Weed Management Options Which Reduce Pesticide Risk

Critical Period of Weed Control

The critical period of weed control is said to be the period of time in which weed control is necessary to avoid significant yield loss (Nazarko et al., 2005). The best time to control weeds and the length of the critical period depend on a number of variables including weed emergence timing, weed densities, the competitive ability of weeds compared to crops, and environmental factors.

Timing of herbicide applications is crucial for controlling weeds effectively and preventing excessive interference with the crop. Different crops are susceptible to interference from weeds at different times. The critical period of weed control for canola is around the 4-leaf stage, or 17-38 days after crop emergence (Martin et al., 2001). The critical period for pea varied between sites but began as early as 2 weeks after emergence (Harker et al., 2001). For a more competitive crop such as barley the timing of weed removal is not as clear cut (O’Donovan et al., 2005). For corn, the critical period depends on nitrogen availability, with the critical period becoming shorter with increased fertilizer rates (read more at U of Nebraska – Lincoln “For Effective Weed Control, Timing Really is Everything” http://ard.unl.edu/rn/0901/weed.html).

Knowledge of the critical period of weed control can also allow for reduced herbicide use, if farmers can manage weeds with one well-timed herbicide application instead of two. For example, an early well-timed application of glyphosate in glyphosate-tolerant canola may eliminate the need for a pre-seeding burn-off, if conditions are not conducive to vigorous early-season weed growth (Clayton et al., 2002).


Control weeds during the critical period for weed control for best results

Weed control operations outside the critical period (i.e. too early or too late) will have little benefit in weed management or crop yield.

Timing herbicide applications well may allow farmers to use one spray operation instead of two in some cases.



Clayton, G.W., K.N. Harker, J.T. O’Donovan, M.N. Baig and M.J. Kidnie. 2002. Glyphosate timing and tillage system effects on glyphosate-resistant canola (Brassica napus). Weed Technol. 16:124-130.

Harker, K.N., R.E. Blackshaw and G.W. Clayton. 2001. Timing weed removal in field pea (Pisum sativum). Weed Tech. 15:277-283.

Martin, S.G., R.C. Van Acker and L.F. Friesen. 2001. Critical period of weed control in spring canola. Weed Sci. 49:326-333.

Nazarko, O.M., R.C. Van Acker and M.H. Entz. 2005. Strategies and tactics for herbicide use reduction in field crops in Canada: A review. Can. J. Plant Sci. 85:457-479.

O’Donovan, J.T., G.W. Clayton, K.N. Harker, A.M Johnston, T.K. Turkington, H.R. Kutcher and F.C. Stevenson. 2005. Barley response to seed placement and herbicide timing. Can. J. Plant Sci. 85:265-270.

Top of page