When To Control Weeds

Controlling weeds in crop is essential to obtain maximum yields. Taking the weeds out of the crop early is the way to attain the maximum benefit. A producer that controls his weeds early reduces competition from the weeds. There is better weed control when weeds are younger with most herbicides, so the herbicide should be more effective when applied early. Knowing when to control weed is something that takes time and effort. Getting out in the field and having a look is the only way that proper weed control can be implemented. When a producer is walking through their field this year, they should take time to pull some weeds. Look at the roots of the plants that are growing in your fields. In general, with most grassy weeds, the plants start growing significantly more roots at the two leaf stage. More roots means that the plants can do a better job of searching out nutrients and moisture and take this away from the crop. With plants that grow in the rosette form, the root development increases significantly when the plants grow bigger than a looney. With plants that grow true leaves rather than grow in a rosette, they start to increase their root mass significantly between the second and third true leaf. If weeds are at this stage, they are significant competitors and should be removed as quickly as possible. The second thing that must be looked at is plant density. Weeds like pygmy weed, yellow yarrow grass and millet all are not that competitive when present in low numbers. When the numbers go up, they need to be sprayed quickly. Other weeds like stinkweed, shepherd’s purse, volunteer canola and narrow-leaved hawks beard are all weeds that are significantly more competitive after they bolt. At this stage rapid growth means that the nutrients and moisture destined for the crop will quickly disappear. If a producer is looking at a field and it just starting to show small weeds, the likelihood of seeing more weeds coming is high. With broadleaf weeds, I like to wait till 80% of the weeds are in in the one leaf stage or better before spraying. With grassy weeds, I like to see 80% of the weeds coming into the second leaf. This will help insure that the most competitive weeds are controlled early. It may mean that some late germinating weeds may get missed, but later germinating weeds are not as competitive with an established crop. Picking the right time to spray is never easy. It is not an exact science, but following the guidelines that are listed here will help improve the results. Remember to consider field history. The fields must be looked at each year and sometimes more than once in the same year. If something doesn’t look right in a field, walk away and come back a day later. Doing this helps me put together a better plan for difficult fields. Always be sure to look at the whole field.


By Dave Cubbon, P Ag