CropTec Innovation Day: What Lies Beneath
by Richard Hull, Rothamsted Research
Chickweed can be a troublesome visitor, especially when it lurks unnoticed below the crop canopy
Walking through a maturing cereal field in late spring is an excellent time to assess how successfully (or not) your weed management strategy is working. It’s the perfect opportunity to assess the progress of some of the most troublesome weed species that UK farmers are forced to control, like blackgrass and cleavers. But what about those weeds that never make it out above the crop canopy, that prefer to hug the ground, stay out of site and complete their life cycle way before the crop matures?
There is a whole raft of species that fit into this category, but the most widespread and prevalent on UK farmland is chickweed (Stellaria media). It’s a tough customer but are there some chinks in its life cycle that can be exploited to achieve better long-term control. This approach has worked for other weed species like blackgrass, where a good knowledge of what happens at key life cycle stages can be used against it.
Changing the drill
Let’s start with its germination pattern and the possibility of altering drilling dates that fall outside that peak period. This works for blackgrass because most of the germination occurs in the early autumn, allowing the plants to be sprayed off before a delayed autumn or spring crop is planted. For chickweed, the germination period is much more protracted and there are two distinctive peak germination periods in the autumn and spring. Therefore, delayed drilling can play a role because it’s always easier to control plants prior to a crop going in the ground than after, but to a lesser extent than with other weed species.
The second area where we have exploited the life cycle of certain weeds to gain better control, is knowing about the persistence of seed in the soil. There is an old adage that “one year’s seeding means seven years weeding”. But is this correct? As a rule, grassweeds have a much shorter persistence than broad-leaved weeds. Chickweed has an annual decline rate of about 30%, with as much as 2% of seed surviving up to 10 years. In the case of chickweed, then, the folk wisdom is pretty much spot on. One of the first experiments I worked on at Rothamsted was looking at the seed persistence of a range of weed species, including chickweed. This involved taking soil cores from a range of plots at our Woburn farm that had been sown with our species the year before. These soil samples where then brought back to the lab, washed out, seeds identified and repeated in the following years.
With these figures in mind is there anything that can be done with cultivations? Very lightly cultivating stubbles to bring old seed to the surface will aid germination as old seed needs light to germinate. This also leaves fresh seed at or near the surface to be predated by birds and insects. Ploughing could work to bury new seed, but with the proviso that you need to make sure you are burying more than you are bringing up. Due to the long persistence in the soil, it would then be a long time before you could use the plough again as a reset button.
Can we exploit aspects of the chickweed life cycle, then, to gain better control? I think this is much more challenging with chickweed and broad-leaved weeds than for grass weeds. Farmers who use herbicides for control in crop have a good range of actives from a wide range of modes of action that are achieving very good reliable control. For growers that don’t, crop control is limited to inter-row mechanical weeding. This has improved massively with the introduction of GPS guidance systems, reducing crop damage, but soil conditions need to be ideal.
A clean crop?
Should we be worried about these weeds that lurk below the canopy, live long in the soil and have a lack of cultural control options? Most of these species like chickweed are not particularly competitive and have wider benefits for the environment, such as being a valuable source of food and shelter for farmland birds and invertebrates. So, it may be worth tolerating low level populations for the greater good.
So, next time you walk through what you think is a clean crop, look below the canopy and have a rummage around. You might be surprised at what is lurking down there.
CropTec Innovation Days
Want to find out more about Chickweed, integrated weed management and sustainable weed control? Richard Hull will be joining us at the Weed Innovation Workshop, brought to you by CropTec Show and Rothamsted Research, to speak about New and emerging non-chemical control technologies. You’ll also have the opportunity to see the Rothamsted field trials for yourself and ask questions!