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24
Forage & Nutrition
Guide 2018
Table 1: Growth habit and control options for some of the most common weeds found in grassland swards
Weed
Growth habit
Control measures
Docks
Seeds germinate rapidly after soil disturbance.
Plants persist and develop
through deep tap roots resulting in dense
populations.
Matureplantscanproduceupto60,000seeds/year
which can survive several years in the soil.
Cutting alone will not control docks since new shoots
regenerate from the taproot.
Sprayingwithaherbicidecanbeeffectiveifdocks
are at a seedling growth stage.
Spraying established docks rarely results in a total
kill even from expensive herbicides.
Usually a follow up spray is required. Optimum time
to spray docks is in late spring during rapid growth
periodbeforeflowering.
Thistles
Plants germinate from seed.
Perennial creeping thistle can be spread by
underground roots which can survive in a dormant
state for some years before pushing up shoots into
poor growing open swards.
Cutting thistles does not kill the plant but can
weaken it.
Sprayingwithasuitableherbicideiseffectivein
controlling thistles especially when they are in the
vegetative growth stage.
Stockmustbekeptoutofsprayedfieldssince
senescing material may be eaten causing digestive
upsets.
Chickweed
Chickweed is an annual plant and infestations arise
from seed previously shed and present in the soil.
It has a rapid rate of growth and becomes very
competitive shading out sown species.
Since it is an annual plant it can be grazed out with
cattle or sheep at a high stocking rate.
Herbicides may need to be used to control high
infestations of the weed in silage swards especially in
firstcuts.
Ragwort
Ragwort is a biennial plant which develops prostrate
leavesinit'sfirstyearandthenflowersandgrowsto
maturity in it's second.
Plants can germinate from seeds which are wind
borne.
Control by cutting is not satisfactory as it can keep
the plant vegetative and encourage it to become
perennial.
While sheep will graze ragwort the toxin will also
affectthemandhencetheyshouldnotbeusedto
graze out infections.
Sprayingcanbehighlyeffectiveincontrolling
infestations especially in late spring when plants are
still at the rosette stage.
Stockshouldnotbeputintoasprayedfielduntil
at least 4 - 6 weeks after spraying to allow the
poisonous decaying material to die.
Rushes
Soft Rush is perennial and establishes from seed in
poached or open swards.
Commonweedinacidic,wetgroundlowinfertility.
Repeated cutting may give some control.
Applying lime and improving nutrient status of the
soil will reduce competitiveness of this weed.
Sprayingcanbeeffectiveespeciallyifcuttingcanbe
carried out 4 weeks after spraying or alternatively if
therushesareverymatureandbrowncuttingfirst
and spraying when the regrowth reaches 12 inches
will improve control.
Author: Martin Reel
Department of Agriculture, Environment and Rural Affairs (Northern Ireland)
A
Weed control on the dairy farm
Grassland weeds, particularly when they reach density levels of 10-20 per
cent in swards, will have a significant effect on sward production
For example, heavy infestations of chickweed have been found
to reduce silage yields by up to 65 per cent and there is also a
direct correlation between dock and spear thistle ground cover
and grass yield every one per cent increase in ground cover
results in a one per cent decrease in grass growth.
In addition, although some weeds have a small amount of
nutritional value, digestibility is much lower than perennial
ryegrass and the structure of the weed can discourage stock
from grazing surrounding grass. To effectively control grassland
weeds, it is first important to understand what species are
present and their individual growth habit.
GRASSLAND WEED SPECIES
There are a variety of weed species that can appear in