Wheat growers interested in direct-seeding (no-till) wheat or barley into cereal stubble back in the 1980s and early 1990s will remember the Yielder™ drill designed by Guy Swanson and manufactured in Spokane, WA. This was a monster drill designed to place seed and fertilizer as a single pass with no prior seedbed preparation. Some jokingly commented that this drill could plant wheat into a gravel road.
One feature of this drill, in addition to its weight, especially when the tanks were full of seed and fertilizer, was the seed openers. These consisted of two steel disks about 24 inches in diameter designed when moving at speeds of 5-6 miles per hour to slice the soil open long enough for seed to drop to a predetermined depth before the soil fell back to cover the seed. Other than the heavy-duty design, this opener was not that different from other disk-type openers. What was different was the spacing between the openers. Whereas the typical drill plants in rows spaced uniformly apart, this drill positioned the openers to plant wheat in pairs of rows spaced 5 inches apart with 15 inches between each pair of rows. Nitrogen as anhydrous ammonia was applied through a separate set of disk openers but as a single band between each pair of rows. Dry fertilizer as phosphorus and trace nutrients could be applied through still another delivery system directly into the seed rows.
The 5/15 paired-row spacing of the Yielder™ drill raised the question in my mind of whether there would be any yield advantage or disadvantage over spacing rows uniformly at 10 inches apart. There would be the same number of rows for each pass of the drill, but the different spacing could affect any number of variables, including ability of the plants to capture sunlight, outcompete weeds for the nitrogen, or change the canopy microclimate enough to affect the severity of diseases.
Any experiment designed to test the effect of paired-row versus uniform-row spacing would need to eliminate all other variables such as seeding rate and depth and fertilizer placement. This could not be done by using two different drills. As it turns out, I had the perfect drill to do the experiments. This was an 8-foot-wide plot drill equipped eight AcraPlant disk-type openers mounted on one tool bar behind eight shanks for placement of fertilizer, staggered on two tool bars so as to help with trash clearance. Seed was distributed through a rotating cone so as to allow the same prepackaged amount to be distributed the full length of each plot, and fertilizer was banded below the seed as a solution of nitrogen, phosphorus, and sulfur at rates determined based on a soil test.
For uniform row spacing, the openers were spaced on the tool bar every 12 inches. For paired-row spacing, every two openers were spaced 7 inches apart (as close as possible) with 17 inches between each pair of openers. Likewise, the fertilizer shanks could be as a band of fertilizer in each row, or single band of fertilizer but at 2X the rate between each pair of rows, all at the same depth.
The first experiment was conducted in 1988 in a 2-acre plot site on the Washington State University experiment station at Lind, WA, where wheat, mainly winter wheat, had been grown annually and with supplemental irrigation since 1968 (see Biological Control—The Rest of the Story). The stubble and other residue of the 1987 crop had been incorporated into the soil with a Lely Rotera™. The experimental site was then divided into four blocks, with half of each block fumigated with methyl bromide under a tarp and half left natural. Each of the four fumigated and nonfumigated plots, in turn, were then seeded half to paired-row spacing and half to uniform-row spacing, with fertilizer placed in each seed row.
No, the better-looking wheat in the paired-rows is not an illusion. It yielded about 10% more, but only in the nonfumigated treatments. Yields were no different in the fumigated treatments. This would tend to rule out a greater exposure to sunlight because of the more open canopy where sunlight would more likely reach more leaves, especially the lower leaves, since that effect should have happened whether or not the soil was fumigated. The results pointed instead to a possible canopy effect favoring a disease or diseases subject to elimination by soil fumigation. Rhizoctonia root rot (see Root Diseases of Wheat and Barley—What do they look like and what do they do to the crop?) was the most apparent disease present on the wheat at this site this year, is subject to elimination by soil fumigation, and is favored by moist soil conditions such as would be more common in the top few inches of soil of rows spaced uniformly compared with rows spaced as pairs, thereby opening the canopy to more drying.
It was possible that year to determine the incidence and severity of Rhizoctonia root rot on roots pulled up with the stubble, immediately after harvest. Using a rating of 0 to 5, where 0 meant no disease and 5 the most severe disease, the average disease rating for uniform-row spacing was 3.5, with 39% of the stubbles rated 4 and 5 in severity, in contrast to paired-row spacing where the average disease rating was 2.2 and only 6% of the stubbles rated 4 and 5 for severity. Almost certainly, the obviously healthier wheat and higher yield with paired-row spacing compared with uniform-row spacing was due at least in part to less Rhizoctonia root rot.
Similar results were obtained in an experiment done in a grower’s field near Colfax, WA, in this case, winter wheat direct seeded into stubble of a spring wheat. At this site, take-all was the dominant root disease. Again, the wheat grew and yielded significantly better when planted as paired rows using the same drill. And again, there was no difference in performance of wheat with these different row spacings in plots fumigated with methyl bromide, as shown by the uniformly good wheat in strips of fumigated soil midway and all the way to the back of this experiment.
Several similar experiments were conducted in the region, including near Fairfield, Wa near the northern Idaho border and near Pendleton, OR, always with the similar results of higher yield with paired than with uniform-row spacing, especially when direct drilling a cereal after a cereal, with the effect nullified by soil fumigation. Moreover, the response was the same for spring wheat and spring barley. Read Abstract here.
One particularly instructive experiment was conducted with spring barley on the WSU station near Lind, WA in a 1-acre plot seeded continuously to spring barley so as to favor Rhizoctonia root rot for tests with varieties and treatments to control this disease. This experiment tested the paired-rows spacing with fertilizer applied in each row at 1X rate or shared by two rows at 2X the rate, all direct drilled. Again, the different drill passes crossed a strip of fumigated soil that served to indicate relative performance of the different treatments with little if any pressure from root disease. As the photo below makes abundantly clear, even rows spaced 7 inches apart need fertilizer placed within each seed row rather than 3.5 inches to one side when under pressure from Rhizoctonia root rot. The taller plants and more advanced heading are clear indictors of Rhizoctonia root rot.
It would be hard to estimate the number of experiments done over the past 30-40 years on fertilizer placement relative to seed placement, whether below the seed or below and to one side of the seed and how far to place the band away from the seed. However, I am not aware of any experiments that have taken pressure from root pathogens into account. Obviously the root disease pressure was atypically high and uniform at this site, but the point is made nevertheless. Clearly, we can expect that results will be different when conducting tests in fields under a long crop rotation compared with a site in 2-year or no crop rotation or continuous crop monoculture. Being able to conduct and especially interpret the results of experiments using soil fumigation as a check has also required extensive studies to prove that the benefits of soil fumigation are due to control of soilborne pathogens and not the flush of nitrogen or other nutrients from the microbial biomass. (See also Why the Increased Growth and Yield Response of Crops to Soil Fumigation?