The majority of winter cropping I walk over is wheat and this autumn I have seen many seedling cleavers trying to emerge through the pre emergence herbicides.
Cleavers are extremely competitive within a crop of wheat. It is reported that only 2 plants per m2 can cause a 5% yield loss and 100 plants per m2 in the autumn a 40% yield penalty.
So why have all these cleavers decide to emerge now?
We have already established that autumn emerging plants are the most significant from a competition perspective but those emerging in spring, while having relatively less effect on crop yield, may produce a significant amount of seed thus maintaining the weed population. (This year saw a large area of spring sown crops with consequently more spring germinating cleavers which in turn produced more than average seed numbers?)
Shed seeds tend to be dormant so relatively few germinate on stubbles immediately after harvest.
Cultivations tend to encourage germination and seedlings can emerge from a depth of 100 mm or more within the soil meaning that ploughing is a less effective technique for control than it is with other weed species.
Most cleavers tend to emerge in a prolonged flush shortly after autumn crops are sown with a second, smaller flush the following spring. However, emergence patterns vary between fields and cropping years for reasons that are still poorly understood.
Cleavers seeds have relatively short persistence in the soil with a typical annual decline rate of 60% for buried seeds. An important consequence of this is that, provided new seed return is prevented, infestations can be reduced substantially within a few years - potentially by over 95% within as little as four years.
This weed does not respond to “Integrated Control Measures.” This means that control is largely dependent on herbicides, so maintaining the availability and efficacy of a wide range of herbicides is essential to UK Agriculture.
No cases of herbicide resistant cleavers have been detected in the UK and no cases have so far been reported in any other European country. Which means poor control is down to a mistake in the herbicide policy not resistance!