Features Agronomy Weeds
Hit weeds early, especially if pre-seed burndown is missed

Missing a spring pre-seed burnoff can mean more difficult weed control. Photo by Bruce Barker.

During the spring of 2008, the cool, dry weather resulted in slow weed emergence, and many farmers put off pre-seed weed control waiting for additional weeds to emerge. Under these circumstances, some farmers either opted to skip the burndown entirely, or got shut out by the weather once they decided to proceed with the burndown. 

In these cases where the burn-off was missed, farmers struggled to control large winter- and spring-annual weeds that were ahead of the crop. This puts additional pressure on post-emergent herbicide products to deal with the weed populations. “Larger weeds that emerge before the crop are tougher to control with a herbicide, so producers need to get in there as soon as they can if they missed their pre-seed burn-off,” explains Clark Brenzil, provincial specialist in weed control with Saskatchewan Ministry of Agriculture in Regina. 
Brenzil’s advice to control weeds early is supported by several research studies. Timing of weed emergence is important, and in a year when weeds emerge before the crop, yield reductions from crop competition are even higher.

Research by John O’Donovan, when he was at the Alberta Environmental Centre at Vegreville, Alberta, showed that when wild oat populations of 20 plants per square metre (two plants per square foot) are one leaf stage ahead of a wheat crop, yield losses of 19 percent can occur if left untreated, compared to a 11 percent yield loss if they emerge at the same time as the crop, dropping to a seven percent yield loss if the wild oats are one leaf stage behind the crop. Similar trends are seen with other crops.

In a research study conducted by Neil Harker of Agriculture and Agri-Food Canada at Lacombe, Alberta, early weed removal also provided greater yield increases than waiting for later emerging weeds before applying herbicides. This is especially important if a pre-seed burndown was missed.

Choose herbicides that allow early application
Herbicides vary in the stages at which they can be applied on the crop, so to get in early requires proper herbicide selection. In herbicide-tolerant canola systems, that choice is easy, as all systems have excellent crop safety at early leaf stages: glyphosate on glyphosate-tolerant canola from the cotyledon to six-leaf stage; Liberty on Liberty Link canola from the cotyledon to early bolting stage; and Odyssey on Clearfield canola from two- to six-leaf stage. 

There are also very good herbicide choices in pulse crops, which allow early application. Odyssey and Solo, for example can be used in peas and Clearfield lentils from the one- to
six-leaf stage. 

On cereals, most Group 1 herbicides, such as Axial, Horizon, Puma 120 Super and Achieve can be applied as early as the one or two leaf stage for control of grassy weeds. For broadleaf weeds in cereals, beware of early timing with full rate phenoxy (2,4-D and MCPA) treatments, since applying prior to tiller initiation can result in yield losses up to 25 percent. The exact timing of crop tiller initiation is difficult to gauge from the external features of the plant, but usually coincides with the four-leaf stage. Since MCPA is milder on most cereal crops, it or products with combinations of MCPA can often be applied as early as two or three leaf stage depending on the product. Some tank-mix products include MCPA at low rates, and these can be safely used at early leaf stages as well. Check label details with these products for the appropriate timing.

Good bets for broadleaf weed control in cereals as early as the two leaf stage include bromoxynil products including Buctril M (bromoxynil and MCPA ester), sulfonylurea products like Refine SG and dicamba (best tolerance at the three to four leaf stage) products. Infinity may be applied as early as the one leaf stage of registered cereal crops.  Check labels for complete recommendations.

Brenzil says it is also important to keep water volumes high when the burnoff is missed, as high weed populations of large weeds makes good coverage of each weed critical, something that could be compromised with lower water rates. Also ensure that water quality does not impact herbicide performance, and use surfactants where recommended. Large weeds are more difficult to control, so using recommended herbicide rates is important. 

Brenzil’s final comment on pre-seed burndown is to conduct it as soon as possible in the spring. Based on research by the University of Saskatchewan, Agriculture and Agri-Food Canada and Dupont Canada, delaying a burndown will result in reduced yields because winter annuals, and early perennials such as dandelion, are using moisture and nutrients up until the time they are treated. Regardless of growing conditions, early pre-seed burndown provided the highest yields, even if seeding does not occur for a few weeks after the burndown. “One of the changes producers should be considering is to do a pre-seed burndown early in the spring, even if they won’t be seeding that field for another week or two,” says Brenzil. “The research suggests that the benefit that we think we get from early seeding may actually come from the weed control operation prior to it rather than getting the seed in the ground earlier.” n