Post-emergence harrowing for weed control

E. Johnson - Scott Research Farm

Problem
Organic crop production practices such as crop rotation, use of clean seed, careful use of tillage between crops, and good crop husbandry reduce problems with weeds. However, some weeds are likely to be found in the crop, and can cause yield loss if allowed to compete with the crop throughout the growing season. Can post-emergence harrowing be used to reduce these problems?

Background
Harrowing after seeding but before the crop emerges can be useful if weeds emerge before the crop. A rod weeder, cable weeder or flexible harrow may be used. Tillage with a drag or flex harrow after the crop emerges can also be effective. A rotary harrow can be used with an excess of trash, where a tine harrow would clog. Pre-seeding harrowing needs an aggressive angle, but post-emergence harrowing should disturb plants as little as possible. Harrowing may not kill all the weeds, but can damage them, to allow the crop a competitive advantage. Extra caution is needed if conditions are very dry. Weeds such as Russian thistle, tumble mustard, wild buckwheat, stinkweed, green foxtail, lambs quarters and redroot pigweed can be controlled well. Control of wild oat with post-emergent harrowing can be quite variable. Kirkland reported that multiple post-emergence harrowing passes reduced wild oat panicles and fresh weight in spring wheat in two years out of a three-year study. However, spring wheat yield was improved in only one year of the study. Three to four passes were required to reduce wild oat fresh weight by 40 to 80%. Crops are generally harrowed with the rows. Limited research done in western Canada and Europe indicates that harrow direction has little effect on selectivity (ratio of weed control to crop injury). Tables 1 and 2 discuss factors and timings for best results when harrowing postemergence for weed control.

Study description
Studies were conducted in 1998 and 1999. Wild mustard and wild oats were seeded as weeds. The experiment included field pea, canola, flax, lentil, and chickpea. Crops were harrowed at a combination of stages including emergence, three-, five- and seven-leaf stage. A check plot received no harrowing treatment. Harrowing was with a tine harrow, and was either a single or double pass at each timing. The experimental design was a split-plot with four replicates.

Major findings
Weed control in the study was erratic, however the study was very useful in determining the relative tolerance of crops to postemergence harrowing. Crop injury was assessed by counting the number of crop plants remaining after the harrowing treatment. Field pea was very tolerant with 85 to 90% of the plants remaining after harrowing (Figure 1). Chickpea was also very tolerant; however, producers should be cautious with this practice since physical injury could make chickpea more susceptible to Ascochyta blight. Lentil showed intermediate tolerance to post-emergence harrowing, while canola and flax did not tolerate harrowing.

Conclusions
Post-emergence harrowing for weed control can be recommended in field pea. Harrowing should be approached cautiously for lentil and chickpea until more is known about the impact of harrowing on disease spread. Postemergence harrowing is not recommended in either canola or flax.

Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund

Contact information:
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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