Post‑emergence harrowing for weed control
E. Johnson - Scott Research Farm
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?
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.
Table 1. Factors affecting the efficacy of post-emergence harrowing
|Factors improving success
|Factors reducing success
|Harrow less than 2 inches deep
|Large amounts of trash
|Soil is dry
|Crop seeded heavy, deep
|Weed seeds deep in soil
|Cool wet conditions following harrowing to let crop recover
|Poor growing conditions following harrowing
Table 2. Optimum timings for post-emergence harrowing for weed control
|Stage for harrowing
|2-4 leaf, before tillering
|up to 6 leaf
|2-6 inches tall
|seedling less than 4" tall
|seedling less than 4" tall
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.
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. 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.
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
Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund