WHEAT Long-Term Effects of Tillage, Nitrogen, and Rainfall on Winter Wheat Yields in the Pacific Northwest

Increased yields with conservation tillage have been attributed to the conservation of soil water (Rao and Sustainable cropping systems are essential for agronomic, ecoDao, 1996; Papendick and Miller, 1977) due to decreased nomic, and environmental reasons. Data from a winter wheat (Tritievaporation and cooler soil temperatures (Gauer et al., cum aestivum L.)/summer fallow rotation experiment, in eastern Oregon, was used to evaluate long-term effects of tillage, N, soil depth, 1982) and increased infiltration (Good and Smika, 1978; and precipitation on yield. The soil is a Walla Walla silt loam (coarseUnger and McCalla, 1980; Allmaras et al., 1985; Schilsilty, mixed, mesic Typic Haploxeroll). The experiment consisted of linger, 1992; Tucker et al., 1971). Papendick and Miller three tillage treatments (moldboard plow, offset disk, and subsurface (1977) reported that wheat yield had the potential to sweep) and six N treatments. Four main time periods (1944–1951, increase up to 20% with conservation of an additional 1952–1961, 1962–1987, 1988–1997), were identified, within which ex2 cm of water in a 25-cm precipitation zone. perimental treatments were consistently maintained. Depth to bedIn contrast, Hammel (1995) found that winter wheat rock ranged from 1.2 to 3.0 m. Yield was significantly greater ( 300 yields under conservation tillage systems were reduced kg ha 1) for the moldboard plow than for the subsurface sweep in all by an average of 565 kg ha 1 compared with conventime periods. Yield was generally greater ( 100 kg ha 1) for the tional tillage methods. In a long-term tillage trial in moldboard plow than for the offset disk, but only significantly in Time eastern Oregon, Schillinger and Bolton (1992) found Periods 3 and 4. For Periods 1 and 2, the addition of N fertilizer that greater quantities of surface residue in the stubbletended to produce higher yields, regardless of quantity or distribution mulch treatment contributed to reduced wheat germinaof rainfall. For Period 3, yield did not increase with the addition of more than 45 kg N ha 1, which we attribute to below-normal tion and stand establishment because of poor seed–soil precipitation. For Period 4, when precipitation was above average, contact and less uniform seedbed conditions compared yield increased with the addition of up to 90 kg N ha 1. Results with plow tillage. As insufficient seed zone moisture is demonstrated that despite beneficial effects on soil properties, consera major limitation in the establishment of fall-sown vation tillage has tended to be less productive for this cropping system wheat in the semiarid PNW, small decreases in seed than moldboard plowing, probably due to lack of downy brome weed zone moisture can decrease yield (Schillinger and Bolcontrol in the conservation tillage systems. ton, 1992). Other reasons cited for lower yields under reduced tillage systems include cooler, wetter soil conditions (Gauer et al., 1982; Papendick and Miller, 1977), T inland Pacific Northwest (PNW) has some of unfavorable interaction between soil physical properties the highest soil erosion rates in the USA (Young et and conservation systems (Cosper, 1983), phytotoxicity al., 1994b). Residue maintained by conservation tillage from previous crop residues (Kimber, 1973; Cochran et systems reduces erosion but, historically, most farmers al., 1977), soil pathogens (Cook, 1980; Elliott and Lynch, have been wary of adopting such systems due to such 1984), and increased grassy weeds (Papendick and perceived drawbacks as poor weed control (Bolton, Miller, 1977). 1983), inadequate planting equipment (Logan et al., One difficulty in interpreting apparently conflicting 1987), and lower crop yield (Cosper, 1983). The develresults on the effect of conservation tillage systems on opment of new farming equipment and chemicals since crop yield is that many studies have drawn conclusions the 1980s have increased the probability of obtaining based on only a few years’ data. Longer-term studies crop yields similar to those of conventional, clean-tillage that include a wider range of weather conditions can systems (Logan et al., 1987), and of lowering input costs. provide data to draw more general conclusions on the However, there are conflicting results on yield response advantages and disadvantages of tillage systems. Longof winter wheat to reduced tillage systems in the PNW. term studies provide perhaps the only way to determine For example, Chastain and Ward (1992) found that whether agricultural practices will sustain or degrade growth, development, and yield of wheat were not afthe productive capability of the soil and allow insight fected by crop residue maintained at the soil surface with into larger trends in crop production. conservation systems, although test weight was reduced. Historically, few if any technologies have increased winter wheat yield more than N fertilization. However, recommending optimum N rates is not an exact science, K.M. Camara, USDA–Natural Resources Conservation Service, 820 especially under dryland conditions (Rasmussen, 1981, Bay Ave., Suite 107, Capitola, CA 95010; W.A. Payne, Texas A&M 1996). Recent concerns over environmental quality, enUniv. System, Texas Agric. Exp. Stn., 6500 Amarillo Boulevard West, Amarillo, TX 79106; and P.E. Rasmussen, USDA-ARS Columbia ergy conservation, and economics have increased the Plateau Conservation Res. Center, 48037 Tubbs Ranch Rd., Adams, need to maximize crop utilization of applied fertilizer OR 97810. Received 1 Oct. 2001. *Corresponding author (Kelli. N and to reduce excess application that may contribute [email protected]). Abbreviations: N, nitrogen; PNW, Pacific Northwest. Published in Agron. J. 95:828–835 (2003).