Cleavers

Key Points

• Cleavers are the most competitive weed in cereals
  • 7 times more competitive than black-grass
  • 37 times more competitive than speedwells 
• Cleavers seed only persists in the soil for 3 – 4 years and can therefore be managed out of the weed bank, unlike grass weeds
• Peak germination occurs in autumn/early winter, but continues until the soil dries in spring
• Cleavers only become competitive once they are above the crop canopy, GS32 onwards, by
  • preventing light interception
  • causing crop lodging
  • causing yield loss through poor grain fill
• Control of cleavers is vital and economically justifiable
  • The necessity to control other broad-leaved weeds should be driven by economic justification — this is where savings can be made.

Introduction

Cleavers — Galium aparine — has been the subject of a great deal of research, partly because of the significance of the plant as an arable weed, but also because of the interesting biology and ecology of the species. The results of much research work was first published in ‘The Starane 2 Cleavers Manual’ produced by Dow AgroSciences with Jim Orson as the original author of part of the manual. A new handbook has since been published incorporating work undertaken by IACR-Rothamsted in collaboration with others, such as ADAS, funded by DEFRA. Some of this work has not been published elsewhere and for this reason must not be reproduced without express permission of IACR-Rothamsted. John Cussans, of the weeds group at IACR-Rothamsted, prepared the updated guide Understanding the Cleavers Problem (134KB PDF).

Biology

Cleavers is a dicotyledonous, or broad-leaved, annual species. At the early seedling stage cleavers produce distinct, large cotyledons with a characteristic notch at the tip. Until the first whorls begin to appear, these seedlings can be confused with those of ivy-leaved speedwell.

The adult plant has a very characteristic appearance because of the way its leaves are arranged in whorls running up the stem and the size of the plant as it scrambles across the field using the crop for support. In summer, typically June, numerous tiny white flowers produce pairs of seeds covered in hooked hairs that can attach themselves to clothing or animal fur. Partly because of the size the adult plant can reach, and its ability to climb over the top of the maturing crop, cleavers infestations have the potential to cause very considerable crop losses.

Seedling Emergence

A very important aspect of cleavers biology that contributes to making it a problem weed is the characteristic emergence pattern observed in autumn sown crops. In autumn, cleavers seedlings emerge in a flush shortly after the crop is drilled. This flush is more prolonged than for many other broad-leaved weeds. In addition, unlike most problem arable weeds, in cleavers a second flush in the spring of the following year often occurs. The precise timing of emergence and the balance between spring and autumn emerging cleavers varies from year to year.

A study of cleavers emergence patterns carried out by IACR-Rothamsted and ADAS confirmed this variability in the balance between spring and autumn germination of cleavers. The average size of the spring flush, as a percentage of the total number of seedlings that emerged, was 7%. The range in percentage of spring germinators was from 2% to 18%.

Often the factor that triggers germination of weed seeds, particularly near the soil surface, is cultivation. The depth of seed burial has an important effect both on the total number of seeds that emerge and also on the speed with which seedlings emerge from germinating seed. For buried seed there is an optimum depth of burial of 2.5cm to 5.0cm with emergence reducing progressively with increasing depth. Little significant emergence occurs from seeds germinating from a depth greater than 15.0cm down the soil profile. This is one of the reasons why ploughing has the potential to suppress weed populations. A number of studies have shown that, as with many other common weed species, cold temperatures can stimulate germination of cleavers seeds. In addition, some available soil moisture is necessary for the germination of all seeds. In cleavers there is some evidence that soil moisture conditions in the spring may be of particular significance.

A study at IACR-Rothamsted has attempted to follow the fate of individual cleavers plants emerging in autumn or spring. The study reveals that autumn emerging plants were much more competitive with the crop, causing considerable yield loss, and also produced the bulk of the viable seed. Spring emerging plants, while not causing significant crop yield loss, did produce a quantity of viable seed.

Emergence Date	Cleavers per m2	Crop Yield Loss	Seeds per Plant
Autumn	28.6	23.0%	151.5
Spring	26.6	0.5%	5.34

It is believed that the late emerging plants play a role in maintaining the population where high levels of control effectively prevent earlier emerging plants from setting seed. Cleavers seeds in the soil decay relatively quickly, lasting only 3 to 4 years, so the successful production of seeds each year is of increased importance. The prolonged emergence pattern causes problems determining the best weed management approach. An application too early risks missing later emerging individuals but control too late will incur crop yield penalties from early crop-weed competition plus potentially poorer control.

