeds? One crop grouping that is often mentioned is catch crops.

Virtually all this seed is imported and the variation in seed sizes and shapes makes them difficult to clean up if grown on infested land.

This is a risk scenario for tillage land but is the problem even bigger than this?

Probably the most difficult crop to manage in this regard is grass seed and all the seed we use is now imported.

So, where does that come from? It seems likely that an amount of it is produced in Britain and most likely in arable areas there where blackgrass is endemic and highly herbicide-resistant.

If this is the case, then we must rightly ask how this is being managed and what standards it must meet if it is to be imported on to this island? In the past, British seed had to comply with the minimum EU standards but what is the situation now post-Brexit?

In many instances, the EU minimum standards are self-regulated, which does not inspire confidence.

A mechanism for spread

We reseed around 250,000 to 350,000 acres annually and the intensity of grassland management has probably increased reseeding rate, especially on the grazing platform.

There is also some grass in tillage rotations and if this is being sown with blackgrass-infested seeds, then it could be increasing the spread footprint of the problem.

Taking a 300,000ac or 121,000ha reseeding figure, if this was reseeded using a 14kg/ha seed rate, it would require around 1,700t of grass seeds. Much more than this is imported and there may be double this amount used in the amenity sector. What is the risk associated with importing infested seed?

Sowing blackgrass seeds

If we take it that the average combined size of diploid and tetraploid grass seeds is 3g/1,000 seeds, then we would have 333,333 seeds/kg.

If sowing at 14kg/ha we would plant 4.6m seeds per hectare or 467 seeds/m2. If a seed lot contained the maximum 0.3% of blackgrass, this would equate to over 100,000 seeds per hectare or over 10 seeds/m2. Even if only half of these established, they are spreading the seeds of disaster.

While the horse has partly bolted, this threat needs to be controlled by officialdom

Given the nature of land use on this island, blackgrass present in a grass sward is ultimately blackgrass in a tillage field. If this has been happening and it is the cause of the rapid recent escalation of the blackgrass problem, it is intolerable.

Tillage farmers would not be allowed to import animals that would endanger the health of the national bovine herd. Why should the actions of the livestock sector be allowed to threaten the viability of the tillage sector, even if it is not intentional?

While the horse has partly bolted, this threat needs to be controlled by officialdom.

Are imported grass seeds routinely tested for blackgrass contamination by either the Department of Agriculture or importers? These questions and challenges must be addressed.

  • Grass weeds like blackgrass can be spread in many ways but most spread is local.
  • Machines, birds, animals, manures, etc, can all contribute to the local spread of a problem.
  • Crops such as catch crops and even grass seed may well be involved in expanding the footprint of the infested area.