Downy Brome Control in Direct Seeding Systems

Downy brome (Bromus tectorum L.) is a troublesome weed in winter wheat and fall rye because its life cycle is similar to those crops. Downy brome has a competitive advantage over fall-seeded crops because it has the ability to resume growth before fall-seeded crops in early spring and to utilize soil moisture at shallow depths. It is also becoming an increasingly troublesome weed in minimum tillage systems.

Downy brome is very competitive in winter wheat, with densities of 50 to 100 plants m -1 emerging within three weeks of the crop. Such an infestation can reduce winter wheat yields by 30 to 40 per cent. Similar downy brome densities emerging in the spring, however, reduce yields by less than 10 per cent. Downy brome is less competitive with fall rye than winter wheat, but it has the potential to reduce rye yield in the range of 15 to 30 per cent.

Description

Downy brome has an erect, slender, 5 to 60 cm tall stem, producing numerous culms (Figure 1). It has a finely divided fibrous root system and reproduces by seed. Leaves are 4 to 16 cm long, 2 to 4 mm wide, light green and covered with fine hairs. The seed heads are 5 to 20 cm long, rather dense, slender and drooping, pale green with purple tinge with up to 8 nodding spikelets that are 2 to 4 cm long and have 2 to 8 seeds each.

In the seedling stage, downy brome can be confused with fall rye; however, it can be distinguished from the crop by the absence of auricles (small hooks encircling the stem at the base of the leaf blade). Japanese brome is becoming more prevalent in some areas and can be confused with downy brome in the seedling stage. Japanese brome has a panicle with the spikelets borne at the ends of long branches.

Japanese brome seed is shorter and has a twisted awn. Downy brome seed tends to be reddish at maturity while Japanese brome is a tan colour.

Growth and Development

Downy brome is generally a winter annual. Under adequate soil moisture conditions, seeds germinate in the fall. Young fall seedlings overwinter in a semi-dormant state and complete their life cycle the next spring. If adequate fall moisture is not available, downy brome can behave like an annual, germinating in the spring soon after the soil temperatures rises above freezing.

The shoots of fall emerged plants grow rapidly in spring; plants generally start heading in late April to early May followed by anthesis within a week. If left unchecked until mid-May, the seedlings can deplete soil moisture reserves. By late April, in most years, downy brome has an extensive root system, and control with tillage can be poor under moist soil conditions.

Seed Germination

Downy brome is a prolific seed producer. Fall-emerged plants, under ideal growing conditions, can produce 450 kg of seed ha-1 (330,000 seeds kg-1). Seeds germinate at or near the soil surface over a wide range of soil temperature and moisture conditions, with most of the seeds germinating within a year of maturation.

The seed matures in mid to late June and shatters within a week of maturity. In minimum tillage systems, seeds can remain viable for up to three to four years if they are deposited on heavy residue.

Management in Winter Wheat and Fall Rye

Cultural practices
Controlling downy brome in adjacent areas - Downy brome spreads into fields along field borders or fence lines and should be controlled when first observed. Crop management practices, such as banding rather than broadcasting nitrogen, and the quick establishment of a competitive crop help control the spread.

Maximizing seed germination - Downy brome seeds remain viable for three to four years. Fall harrowing may stimulate more seeds to germinate by increasing soil-to-seed contact. These plants can then be controlled in late fall or early spring to help reduce the seed bank.

Managing crop residue - Good chaff spreading is important because the practice evenly distributes downy brome seeds in the field. This approach helps prevent the formation of dense, sod-bound patches of downy brome, which can be difficult to control with tillage or herbicides.

Crop rotation - Restricting winter wheat or fall rye to once every three to four years helps keep downy brome at low infestation levels. Rotating fall crops with spring crops is advised because downy brome can be controlled with a pre-seeding herbicide application or tillage before spring seeding.

Using certified seed - Small grains and grass seeds are often contaminated with downy brome seed. Planting clean seed reduces the risk of re-infestation.

Using optimum dates - Use the optimum seeding date for the area to get early establishment of winter wheat or fall rye. In light to moderate downy brome infestations, a crop emerging before the weed can significantly reduce yield losses.

Maintaining a good vigorous crop stand - A healthy crop has a competitive advantage over downy brome.

Chemical control

Post-emergent applications of Metribuzin (Sencor) - In Canada, chemical control options in winter wheat and fall rye are limited. Sencor controls downy brome when applied in the fall. Apply after the winter wheat crop has started tillering (past the three leaf stage) and initiated secondary roots. Sencor is only registered for Norstar winter wheat. There are no herbicides registered to selectively control downy brome in fall rye.

Management for Spring-seeded Crops

Downy brome is also a concern in minimum tillage systems; however, the weed can be effectively managed by following all the above cultural practices plus the use of the following herbicides.

Pre-seeding applications of glyphosate
Glyphosate (trade names: Credit, Factor, Glyfos, Maverick, Renegade, Roundup Dry, Roundup Original, Roundup Transorb, Roundup WeatherMax, Touchdown iQ, Vantage, Vantage Plus and Victor) applied in late fall or early spring is an effective way to control downy brome.

A pre-seeding application (before crop emergence) of glyphosate at 0.51 to 0.77 L/acre, applied before the onset of heading, generally provides good control. Control may be reduced if growing conditions are poor, e.g. drought or plant competition within dense, sod-bound patches, or if the plants are covered with dust.

Pre-emergent applications of trifluralin
Trifluralin (trade names: Rival, Advance, Treflan and Bonanza) provides reasonable pre-emergent control of
downy brome in oilseed and pulse crops.

Management on Summer-fallow

Glyphosate or Rustler (mixture of glyphosate and dicamba) can also be used for downy brome control on fallow land; however, the chemicals do not provide residual control.

Summary

Downy brome is a winter annual, grassy weed that is becoming increasingly troublesome in southern Alberta. Its rapid increase in density and distribution on cropland is associated with fall-seeded crops and minimum tillage systems.

Key control practices for winter wheat or fall rye include leaving an interval of at least three to four years between fall-seeded crops and adding an oilseed or pulse crop to the rotation.

For minimum tillage systems, glyphosate applied in late fall or early spring provides better control than tillage, especially under cool, wet conditions. Higher rates of glyphosate are necessary to control downy brome after heading.

References

Ali, S. 2004. Crop Protection 2004. Agdex 606-1. Edmonton, Canada. Information Packaging Centre, Alberta Agriculture, Food and Rural Development. 508 p.

Blackshaw, Robert E. 1993. Control of Downy brome (Bromus tectorum) in conservation fallow systems. Weed Technology. 5: 557-562.

Upadhyaya, M. K., R. Turkington and D. McIlvride. 1986. The biology of Canadian weeds. 75. Bromus tectorum L. Can. J. Plant Sci. 66: 689-709.

Prepared by:
Rob Dunn
Conservation and Development
Alberta Agriculture, Food and Rural Development

Bob Blackshaw
Lethbridge Research Station
Agriculture and Agri-Food Canada

Source: Agdex 519-14. Revised March 2005.