Crop Competition

Compared with other species the growth pattern of cleavers is very late, i.e. significant biomass accumulation occurs relatively late in the season. The later growth pattern of cleavers translates into a late onset of competition with the crop. Cleavers, except in extreme conditions, appear not to influence crop growth until at least the end of April.

The variation in weed growth and crop yield loss caused by cleavers seedlings is not random but, in the main, is due to different weather conditions from year to year as well as soil type and crop agronomy differences from site to site. For example, cleavers are clearly much more vigorous and have a greater effect on crop yield when rainfall in June, July and August is higher.

Because of the potentially very high levels of crop yield loss that can result from cleavers populations a great deal of attention tends to be focused on the growth and effects of the weed in the current year. In reality, a successful weed management strategy for cleavers must address the long-term population dynamics of the species and be based on a sound understanding of the population biology of the weed over the course of the complete rotation.

Model for Long-Term Population Changes

Models such as this should be seen not as predictive but rather as tools that allow researchers, agronomists and growers to look at the potential long-term effects of changing different parts of the life cycle of a weed either through agronomy or weed control practice.

 

These sorts of studies indicate that a minimum of 95% control, but preferably 98 – 100% effective and consistent control of cleavers, is required in winter wheat to contain or reduce the population level. The model also highlights that herbicide is not the only way of managing cleavers populations. Cultural control methods can help ease the pressure on herbicide programs.

Competitive Weed Indices

Cleavers are the most competitive weed in cereals, 7 times more competitive than black-grass and 37 times more competitive than speedwells. To compare the relative competitiveness of many commonly occurring arable weeds refer to the Competitive Weed Calculator (401KB PDF). An example of some competitive indices and the formula for calculating the value of potential lost yield per hectare is shown below.

Weed	Competitive Index (% yield loss per weed)	Weeds per m2 to cause a 5% yield loss
Cleavers	3.0	1.7
Wild-oat	1.0	5.0
Black-grass	0.4	12.5
Mayweed	0.4	12.5
Poppy	0.3	16.7
Redshank	0.2	25.0
Common chickweed	0.2	25.0
Knotgrass	0.1	50.0
Crane’s-bill	0.08	62.5
Speedwell	0.08	62.5
Field pansy	0.02	250.0

Number of weeds per m² x Competitive Index = % yield loss
% yield loss x average yield in tonnes per hectare x average £ per tonne
= VALUE OF LOST YIELD PER HECTARE

Product Choice

There are two priorities for selecting a cleavers control product: final level of control and speed of control. To prevent yield loss and minimise seed return products need to deliver a consistent 95 – 98% control, whatever the time of year, whatever the climatic conditions. After crop GS30-32, in addition to the level of control, speed of control is essential to prevent cleavers smothering the crop and competing for light. Most damage to yield and profitability is done after this stage.

The longer the control of cleavers is left after GS32 the lower will be the return, despite the fact that cleavers can still be controlled successfully from applications of Starane 2 later in the season. Late emerging cleavers, i.e. those that appear around GS32, have less impact on final yield. They are more likely to be the source of seed return.

To maximise profitability from the control of cleavers applications should be made at or around crop GS32. Apply a cleavers herbicide with the T1 fungicide program. For products to deliver the required control at the appropriate time of the season they need to have certain key attributes.

Early season control – February to March

• Must deliver consistent, high level of control of cleavers
• Need to be able to control slow-growing cleavers and plants affected by autumn treatments
• Must be able to work in a range of temperatures and weather conditions, especially cold and slow growing conditions
• Speed of control is not important

Products based on active ingredients such as florasulam are most effective, namely Boxer* or Galaxy*.

Mid season control – April

• Must deliver consistent, high level of control of cleavers
• Need to be able to control slow-growing and fast-growing cleavers
• Must be able to work in a range of temperatures and weather conditions, especially fluctuating diurnal conditions with high daytime temperatures and low night temperatures
• Speed of control is important

Use products based on a combination of active ingredients, florasulam and fluroxypyr, to give rapid and consistent control under fluctuating conditions, such as Starane 2 or Galaxy.

Late season control – May onwards

• Must deliver consistent, high level of control of cleavers
• Need to be able to control fast-growing cleavers
• Low and/or fluctuating temperatures are not normally an issue
• Speed of control is essential

Use products containing fluroxypyr to give rapid and consistent control under fast growing conditions, such as Galaxy or Starane 2.

Visit the product pages for more comprehensive information about Boxer, Galaxy and Starane 2.