Field pea – manage seeding for best pre-emergence weed control

E. Johnson - Scott Research Farm

Problem
It is recommended that field pea be seeded early and at a depth of 5 - 7.5 cm. Organic producers often delay seeding and use pre- or post-seeding tillage to remove germinated weeds. Crops that emerge before weed emergence are more competitive than crops emerging at the same time or after weed emergence. Delayed seeding to control lateemerging weeds and seeding at shallower depths to promote rapid crop emergence may be an alternative weed management strategy. What combination of seeding date, seed depth, and pre-emergence weed control optimizes field pea yield when grown without herbicide use?

Background
Organic producers use several cultural and mechanical methods to manage weeds. Prior to introduction of selective herbicides, secondary tillage combined with delayed seeding was commonly recommended to control wild oat. Research conducted from 1952 to 1957 at seven locations in western Canada found that tillage and delayed seeding controlled greater than 70% of wild oat in wheat and barley. A Minnesota study found that pre-plant tillage just prior to delayed soybean planting resulted in reduced weed populations. Delayed seeding may be effective in controlling some weed species, but it normally results in lower yields for many crops. Field pea production has been shown to be viable in the brown and dark brown soil zone of Saskatchewan, provided that seeding is completed before mid-May. In British Columbia, delayed seeding reduced processing pea yield at a location where the mean maximum growing season air temperature exceeded 21° C, but had no effect at location with lower air temperatures. Delayed seeding and secondary tillage is commonly used by organic growers in spite of the potential yield decline. Most weed seeds emerge from shallow depths (< 2 cm) and large seeded crops can be seeded deep allowing for selective mechanical weed control between seeding time and crop emergence. Pavylchenko observed that preemergence tillage gave crops about a ten-day weed free period.

Study description
Our field experiment was conducted on cultivated cereal crop stubble at the Scott Research Farm from 1999 to 2001. Treatments of seeding date (early May, mid-May and late May), seed depth (2.5 cm and 7.5 cm) and weed control (none, pre-emergence tillage, and herbicide) were applied in a split-plot experimental design. Each treatment was replicated four times. Wild oats and wild mustard were seeded just before the earliest seeding date followed by a shallow cultivation to distribute weed seeds and manage crop residue. We seeded field pea ( Grande) at a target plant density of 80 pea plants m with a narrow opener hoe-drill equipped with on-row packing. We did pre-emergence tillage about three to five days after seeding, but the second tillage sometimes had to be delayed up to 10 days depending on weather conditions. Preemergence tillage was accomplished in the shallow seeded plots (2.5 cm) with a tine harrow while a rodweeder was used for the deep seeded plots (7.5 cm). Data collection included field pea density, weed density, weed biomass, and crop yield.

Major findings
Crop establishment was very good in all years with plant densities exceeding 70 plants/m (data not shown). Overall, seeding date and seeding depth did not have an effect on crop establishment. Field pea was not damaged by pre-emergence tillage as the treatments did not reduce field pea density. In some cases, deep seeding delayed crop emergence (date at which distinct crop rows were visible) by one day. However, emergence differences did not affect time to crop maturity. Pre-emergence tillage did not result in a detectable reduction in weed biomass at the early May seeding date since very few weeds had emerged (Table 1). Herbicides effectively reduced weed biomass at all seed dates. Pre-emergence tillage was effective in reducing weed biomass at the delayed seeding dates, with the pre-emergence rod-weed treatment resulting in weed fresh weight reductions similar to herbicide treatments. Pre-emergence harrowing was not as effective as pre-emergence rod-weeding in reducing weed biomass at delayed seeding dates. Deep seeding provided some weed control effect at the late May seeding date since the seeding implement was able to uproot and bury more weeds when set at a 7.5 cm depth. Seeding delayed until late May resulted in yield declines of up to 37% (Table 2). Previous research has shown that seeding field pea in late May compresses the flowering period resulting in lower yields. Pre-emergence tillage resulted in small crop yield increases at the early May seeding date; however, it provided yields equivalent to herbicides at the mid-May seeding date. At the mid- May seeding date, yields achieved by pre-emergence tillage resulted in 80% of the highest yield achieved with early seeding and herbicide application. At the late May seeding date, pre-emergence tillage improved field pea yields by 30 to 35% with preemergence rod-weeding providing higher yields than pre-emergence harrowing. Though pre-emergence tillage improved field pea yield at the late May seeding date, these yields were still lower than the untreated checks seeded in early May. Shallow seeding did not improve field pea competitiveness at any seed date. Deep seeding resulted in higher yields at both the early and late May seeding date, but not at the mid-May seeding date. We don't fully understand the reason for higher yields with deep seeding at the early seed date. However, we believe that deep seeding may provide a better environment for Rhizobium bacteria survival, resulting in improved nitrogen fixation. Improved yield from deep seeding at the late May date is likely due to the weed control effect described earlier. Organic growers may be able to achieve adequate weed control and satisfactory yields with a slight delay in seeding date followed by pre-emergence tillage, but a long delay in seeding is counter-productive.

Conclusions
Pre-emergence tillage between seeding time and crop emergence can be very effective in reducing weed density and preventing yield loss provided there is adequate weed growth at tillage time. A strategy for producers who choose not to use herbicides would be to conduct early spring tillage to stimulate weed emergence, wait 10 - 12 days and then seed at a 7.5 cm depth. Follow this with preemergence rod-weeding five to seven days after seeding.

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


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