Steep III - Pacific Northwest Conservation Tillage Systems Information Source

PNW CONSERVATION TILLAGE HANDBOOK SERIES STEEP
Chapter 5 - Weed Control, No. 10, Summer 1988

Cheatgrass or downy brome (Bromus tectorum) is a winter annual grass that was introduced into this region during the last century. The growth habit of cheatgrass makes it a difficult weed to control in fall-planted cereals. It can emerge with cereal seedlings or later and matures in June. Chemical control of cheatgrass is expensive and frequently inconsistent. Improved consistency of chemical control can be achieved when cereal seedlings are ahead of cheatgrass seedlings in growth stage. However, rarely does cheatgrass emerge in a uniform stand. Plowing buries a portion of the cheat seed deeply enough that it will not emerge. Cheatgrass has only limited dormancy so seed buried by plowing is not likely to be uncovered at a later date and germinate.

The lack of deep soil mixing in reduced tillage and no till systems causes more weed seed to be at or near the surface. Thus cheatgrass can be more of a problem in these systems. This is particularly true in annual cropping situations. A common practice when cheatgrass is a problem in these systems is to delay seeding until the cheat has germinated and can be killed by glyphosate. This postpones seeding until after fall rains, which can be a problem in dry years.

At present there is no single cultural practice or chemical that will effectively and consistently control cheat in winter grains. The current strategy of most growers is to keep cheatgrass at some tolerable level by measures that seem most affordable and practical. This includes herbicides, spring cropping, burning residue and other methods. This fall a new chemical method will be available to assist in cheat control strategies. The chemical is atrazine and the method is ''atrazine inversion." Atrazine is not a new chemical by any means, but a new registration is being sought in Washington and Oregon for its use on winter cereals using the inversion method. The registration is expected sometime this fall.

The inversion technique involves applying a root absorbed or soil-activated herbicide before seeding and then relying on a hoe or deep furrow opener to move treated soil out of the seed furrow before seeding. Atrazine is one of several products that can be applied in this manner. The atrazine inversion method was developed and refined by several STEEP researchers including Dean Swan, WSU weed scientist, Pullman; Arnold Appleby, OSU weed scientist; Corvallis; Don Rydrych, OSU weed scientist, Pendleton; and Floyd Bolton, OSU crop scientist, Corvallis. Several producers have also participated in testing and refinement of the technique. The advantage of using atrazine is that it is relatively inexpensive (about $2.00/ acre), and a reasonable high level of control can be achieved.

Table 1. Observational assessment of cheatgrass control and crop injury from atrazine inversion at 15 locations In eastern Oregon, April 1986 (Appleby, OSU, Corvallis).

Site	Crop	Rate of atrazine (lb ai/acre)	% cheat control	% Crop injury	Comments
Wasco County
1	Wheat	0.5	85	0
		1.0	98	5
2	Wheat	0.5	90	2	Knotweed Controlled, possibly sprayed with another herbicide
		1.0	99	50
Sherman County
3	Wheat	0.5	87	0	Some fiddleneck starting to invade
4	Wheat	0.5	90	0	Effective control of broadleaves
		1.0	99	0
5	Barley	0.5	95	0
		1.0	80	0	No explanation for the decreased control at 1.0 lb rate
Gilliam County
6	Wheat	0.5	85	0
		1.0	85	0
7	Wheat	0.5	65	0	Seeded 2 to 3 weeks after treatment
		1.0	65	0
8	Wheat	0.5	30	0	Blue mustard unaffected, suspected 2 to 3 week delay in seeding
		1.0	55	0
Morrow County
9	Wheat	0.5	95	10-15	Difficult to assess injury because of poor stand
Umatilla County
10	Wheat	0.4	-	30-60	Cheat not present in either treated or untreated areas, injury intensified because of poor growth.
11	Barley	0.4	90	2-5	Excellent fiddleneck control
12	Barley	0.4	85	15-20	Steep slope, injury observed in small pathces
13	Wheat	0.4	90	0	Soil organic matter approximately 0.9%
14	Wheat	0.4	99	25	Seeded with semi-deep furrow drill with one disk
15	Wheat	0.4	5	0	No-till seeding

In fall 1985, Appleby established 19 locations in eastern Oregon where atrazine was applied by inversion. Strips in grower's fields were treated with atrazine, and the growers seeded across them using their own equipment. In April 1986, cheatgrass control and crop injury were assessed at 15 of these sites (Table 1). The degree of control was evaluated by visually comparing cheatgrass populations from inside and outside the plot. Crop injury was determined by comparing the estimated percentage of plants that showed adverse symptoms, such as yellowing or stunting, from treated and untreated areas. These symptoms may or may not have been apparent at a later date.

Yields from these plots were not available, consequently neither the influence of cheat control nor herbicide injury on production can be assessed. However, Table 2 presents some data from work done by Bolton at the Sherman Experiment State at Moro. Bolton assessed the influence of atrazine, carrier solution and nitrogen fertilizer sources. Atrazine treatments yielded 2 to 6 bushels per acre better than non-atrazine treatments. Presumably the increased yields resulted from control of cheatgrass. The actual response to cheatgrass control will depend upon the severity of the infestation. A heavy infestation that is controlled by 90 percent will have greater impact on yields than a light infestation controlled to the same level.

Table 2. Influence of atrazine, carrier solution and nitrogen source on yields of Stephens soft white winter wheat at the Sherman Station, Moro, OR, 1987 (Bolton, OSU, Corvallis).

In general over the 15 sites atrazine inversion did a reliable job of controlling cheatgrass. However, there is risk of plant injury when this method is used. Injury may occur when drill openers do not remove a sufficient amount of treated soil away from the seed zone or when seed fi.u-rows slough down. The latter can happen because of insufficient soil moisture to hold the furrow, excessive tractor speed when drilling or soil slippage on steep slopes. Other factors such as stand hardiness, winter conditions and soil conditions also appear to influence the chance of plant injury.

The feasibility of atrazine inversion should be evaluated from the perspective of individual farms and fields. Such factors as severity of cheatgrass infestation, soil conditions and types of seeding equipment should be considered. Obviously, for producers who normally seed with double disk drills, inversion is not an option. Under extremely dry conditions, on steep slopes where furrows can slump, or if you prefer to seed at high ground speeds, inversion is not advisable. Producers and researchers who have worked to develop the inversion technique offer the following advice:

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide, Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.