Fertilizer Placement-Row Spacing Effects on Wild Oat

Chapter 5 — Weed Control, No. 14, Winter 1990

Roger Veseth

The number of different models of commercial conservation tillage drills available in the Northwest has increased dramatically over the past 15 years. Most of the current models have the capability of deep banding all or most of the nitrogen fertilizer below seeding depth near seed rows. This is in contrast to early models of conservation tillage drills, particularly no-till drills, which were generally restricted to placing a limited amount of nitrogen with the seed as starter fertilizer, and surface broadcasting the bulk of the crop’s nitrogen fertilizer requirement. Many of the current drills also have the option of different row spacings in combination with deep-band fertilizer placement. Northwest research and farmer experiences indicate that the method of nitrogen fertilizer application and row spacing selection often affects weed competition and crop yield.

One row spacing-fertilizer placement arrangement, called “paired-row,” is an option on a substantial number of conservation tillage drills today. Two seed rows are spaced close together (3 to 7 inches apart), one on each side of a deep fertilizer band. The paired-rows are separated by wider interrow spaces (8 to 18 inches). Preferred widths of paired seed-rows and interrows vary with the precipitation zone, crop and other factors, An example of one paired-row spacing (6:14-inch) is shown in Fig, 1.

There has been considerable interest in the potential for reducing competition from grass weeds and improving crop yields,,with the combination of paired-row spacing and deep-band placement of nitrogen fertilizer. Theories to explain the row spacing-nitrogen fertilizer placement effects have included that the crop is more competitive against weeds, due to improved access of the crop roots to the fertilizer, and that the weeds in the interrow (without the fertilizer band) are less competitive because their roots are further from the fertilizer band early in the growing season. In addition, surface broadcast nitrogen has also been shown to commonly increase the plant densities of some grass weeds, such as wild oat and downy brome, compared to deep banding.

Fig. 1. Fertilizer band spacing Schematic illustration of a cross-section view of 6:14-inch paired-row spacing showing development of wheat saminal roots (roots formed at the seed) at the early 1 -leaf stage. Fertilizer bands are centered within the pairs, 2 inches below seed depth (from PNW Extension Bull. PNW 263).

Research Effort

Several research efforts are underway in the Northwest to develop a better understanding of the interactive effects of fertilizer placement, row spacing and grass weed competition on cereal growth and yield under dryland cropping. Most of the efforts are part of the STEEP (Solutions To Environmental and Economic Problems) program. Since 1975, this conservation farming research effort has annually involved more than 100 scientists from at least 14 different disciplines at the USDA-Agricultural Research Service (ARS) research centers and land-grant universities in Idaho, Oregon and Washington.

One study is being conducted with spring barley near Moscow, ID, by Joan Lish, University of Idaho weed research associate, and Dorm Thin, UI weed scientist. Their field experiment focuses on the effects of row spacing, fertilizer placement and wild oat densities on wild oat competition with spring barley. Field research was conducted in 1988 and 1989 at the UI Plant Science Farm near Moscow, a 22-inch annual precipitation area. Previous crops were winter wheat in both years of the experiment. The research site was chiseled and disked before planting.

Steptoe spring barley was seeded in April each year using a double-disk, no-till drill (modified Haybuster 1,000 series drill). Two row spacings were compared. Both row spacing arrangements had 6-inch spaced paired-rows with either 8- or 15-inch interrow widths (Fig. 2).

Wild oat was seeded in 3.5-inch row spacing, perpendicular to barley rows, at densities of 0, 13, 21, 27 and 37 plants/ft2. Three weed sampling areas were uniformly selected in each row spacing-fertilizer placement treatment combination over the different wild oat densities. These included (1) within the paired-row, (2) interrow adjacent to the row and (3) middle interrow (Fig. 2).

Based on soil tests, nitrogen (N) was the only fertilizer nutrient required both years. Nitrogen fertilizer was either surface broadcast and incorporated before seeding or deep-banded with the drill within the 6-inch paired-rows (Fig. 2). Fertilizer coulter-knives, mounted in front of and in between the pairs of seed openers, deep-banded the nitrogen 3 inches below seed depth (about 5 inches deep). Ammonium nitrate/sulfate (ANS) was applied at a rate of 70 pounds N/acre in 1988 and 85 pounds N/acre in 1989. The fertilizer coulter-knives were also pulled through the ground in the broadcast treatments, although no fertilizer was banded. This eliminated the ‘ ’tillage effect” of the fertilizer openers as an experimental variable.

Fig. 2. Overhead-view schematic Illustration of the four combinations of 6:8-or 6: 15-inch paired-row spacings and band or broadcast nitrogen fertilizer applications, and the names, locations and sizes of the weed sampling areas (Lish and Thin, Ul).

Preliminary Results

Density of Wild Oat Stems

Lish and Thin found that the density of wild oat stems (main shoots and tillers) was significantly influenced by the interactions of nitrogen fertilizer placement and interrow spacing (Fig, 3). Density of wild oat stems was lowest in the 15-inch interrow where nitrogen was banded and highest in the 15-inch interrow where nitrogen was broadcast. In both interrow spacings, differences between wild oat stem densities in the middle and adjacent-to-row sample areas were generally not significant. The effect of nitrogen fertilizer placement was stronger than the interrow width effect. If nitrogen was banded, interrow width had little effect on the density of wild oat stems. But if nitrogen was broadcast, increasing the interrow width increased the density of wild oat stems.

Wild Oat Height

Wild oat height in mid-July, after heading of the barley and wild oat, followed the same trend as wild oat stem density in the June sampling. Wild oat was shortest in the 15-inch interrow where nitrogen was banded, and highest in the 15-inch interrow where nitrogen was broadcast (Fig. 4). Also, in both interrow spacings, differences between wild oat height in the middle and adjacent-to-row sample areas were generally not significant.

