AgFACT no. 20, November 1995 ISSN 1172-2088 Managing pastures to prevent weed invasion Weed-free   pastures   are   more   productive   and give   better   animal   performance.  They  require less herbicide control, reducing the development of    herbicide    resistance.    Chemical    and    non- chemical   methods   of   preventing   weed establishment   in   new   pastures,  and  managing weeds in older pastures are discussed.   Importance of the problem Weedy   pastures  reduce  dry  matter  production and   therefore   animal   performance   and production per hectare. Control can be costly and doesn’t always work. The most effective method of preventing weeds from invading pastures is by developing  and  maintaining  a  dense,  vigorous and  uniform  sward,  because  continuous  pasture cover  inhibits  the  germination  of  many  weed seeds. Of the weed seedlings that do emerge, few can  compete  with,  and  survive  in,  a  strongly growing pasture.   Weed control in establishing pastures Weeds   can   slow   down   pasture  establishment, especially  in  slower-establishing  species.  Once established,   they   can   be   difficult   to   remove. When  sowing  a  new  pasture  always  remember to: · Prepare the paddock well so your pasture will have the best possible seed-bed · Deal  with  any  existing   weed   problems;  a fallow   period   followed   by   herbicide application pre-drilling may be effective    · Use  the  right  amount  of  the  right  kind  of fertiliser · Use   certified   seed   to   avoid   sowing   weed seeds along with the desired mix · Use cultivars suited to your soil and climate; weeds will quickly fill any gaps left by poor establishment · Control   insect   pests,   especially   Argentine stem weevil, grass grub and porina Quick   intense   grazing   of   newly   established pastures  eliminates  many  weeds,  but  herbicide use  should  be  considered  if  potentially  serious species such as Californian thistle or ragwort are present. Weed control in established pastures Once weeds have become established in pasture they  can  be  controlled  by  chemical  or  cultural (non-chemical)  means,  or  a  combination  of  the two. Cultural control techniques Mowing  or  topping  tall-growing  pasture  weeds makes   them   less   unsightly,   prevents   seed-set, helps  stock  gain  access  to  the  pasture  and  can reduce possible damage to stock (e.g., eye or pelt damage from seeds). However, these techniques seldom kill weeds.   Grubbing   or   pulling   weeds   in   small,   sparse infestations can help to prevent spread. However, some   weeds,   such   as   Californian   thistle   and ragwort, will regrow from root fragments. Others such as nodding thistle will regrow unless cut off at least 5 cm below ground level. Hard grazing of weeds, especially when they are young  and  likely  to  be more palatable, can also be   an   effective   control   technique.   Rotational grazing  by  large  numbers  of  animals  for  short periods  of  time  can  help  control  many  weeds, including annual and biennial thistles. Herbicide control Few  herbicides  can  be  used  for  selective  weed control in pastures containing clover. MCPB and 2,4-DB  are  the  exceptions.  However,  they  are effective  only  on  a  limited  range  of  weeds  and work best on seedlings rather than larger plants. MCPA and 2,4-D are commonly used for broad- leaf  weed  control  in  established  pastures,  but both  severely  damage  white  clover.  MCPA  is less harmful than 2,4-D, to red and subterranean clovers.   Within   a   month   of   spraying   pasture