In recent years, we have become increasingly conscious of resistance to the products we use and we are becoming more conscious of the need to protect them. That holds whether the target is a weed, disease or insect and the challenge has become increasingly real. Everyone looking in suggests that integrated pest management (IPM) is the way to go, but what does that entail in the case of weeds?

Are there really any IPM tools when it comes to weeds? In the case of diseases we talk about variety, sowing date and rotation. Variety doesn’t apply in the case of weeds, but then perhaps it does. Sowing date doesn’t apply either, or does it? Actually, all things that can be considered as IPM in general husbandry can also be considered as IPM for weeds, and more.

Herbicide resistance

As is the case with diseases, herbicide resistance is more likely to develop in big populations because chance random events are a numbers game. The more there are, the greater the chance of reproduction throwing up something that is no longer controlled by a pesticide agent. So, anything and everything that one can do to get and keep weed numbers down is useful in the fight against resistance development.

For weeds to be a problem in a field, the seeds must be in the ground in that field to begin with. But, of course, they must come from somewhere to begin with. Weed seeds can be present in the seed you are sowing but numbers here would be very small and barely noticeable in a field where that weed had not already been present.

Some weeds may have begun from this source years ago but now those few initial weed plants that were ignored or unseen have since become a serious annual problem and cost.

Sources of infestation

Weed seeds can also come into fields with other inputs such as fertiliser but this is highly unlikely. What is more likely is that they could come in via organic manures from other farms. This is why I would always encourage growers to only take farmyard manure from farms where they were the sole supplier of straw to that farm in that season.

In recent years, we have heard people complain about spent mushroom compost as a source of unwanted weed seeds, specifically in conjunction with blackgrass. It goes without saying that any material that had, or may have had, seeds of an unwanted species present represents an unacceptable risk.

I say this because researchers indicate that proper composting will generate heat levels that can kill any seeds during composting.

That may be OK in theory but any self-respecting tillage farmer should never allow seeds of an unwanted species on to the farm. And this applies even if they are supposed to be dead, because one never knows. So spent mushroom compost, especially if derived from imported UK straw, is something that, unfortunately, clean fields should be wary of.

Unwanted weed seeds can also be brought on to clean land by birds, by wild animals or by humans themselves. How often do we visit areas with problems to return home indifferent to any level of biosecurity? How often have Irish growers gone to events such as Cereals in the UK and been indifferent to the possibility of seeds being caught in their clothes or shoes?

Physical transfer remains one of the most likely routes for both infestation and resistance transfer in weeds.

Then there is what may be the biggest single cause of transfer – machinery. Discussion on this topic generally drifts to the combine and the baler, especially if either was recently imported. These are huge risks for sure, but they are not the only causes.

Seed already present in the soil survives in the clay and this regularly moves from field to field on tractor tyres and on machines. It is always only a few seeds initially but if these are not controlled, then a small infestation can quickly become a big problem.

And it is not just the machines or tyres that can cause a problem. Cultivators, drills, rollers and even fertiliser spreaders and sprayers can take soil from field to field and, in so doing, also take a new weed species or a resistant weed.

It is increasingly recognised that physical movement is a major cause of resistance spread, so significant biosecurity is needed once any level of resistance appears in an area.

Physical controls

There are many possible sources of infestation but it is how you deal with an initial problem that matters. The most obvious response here is to rogue. It is unquestionably the best thing to do as it is the most effective way to prevent seed return. But once a problem gets beyond two to three plants per square metre on average, it quickly becomes too big for quality rogueing. And to rogue it, you must know it is there, so the person walking the crop or on the sprayer needs sharp eyes.

The particular weed present is also a factor here as some weeds are easier to rogue than others. Wild oats versus sterile brome is an immediate example. But the biggest obstacle here is that most people are just not prepared to rogue. Too few people are willing to go up tramline after tramline to pull and remove weeds such as wild oats, canary grass, sterile and meadow brome or even blackgrass.

Remember, physical control is about two things, reducing seed return and growing out the seed bank. While the prevention of seed return should be the only way to look at a situation where there are a few strange plants here and there, growing out the seed bank is an important option where populations have gone well beyond rogueing.

In the past, it was common for people to watch a problem build until it justified spraying and then spray annually. Now, with the realisation that these problems are perennial and with resistance popping up around the place, this is no longer an option for any weed.

Farm example

I will give one farm experience in this regard. Somewhere in the 1980s, I was asked to advise on a farm that was coming back from years in conacre which was badly infested with wild oats. The technology at the time was Commando. This was a good spray to kill the main stems of wild oats but, more often than not, this just encouraged tillers to thrive resulting in a second surge of seed heads.

So the wheat was sprayed with Commando at the recommended timing. It was sprayed a second time a few weeks later, post crop flowering, when the surviving tillers had headed out. This was common practice at the time to help reduce seed survival and seed set. Then the crop was weed-licked using a 12m weed licker and, lastly, it was sprayed with Roundup pre-harvest. This was obviously unsustainable and the pressure was huge.

That same autumn, I persuaded the grower to invest in a stubble cultivator. Having cultivated after straw removal a volunteer wild oat population of between 600 and 800 wild oats plants per square metre was ploughed down. For many years that followed, the continuous wheat was sprayed for wild oats with the chemistry of the day with a high level of success. Cultivation continued every year and, in 2005, oats was introduced as one crop in a new rotation with a level of trepidation.

