Weeds are a fact of life on farm, but it is important to remember that all cultivated crops originated from wild populations. So how do we deal with them? 

The best option is to prevent new weeds coming onto your property. In most cases, farms have a resident population of weeds but do keep an eye out for new weeds appearing on your farm. Spot spraying small areas is more cost effective than having to treat large areas later on. The number of different weed species changes from paddock to paddock, so each paddock needs to be monitored and the weeds identified so that the correct method of control is used. Use all control options available, this is particularly important for difficult weeds like established Californian thistles (Cirsium arvese) which takes a couple of years to get on top of. The use of crop rotation and cultural control methods like mowing can assist with this particular weed.

Here is a to-do list to help control weeds in the long-term:

• Avoid bringing weed seeds onto your farm.
• Identify weed species.
• Eliminate if possible (for example, if it is only around the stock yards spray with a comprehensive herbicide).
• Avoid weeds seeding as they produce many seeds each year and increase the problem.
• Target weeds when they are small as they are much easier to control.
• Use crop rotation to ensure each weed species has a chance to be controlled.
• Two common weeds found in the South Island are shepherd’s purse (Capsella bursa-pastoris) and annual poa (Poa annua). 

Shepherd’s purse

Easy to spot once flowering, shepherd’s purse has a white flower and triangular/heart shaped seed pod. It is a member of the brassica family and is a prolific seed producer which has been successful in our current farming systems. Control strategies in pasture include comprehensive spray-out, stale seedbed technique to control germinating weeds, pasture establishment herbicide applications of flumetsulam to allow pasture to dominate, and good grazing management to avoid pasture opening up and allowing weeds to establish. Crop herbicide options differ for each crop so it is best to discuss this with your local PGG Wrightson representative.

Annual poa

Annual poa is a grass weed found throughout the South Island which has a distinctive leaf shape and found flowering at pretty much any time of the year. There is no comprehensive control strategy in pasture so focus on spray-out, seedbed preparation management and grazing management. In crops, there are many control options to ensure this weed does not compete. 

For help identifying and controlling annual poa, shepherd’s purse and other weeds on your farm, contact your local PGG Wrightson Technical Representative.