PROJECT RESULTS

Critical Period of Weed Control in Canola

Table of Contents:

• Background and Objectives
• Procedure and Project Activities
• Results and Discussion
• Conclusions

ARDI Project:

Total Approved:

Date Approved:

Project Status:

#98-112

$16,600

May 27, 1998

Completed September, 2000

Background and Objectives:

With the advent of herbicide tolerant canola and the increased use of in-crop herbicides for weed control in canola, producers want to know the best timing for weed control in canola. Some of their primary concerns include the sensitivity of canola to early competition, and the period of weed control in the canola crop necessary to prevent yield loss from late emerging weeds.

The critical period of weed control is the time during the lifecycle of a crop in which it must be kept weed-free to prevent yield loss. Critical period research in canola is important to determine the length of time that weeds can be left in the crop, especially with the increased use of herbicide-tolerant canola varieties, and to determine the length of time that soil-residual herbicides need to remain effective to prevent yield loss.

The objective of this study was to define the critical period of weed control for canola. Natural mixed-species annual weed populations were used, and the timing of weed control was related to development stages of the canola, as well as to days after canola emergence.

Procedure and Project Activities:

Trials were conducted at three distinct locations in 1998 and 1999 in southern Manitoba. The sites were located at the University of Manitoba research station near Carman, at Kelburn Farm owned by James Richardson International near Winnipeg, and at the Cyanamid Research Farm near Homewood.

The plots were cultivated and, in some instances, sprayed with herbicide shortly before seeding. In this way, each site was weed-free at seeding. The crop was seeded at a rate of 6 kg ha-1, along with 6 kg ha-1 of Counter 5G (terbufos) granular insecticide (Counter 5G is a product of BASF). The timing of weed control was based on the development stages of the canola crop. The critical period of weed control was determined by using two sets of treatments. In the first set, the crop was kept weed-free up until the cotyledon, 2nd leaf, 4th leaf, 6th leaf, 8-10th leaf, and early flower stages of the canola. In the second set of treatments, weeds were allowed to interfere with the crop for increasing durations up until the cotyledon, 2nd leaf, 4th leaf, 6th leaf, 8-10th leaf, and early flower stages of the canola.

The variety of canola used was Innovator (a relatively non-competitive variety), which is tolerant to Liberty (glufosinate) herbicide (Liberty herbicide and Innovator canola are products of Aventis crop protection). Weed control was accomplished using this herbicide.

A natural, mixed weed species population was utilized in order to test for general weed interference. At late flowering, samples of weedy material were removed from the weed-free treatments to determine the amount of weed re-growth that occurred after increasing durations of weed-free period. Non-crop dockage was removed from the yield samples, and this yield data was expressed as a percentage of the weed-free control. The length of the critical period was determined from the equations fitted to the data for two levels of yield loss (5 and 10%) (Figure 1).

Results and Discussion:

Currently in western Canada, there are two main options for herbicidal weed control in canola crops. The first is to use pre-emergence herbicides. In order to prevent considerable yield loss, these herbicides must provide weed control for a time period equal to or longer than the critical weed-free period. This study showed that canola must be maintained weed-free up to the 4th to 6th leaf stage (17 to 39 days after emergence (DAE)) to consistently prevent greater than 10% yield loss. For lower levels of yield loss, greater variability occurred in the calculated weed-free period to the point where maintaining a weed-free period was not feasible for some site-years in this study (i.e. the crop needed to be kept weed-free for the entire season). The critical weed-free period was greatly influenced by the nature of the weed infestation (species, densities and emergence periodicity) and the crop seeding date relative to the emergence timing of the weeds. Delayed seeding significantly reduced the length of the critical weed-free period. At the Carman site in 1999 for example (Table 1), the canola needed to be kept weed-free up to the early flower stage if the crop was seeded on April 27, but it needed to be kept weed-free only until the cotyledon stage if the crop was seeded on May 14.

Table 1.  Critical length of weed-free period and critical timing of weed removal for canola in days after emergence (DAE) and development stages of the canola crop for 5 and 10% yield loss levels.

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