Oregon State University
Washington State University
University of Idaho
Direct Seed Tillage Handbook
   Return Tillage Handbook

Chapter 5 - Weed Control, No. 11, Summer 1988

Crop Density - A Weed Management Tool

Roger Veseth

An effective weed control strategy should incorporate all the management options that the producer has available. Although the application of herbicides is the most common option that comes to mind for weed control, there are a number of other considerations which can complement or reduce the need for herbicides.

Donn Thin, University of Idaho weed scientist, points out that one critical aspect of a weed control strategy is maximizing the competitive ability of the crop in relation to the weeds. Management strategies should allow producers to manage for the strong points of the crop and against the weak points of the weeds. Thin is one of more than 100 Northwest researchers involved in the STEEP (Solutions to Environmental and Economic Problems) conservation farming research effort underway in Idaho, Oregon and Washington.

Space Capture

Thin emphasizes that an important part of improving the crop's ability to compete with weeds is what he terms "space capture. " The plant which is first able to occupy space in the field, whether it is the seeded crop or a weed, will probably have a competitive advantage in utilizing the available water, nutrients and light needed for growth. Several management and environmental factors influence the space capturing potential of the crop in relation to the weeds.

Some of the factors which determine the potential for early, rapid emergence, and a vigorous competitive crop, include seed quality, varietal adaptability, seeding date, seeding rate and plant density, and early access to adequate amounts of nutrients and water. Important cultural management factors such as crop rotation, combine residue distribution and tillage practices must also be considered in addition to herbicide applications to reduce weed populations. Thin has research underway to evaluate the effects of several of these factors influencing the crop's ability to compete with weeds, including plant density, paired row vs. uniform row spacing and fertilizer placement.

Plant Density

Preliminary results from Thin's research indicate that increasing the crop plant density, through adjustments in seeding rate and row spacing arrangements, improves the crop's ability to compete with weeds. Thin points out that growers can begin to evaluate the effect of a higher crop density on the ability of the crop to compete with weeds by looking in their own fields.

Corners and ends of fields typically have double-seeded areas. Weeds are commonly less of a problem in these double-seeded areas because of the higher plant density. Thin reports that some growers have avoided the usual application of wild oat herbicides required on fields in the past by seeding the field twice in two directions and using a higher than normal seeding rate.

When experimenting with crop density, however, he cautions producers not to increase seeding rate without changing row spacing or other seeding arrangement. Increased intra-crop competition (among crop plants) within the rows could reduce yield, particularly under limited moisture and nutrient conditions.

Research Effort

Thin's plant density research has focused on spring barley and wild oat, Field experiments were initiated in 1987 at the University of Idaho Plant Science Farm near Moscow in a 22-inch annual precipitation area. The objective of the study is to determine the effect of crop and weed plant density on intra- and inter-species competition. The project is being conducted with the assistance of UI graduate students Larry Tapia and Richard Evans. Steptoe spring barley and wild oat were seeded under conventional tillage at a depth of 2 inches in rows 3.5 inches apart. Five barley planting densities were used: O, 131,246,364 and 420 plants per square meter (pit/m2). This corresponds to barley seeding rates of 60, 114, 174 and 198 pounds per acre (lb/acre),

Wild oat plant densities were 0, 62, 98, 132 and 189 plt/m2. For an approximate conversion of plt/m 2 to plt/ft2, multiply plt/m2 by 9.3 percent (.093).

The barley and wild oat seed rows were perpendicular to each other. Narrow row spacings of 3.5 inches were used to allow the use of higher plant densities than would be feasible with standard 6- or 7-inch row spacings. Whole plant samples were taken 5 times (14-day intervals), beginning 14 days after emergence until maturity,

Preliminary Results

Analysis of preliminary results from the first year of this 2-year study suggests that the spring barley plant density was not affected by wild oat plant density. However, wild oat density was reduced with increasing spring barley density. By the third sampling, wild oat plant biomass (weight per area) also decreased with increasing spring barley density (Fig. 1).

Spring barley yield was influenced by both barley density and wild oat density. When averaged over wild oat densities, barley grain yield was highest at the 364 plt/m2 barley density (Fig. 2), which corresponds to a 174 lb/acre barley seeding rate. At crop densities above 364 plt/m2, yields again decreased, even without wild oat competition (also see Fig. 4). When averaged over barley densities, barley grain yield decreased as wild oat density increased from 0 to 98 plt/m2, but not beyond that population (Fig. 3).

Fig. 1. Effect of barley density at five sample times In 1987 on wild oat biomass averaged over wild oat densities at Moscow, Idaho (Thill, Tapia and Evans, UI).

Fig. 2. Effect of barley density on barley grain yield (multiply by 8.9 to convert to lb/acre) averaged over five wild oat densities in 1987 at Moscow, Idaho (Thill, Tapia and Evans, UI).

Fig. 3. Effect of wild oat density on barley grain yield (multiply by 8.9 to convert to lb/acre) averaged over four barley densities in 1987 at Moscow, Idaho (Thill, Tapia and Evans, UI).

Fig. 4. Effect of both barley density and wild oat density on barley grain yield (multiply by 8.9 to convert to lb/acre) in 1987 at Moscow, Idaho (Thill, Tapia and Evans, UI).

Barley yield decreased more from increasing wild oat densities at low barley densities than at high barley densities (Fig, 4). For example, at the 131 plt/m2 barley density, decreases in barley yield ranged from 19 to 45 percent with the wild oat densities from 62 to 189 plt/m2. At the 364 plt/m2 barley density, however, decreases in barley yield ranged from only 9 to 19 percent with the same range of wild oat densities. The three-way interaction of barley grain yield, barley density and wild oat density was statistically significant at the 94 percent probability level (P= 0.06).


The researchers concluded that spring barley has a greater competitive ability over wild oat at higher crop densities as compared to lower crop densities. Adjustments in crop density may be a relatively simple and complementary addition to producer's weed management strategies. They emphasize, however, that these are preliminary results from the first year of the study. Continued research will be needed to confirm these results over annual climatic variations and different growing conditions. The second year of the study is underway. Similar studies are planned at other locations in Idaho.

Thin stresses that manipulating crop density and other factors for improving the crop's competitive advantage over weeds will probably not replace herbicides in most cases, but will hopefully minimize the amount and frequency of herbicide use. For example, if crop management options such as a higher crop density can significantly reduce the weed population or size of the weeds, use of lower recommended rates of herbicide maybe possible. Generally, the higher the weed population and the larger the weeds, the higher the rate of herbicide required to achieve effective control. In the future, Thin plans to include economic evaluations of increased crop density and other weed management options as weed control tools. Changes in weed seed production, weed populations, species shifts and other short and long term results of the management options also need to be evaluated.


Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
Redesigned by Leila Styer, CAHE Computer Resource Unit; Maintained by Debbie Marsh, Dept. of Crop & Soil Sciences, WSU