It was planted late (early November) to minimise any wild oat infestation risk. In the following May/June, it was comforting to see that the crop only had about one wild oat plant per 100m2 or more. A few years later, all specific wild oat spraying was discontinued except on broadleaved crops where volunteer cereals were also a problem.

The original cultivator was a Kongskilde vibroflex but, in later years, this was changed for a Lemkin disc. It gave a lot more control over cultivation depth and it also provided more establishment options. It is also important to state that the cultivations helped to control other weeds such as sterile brome and cleavers which were also present. It had been used in a way that minimised slug numbers over time – that was another cost that had been removed completely even though winter rape was one of the crops grown.

Ploughing

It is generally acknowledged that ploughing is one of the best forms of IPM for weeds. This comment is both correct and incorrect for good reasons. Ploughing and burying weed seeds will certainly reduce the number of weeds that appear in the following year. However, that is not always the full story because seeds that remain viable can be turned back up again in the following season.

For weeds, a plough-based system is a bit like the classical sand-based egg timer – when the sand has run through, just turn it over and start again.

In terms of weed control and the use of a plough as an IPM tool, it would be far more effective to leave one or two years between the use of a plough and opt for a stale seedbed with min-till establishment in between. This does not have to be on a whole farm basis as it can fit best in a rotation.

Regardless of how one establishes oilseed rape, it should never be necessary to plough for the following crop as the rape has already done the job of opening up the soil. The same might be said for beans. So these are two opportunities that one should take to not plough to enhance weed control.

The reason for not ploughing is that seeds undergo a natural degradation process in the soil. There is a natural loss of a proportion of seeds every year where they do not have the opportunity to grow. So the longer they are buried, the fewer that will be viable when they are turned up again. This is why there are always fewer arable weeds following good grass leys.

Natural seed degradation

I am not exactly sure why seeds “die” in the soil, but it may be safe to assume that a proportion of the seeds buried deeply will germinate but not emerge from depth. Also, the dormancy which prevents germination will run out at different times in individual seeds, allowing them to germinate when buried. It seems probable that some may rot because seed coats will have been damaged. Others may be taken by birds or soil animals. And, then, some will survive for a long time but, generally, the numbers in that category are low.

Let’s compare two individual grasses. We are generally told that the annual loss of viability in something such as sterile brome tends to be in the 88% to 92% bracket. So good ploughing that will bury the seeds is very effective. Indeed, the reason this weed was named “sterile” was that so few seeds normally reappeared in the season following ploughing – it was thought to be largely sterile. However, this is well-proven not to be the case by anyone involved in min-till.

Even where ploughing is practised, one can often see patches of sterile in places where the sods are not fully turned over for burial. This is very evident at ins-and-outs, in stoney patches, close to obstacles and along by hedges where the outside furrow is often poorly turned.

Stay with sterile brome for a moment. If you physically move the location of the ins-and-outs for a few years you should get rid of brome in that area. Leave another one or two runs of the drill on the headland side. For the infestation by the hedge, could you plough the headlands into the hedge for two to three years to secure complete burial? Alternatively, leave an uncultivated strip by infested hedges or plant these areas with grass or pollinators.

Now, take a second species such as blackgrass. The natural seed loss here is roughly 64%. This is much lower than brome but we might ask why. Is it because it is a much smaller seed? Because it has a much tougher seed coat? Is it slower to break dormancy? Is it less prone to having its dormancy negated by light or some other trigger? Is there something about it that makes it less prone to rotting? Unravelling these questions would help us to secure more IPM options to help tackle the problem. While ploughing it down does have an effect, leaving it unploughed for one or two more years has a much stronger impact.

Healthy soil with lots of life

That said, I sometimes wonder if we can do something to the soil that would cause more weed seed deaths over time – let nature do more for us. Is a healthier soil full of biological life a place where fewer weed seeds survive? After all, there are always far fewer weeds when one ploughs up old ley. But do they rot or are they eaten?

If the general health of the soil is improved, it will contain more bugs and fauna. If you look back to the days of growing sugar beet, dressings of insecticide were frequently required to kill a range of creepy crawlies that would eat seeds or the plants. There seem to be fewer of those critters now.

If our soils contained more of those little fellows with muscular jaws that wriggled through an open soil, might they nip some of these weed seeds and break their seed coats to enable them to rot? I do not know the answer to this, but it seems plausible. A soil full of life needs feeding and it will generally feed on any organic material that is available.

We can do more to help

Many growers have not given the issue of reducing weed seed numbers in the soil much thought up to now. But they will appreciate that lowering weed pressure is useful for prolonging the lifespan of herbicides that are still working effectively.

I am sure that there are many other angles that can be considered as no-till might be expected to have minimal weeds in time as there is so little soil disturbance to encourage germination. While history shows that there will always be a weed to exploit every situation, being aware of that is the best way to help prevent it from happening.

Thinking through a problem can help find other solutions, at least for individual fields. Most of our serious grassweeds tend to germinate and grow following cultivation in the autumn – this is why early drilling is so favourable for grassweeds. For this reason, stubble cultivation immediately post-harvest is one of the best IPM options for most weeds. And to be even more effective, it might incorporate straw to help drive soil biology.

Planting catch crops may bring two benefits – some weeds will grow in the crop so these seeds will be gone and putting back organic matter may indirectly help seed loss in the long-term.

If you are tempted to put in a wild-flower strip, put it along a hedge where brome is a serious problem. Brome does not tend to propagate within a population of plants – it much prefers open spaces. Indeed, it took off following a period when we burned off hedges to stop the ingression of scutch. This was a classical case of where getting rid of one problem left us with an even bigger one.