William F. Schillinger, Washington State University, Lind, WA and Stewart B. Wuest, USDA-ARS, Adams, OR
Farmers in the low precipitation (150-to 300-mm annual) region of the inland Pacific Northwest plant winter wheat (Triticum aestivum L.) as deep as 20 cm below the soil surface of summer fallow to reach adequate moisture for germination. Deep-furrow drills are used to stack soil in furrow ridges to somewhat minimize the depth of soil covering the seed. Row spacing on conventional deep-furrow drills is 41 and 46 cm, but such relatively narrow spacing limits the quantity of loose, unanchored residue that the drills can pass through and is an impediment to the practice of conservation tillage fallow. The objective of our experiment was to determine whether row spacing could be widened without detrimental effects on winter wheat yield and weed control. Experiments were conducted at three sites during three years (9 site years) with rows spacing treatments of 41, 46, 51, 56, 61, and 81 cm. Two sets of experiments compared the six row spacing treatments with: (i) the same number of seeds per unit length of row, and (ii) the same number of seeds per unit area (i.e., kg/ha). Although results varied with location and year, overall there were no differences in grain yield for wheat planted at 41, 46, and 51 cm row spacing with a gradual, sequential reduction in yield with row spacing of 56, 61, and 81 cm. Yield component data showed that the yield decline with the widest row spacing treatments was due to fewer spikes per unit area compared to narrower row spacing. Our study showed that row spacing of deep-furrow drills can be widened to 51 cm with no overall reduction in grain yield. Wider spacing provides better residue clearance for deep-furrow drills and, therefore, allows farmers to practice of conservation tillage fallow with fewer drill plugging problems during planting.