Among the many issues growers must consider when selecting a drill for direct seeding is seed row spacing. With conventional tillage, most cereals in the inland Pacific Northwest are planted at row spacings between 7" and 16". The narrow 7" spacing is common among single and double disc drills while the widest row spacings are found with deep furrow hoe drills. When considering row spacing for a direct seed drill, one should consider both "operational" and agronomic issues. As row spacing increases, there will be more room for straw to pass between the openers, there is more room to mount various row cleaning attachments and fertilizer openers, and there are fewer actual moving pieces and numbers of openers, hoses, and bearings. However, in terms of agronomic issues, at some point as row spacing increases, yield potential will drop, as the rows become far enough apart that the crop does not optimally utilize sunlight, water, and nutrients. Furthermore, if you try to maintain an equal seeding rate per acre, you must plant more seeds per foot of row as the row spacing increases. This results in increased plant to plant competition within a seed row, which reduces tillering in cereals. In addition to direct effects on the crop, a narrower row spacing will result in quicker and more complete canopy cover which will increase the crops competitiveness with weeds and provide greater soil surface protection.
To help growers evaluate the tradeoffs between the operational and agronomic advantages of various row spacings, we initiated an experiment to determine the effect of row spacing on yield of direct-seeded spring wheat and barley. This report summarizes the first year (1998) of this experiment conducted at the Washington State University Dryland Research Station at Lind. This site receives an average annual precipitation of 9.5".
An experiment was designed to test the effect of rows spaced 7.5", 9", 12", and 16", on the yield and yield components of Alpowa spring wheat and Baronesse spring barley. To ensure that we only tested the impact of row spacing, seeding rate, fertilizer rate, and fertilizer placement relative the plant row were kept constant across all row spacings. We used a research no-till drill that allowed us to change row spacing and always place the fertilizer band 2" directly below the seed row. All plots were sown at a 70 lb/acre seeding rate and were fertilized with 40 lb N, 5 lb P, and 5 lb S per acre applied as a dry blend at planting using a deep band fertilizer opener running directly in front of the seed opener. The openers for both seed and fertilizer where an offset double disc type which resulted in very low soil disturbance.
Experimental measurements included plant stand, yield components (heads per area, seeds per head, and weight per 1000 seeds), grain yield, grain test weight, and grain protein.
Results and Discussion
Even though we planted the same number of seeds across all row spacings, stand counts taken 25 days after seeding showed that plant stands were lower on an area basis in the 16" row spacing treatments (data not shown). On average, plant stands in the 16" row spacing treatments were 27% less for the barley and 36% less for spring wheat compared to the 7.5", 9", and 12" treatments. To plant the same number of seeds per area, there were 2.1 times as many seeds planted per foot of row in the 16" treatments compared to the 7.5" treatments. The resulting seedling-to-seedling competition may have reduced seedling survival.
Yield results (Figure 1) for both spring wheat and barley indicate that the 16" row spacing decreased yields compared to narrower row spacings. For spring wheat, yields were statistically equal for the 7.5", 9" and 12" spacings. However, for the barley, the 7.5" spacing produced 220 lb/acre and the 9" spacing produced 333 lb/acre greater yield than the 12" spacing.
Analysis of the yield component data shows a strong relationship between heads per area and yield in both the spring wheat and spring barley (Figure 2). The other two yield components, kernels per head and 1000 kernel weight did not vary significantly across row spacings and were not related to yield. Thus, it would appear that the loss of yield potential with increasing row spacing is due to the inability of the crop to maintain head numbers per area because of within row crowding. Our data agree with published research conducted in other low-rainfall dryland regions of the world, which show that head density is the most important yield component for cereals when extreme drought is not a factor.
The results presented in this paper are from the first year of a two-year experiment, and hence it is difficult to draw strong conclusions. However, there seems to be good evidence that a 16" row spacing is too wide to produce maximum yields of spring wheat or barley at the Lind location. Furthermore there is some evidence that Baronesse spring barley which produced greater tiller numbers per area than the Alpowa spring wheat is more sensitive to within row crowding and that tiller numbers per area were reduced by a 12" row spacing as compared to a 7.5 and 9" row spacing. Because of this, there appears to be a yield benefit from seeding Baronesse barley at the 7.5 or 9" row spacing at least in 1998.
Figure 1. Yield of Alpowa spring wheat and Baronesse spring barley versus row spacing. Error bars represent the least significant difference at the 5% probability level.
Figure 2. Yield of Alpowa spring wheat and Baronesse spring barley versus head number per ft2.