Varying Winter Wheat Seeding Rates among Landscape Positions

Abstract

Recent research in the Palouse region of eastern Washington has suggested that traditional seeding rates may not be adequate to maximize winter wheat ( Triticum aestivum L.) yield on all landscape positions. Low spike densities due to reduced light intensities, longer snow cover, and cool temperatures result in lower than expected yields on north‐backslope (N‐backslope) landscape positions. The objective of this experiment was to determine if higher seeding rates could increase spike density and yield. Three seeding rates, 1.0X standard grower rate (1.0X), 1.5X standard rate (1.5X), and 2.0X standard rate (2.0X), were tested on toeslope, south‐backslope (S‐backslope), shoulder, and N‐backslopes at two locations, Farmington and Pullman, WA, in 1992 and 1993. The 1.5X and 2.0X treatments were established with an additional seeding operation perpendicular to the 1.0X drill passes. Increasing seeding rates resulted in greater spike density on all landscape positions. Spike density of wheat seeded at the 2.0X rate was from 11% greater on the S‐backslope to 24% greater on the N‐backslope in comparison with the 1.0X rate. Yield, however, only increased on the N‐backslope with wheat seeded at the 2.0X rate yielding 10.3% more than wheat seeded at the 1.0X rate. On the three other landscape positions, a reduction in kernel number per spike resulted in no yield increase. Based on expected costs, the 2.0X seeding rate would increase net return on the N‐backslope by $2.35/acre given $3.55/bu wheat. Research is needed to determine if similar yield increases could be achieved without making a perpendicular drill pass to double the seeding rate, which would increase net returns by $13.23/acre. Research Question Recent research in the Palouse region of eastern Washington has demonstrated that low spike density may limit winter wheat yield on north‐backslope landscape positions. Spike density typically increases with more plants per area. Thus, it may be possible to increase yield on north‐backslopes by increasing the seeding rate. If yield is responsive to seeding rate, growers may want to consider varying seeding rates within fields. Literature Summary North‐facing slopes receive less solar radiation, have snow cover longer and are cooler than other slopes during late winter and early spring when active plant growth resumes. At the latitude of the study sites, irradiance on north‐facing slopes can be less than one half that on south‐facing slopes in early February. Both light intensity and temperature are important determinants of tiller production, with low light intensity and cool temperatures reducing the tillering capability of winter wheat plants. Given the limitations to tiller production per plant, increasing plant number per area could increase final spike density and grain yield unless some other factor is limiting. If spike density is not limiting yield, increases in spike density usually result in lower kernel number per spike or lower kernel weight, and yield remains constant. Study Description Three seeding rates (standard rate [1.0X], 1.5X standard rate [1.5X], and 2.0X standard rate [2.0X]) were evaluated on the N‐backslope and three other landscape positions. The experiment was conducted near Farmington and Pullman, WA, during the 1991–1992 and 1992–1993 growing seasons using separate fields each year. An area representative of each landscape position was selected within each of the four fields. Varieties: ‘Madsen’ at Farmington and ‘Cashup’ at Pullman (common soft white winter wheats). Crop rotations: winter wheat‐lentil at Farmington and winter wheat‐spring barley‐spring pea at Pullman. Sept. to Sept. precipitation: 16.7 in. in 1991–1992 and 17.8 in. in 1992–1993. Soils: Palouse‐Thatuna silt loams at Pullman and Palouse‐Thatuna‐Naff silt loams at Farmington. N application rates: 105 lb/acre at Farmington and 85 lb/acre at Pullman in fall 1991 and 116 lb/acre at Farmington and 95 lb/acre at Pullman in fall 1992. Landscape positions: toeslope, south‐backslope, shoulder, and north‐backslope. Seeding rates: 1.0X, 24 seeds/sq ft average; 1.5X, 38 seeds/sq ft average; and 2.0X, 48 seeds/sq ft average. The 1.5X and 2.0X seeding rate treatments were established by making an additional drill pass (0.5X and 1.0X) perpendicular to the 1.0X standard rate. Applied Questions Did higher seeding rates alter grain yield, grain protein, and test weight on the north‐backslope? Increasing the seeding rate to the 1.5X rate increased grain yield by 3.6 bu/acre, while doubling the standard seeding rate increased grain yield by 7.5 bu/acre. The yield increases resulted from a 24% increase in spike density as the seeding rate was doubled. Doubling the seeding rate decreased grain protein slightly while the effect on test weight was mixed, with test weight increasing at one site and decreasing at the other three. Did higher seeding rates alter grain yield and yield components on toeslopes, south‐backslopes, or shoulders? There was no change in yield on these landscape positions as seeding rate was doubled. The higher seeding rates did increase spike density on all three of these landscape positions, but kernel number per spike decreased as spike density increased resulting in no change in yield. Does the yield increase from doubling the seeding rate on the north‐backslope justify the added cost? Based on 1992 Washington State University crop enterprise budgets, a 7.5 bu/acre increase in yield would increase profit by $2.35/acre of north‐backslope given $3.55/bu wheat, a moderate price historically. Further research is needed to determine if similar yield increases could be achieved without making a perpendicular drill pass to double the seeding rate as the second drill pass accounted for over half of the cost of doubling the seeding rate. If similar yields could be achieved in one drill pass, the net return would increase by an additional $13.23/acre. Should growers begin doubleseeding north‐backslopes? The results from the four site‐years included in this study are encouraging. The best option for growers is to experiment with doubleseeding on their own farms if their equipment will allow them to make a second seeding pass and increase plant density. A double disc drill was used during the experiment and the number of plants per area in the doubleseeded treatment was 1.84 times that of the single seeded treatment, indicating only a slight reduction in emergence as the result of the second drill pass. Promising areas for doubleseeding are north‐backslopes where spike densities are low enough that you can easily see the ground between the rows after heading. All the experimental sites were in the 21 to 23 in. annual precipitation area and our conclusions may not be applicable in drier areas where moisture is more critical in determining yield potential.

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