Light Penetration of Plant Canopy

Penetration of light to the bottom of the canopy was measured in the first week of July with a line quantum light sensor placed diagonal to the barley rows. This device measures the amount of photosynthetically active radiation (PAR). Increasing the density of plants (crop and weeds) reduces light penetration through the canopy.

Fig. 3. Influence of 8- or 15-inch interrow spacings and broadcast or band nitrogen fertilizer application on density of wild oat stems (when averaged over the five wild oat densities) in early June 1986 at the barley 5-leaf stage and wild oat 4-leaf stage, Moscow, ID (Lish and Thill, Ul).

Fig. 4. Influence of 8-or 15-inch interrow spacings and broadcast or band nitrogen fertilizer applications on wild oat height (when averaged over the five wild oat densities) after barley and wild oat heading in mid-July 1988, Moscow, ID (Lish and Thill, UI).

Fig. 5. Influence of nitrogen fertilizer placement in spring barley on light (percent of photosynthetically active radiation – PAR) penetration to the bottom of the plant canopy (when averaged over two row spacings and five wild oat densities) after barley and wild oat heading in mid-July 1988, Moscow, ID (Lish and Thin, Ul).

Fig. 6. Influence of wild oat density in spring barley on light (percent of photosynthetically active radiation – PAR) penetration to the bottom of the plant canopy (when averaged over two row spacings and two nitrogen fertilizer placements) after barley and wild oat heading in mid-July 1986, Moscow, ID (Lish and Thin, Ul).

There was an average of 40 percent less light penetrating the canopy where nitrogen was broadcast compared to band applications (Fig. 5). Lish and Thin point out that this reduction correlates well with the higher number of wild oat stems and taller wild oat plants in broadcast nitrogen treatments. There was also a consistent reduction in light penetration with increasing wild oat densities at the same sampling time (Fig. 6).

Barley Grain Yield

In 1988, barley grain yield was significantly greater under banded nitrogen fertilizer than broadcast applications with the 15-inch interrow width (Fig. 7). However, fertilizer placement did not influence barley yield with the 8-inch interrow. Spring barley yields were progressively reduced with increasing wild oat densities (Fig. 8). The reductions in barley yield with increasing wild oat densities was highly correlated (r = .99) with reductions in light penetration through the canopy (see Fig. 6).

Preliminary 1989 Results

Barley Grain Yield

In 1989, barley yield was significantly increased with banded nitrogen fertilizer (Fig. 9) compared to broadcast applications. Yield also progressively decreased with increasing wild oat density (Fig, 10). However, Lish and Thin found no effect of interrow spacing on barley yield and other experimental components. The researchers attributed this to the dry spring conditions in 1989, in contrast to the relatively “normal” spring in 1988.

Nitrogen Content of Wild Oat

Wild oat plants were sampled in early June and analyzed for plant nitrogen content to evaluate the affect of fertilizer placement, interrow spacing and wild oat densities on wild oat nitrogen uptake. The researchers found no effect of interrow spacing and wild oat density on nitrogen content of wild oat plants. Nitrogen fertilizer placement did, however, have an effect (Fig, 11). With banded nitrogen fertilizer, wild oat nitrogen content significantly decreased with increasing distance from the fertilizer band — from the paired-row to the middle of the interrow. In contrast, wild oat nitrogen content in broadcast nitrogen treatments was highest in the’ middle of the interrow. Lish and Thin found no significant differences in wild oat nitrogen content with nitrogen placement at later sampling dates. Still, nitrogen placement affected grain yield at harvest (Fig. 9), indicating that differences in early-season wild oat competition may have influenced barley yields.

Fig. 9. Influence of nitrogen fertilizer placement on barley grain yield (when averaged over two interrow widths and five wild oat densities), 1989, Moscow, ID (Lish and Thill, Ul).

Preliminary Conclusions

Lish and Thin conclude from the results of the 2-year study that deep-banding nitrogen fertilizer in spring barley can significantly increase barley yields, and reduce wild oat growth and competition against the barley compared to broadcast applications. Banding nitrogen fertilizer increased spring barley yield with the 15-inch interrow spacing in 1988 and both interrow spacings in 1989.

In 1988 the importance of interrow width in the paired row spacing on barley yield and wild oat competition depended on the method of nitrogen application. With banded nitrogen, interrow width had little effect. With broadcast nitrogen, however, wide interrow width increased wild oat competition and reduced barley yield. The lack of effect from interrow width in 1989 was attributed to the unusually dry spring conditions. Because of the different results from the 2 years, the researchers point out the need for further research on the influence of environmental factors, such as precipitation, on the effects of different interrow widths in paired-row spacing, The effects of different combinations of interrow widths and banded fertilizer also need to be further evaluated across a range of precipitation zones, crops, weed species, and other environmental and management variables.

Fig. 10. Influence of five wild oat densities on barley grain yield (when averaged over two interrow widths and two nitrogen fertilizer placements), 1989, Moscow, ID (Lish and Thill, Ul).

Fig. 11. Influence of nitrogen fertilizer placement in spring barley on percent nitrogen content (dry wt.) of wild oat plants (when averaged over two interrow spacings and five wild oat densities) at the barley 5-leaf stage and wild oat 3-leaf stage in three row-spacing sample locations on June 2, 1989, Moscow, ID (Lish and Thill, UI).

From these and other related research results, Lish and Thin feel that changes in cultural practices, such as deepbanding nitrogen fertilizer and reducing row spacing, may permit reduced frequency or rates of herbicide applications, or at least increased herbicide effectiveness. Thin has additional research planned to continue evaluating the effects of various cultural practices such as row spacing, fertilizer placement and crop plant densities on weed competition and yield of spring barley and wheat.