Weed Control

Summer 2009
Spring Growth Conference 2009

Ellen Mallory of the University of Maine reported on weed control tactics in organic cereals for graduate student Lauren Kolb, research associate Tom Molloy and associate professor Eric Gallandt.

Mallory said that the primary strategy for weed control in organic cereals now is tine harrowing when weeds are in the white thread stage, which can be very effective when the soil is dry. “You can get weed reduction on the same order as herbicides – maybe 90 percent control.” But if the soil is wet and/or weeds are bigger, control can be as low as 20 percent.

Another tactic is to seed in patterns that allow wheat to compete more with weeds than with itself. Spring wheat is typically planted in rows, so it competes against itself first. But seeding uniformly in all directions increases the chance that a wheat plant will compete against a weed rather than another wheat plant. Mallory’s former professor had a drill made to do this on his experimental plots and found a 30 percent decrease in weed biomass and a 9 percent increase in spring wheat yield. Lacking such a drill, farmers can broadcast and cultipack seed or sow half their seed in one direction and half in another.

When Kolb looked at standard (250 plants per square meter; about 145 pounds of seed per acre), broadcast, two-pass (200 plants per square meter; about 120 pounds of seed per acre) and high density seeding (500 plants per meter square; about 290 pounds of seed per acre), all three nonstandard plantings had better weed control than standard. Making two passes cost an extra $16 per acre; and seeding at the higher rate in the high density plots cost $30 per acre more. Broadcasting did not give the best germination because of some soil crusting and lack of control of seeding depth; and on the second of the two-passes, the tractor wheels may have pushed some seeds too deep. “So the strategy of higher seeding may be OK, but the way of doing it may be a problem,” said Mallory.

The researchers also tested planting in narrow (4 1/2-inch) and wide (9-inch) rows (the latter by plugging every other opening on the drill). Yields were significantly greater in the narrow row/increased density system, and the wide row was intermediate between standard and narrow treatments. The narrow row/increased seeding resulted in a net profit of $270/acre, following by wide row ($240/acre) and standard ($175/acre). (Profit data have not been analyzed statistically yet to see of these differences are significant.)

Narrow rows increase the competitive ability of the crop; but wide rows enable the farmer to cultivate with a tractor-mounted hoe (Gallandt imported one from Denmark for this study) – treating the small grain more as a row crop. The wide-row system enables better weed control through inter-row hoeing; timing of weed control is less critical, since weeds can be hoed beyond the threadleaf stage; perennial weeds can be controlled; soil crusts are broken; and multiple passes with the hoe are possible. The disadvantages are poor intra-row weed control; and steering is critical. (The tractor-mounted hoe can be steered robotically or by someone sitting on the hoe.)

Weed biomass was reduced by two-thirds to three-quarters in the narrow- and wide-row strategies, compared with standard planting; but costs were greater for narrow-row planting.

These treatment plots had two Lely cultivations; only the wide-row planting was hoed. The control, standard treatment was not Lely-cultivated, to mimic dairy farmers lack of time in spring and/or their inability to weed when soils are wet.

Mallory noted that hoeing weeds could add 10 to 15 pounds of nitrogen to the soil, and that such an increase when the crop is flowering could increase the protein concentration in the grain.

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