Crop Rotation and Nitrogen Effects on Normalized Grain Yields in a Long-Term Study

information is available on their effect on year-toyear variability. Effects of year-to-year variability in agricultural production systems There have been several attempts to analyze what have always been a concern, but few studies are conducted for a long effect these different aspects of management may have enough period of time where management system evaluations and assessments can be made. Given this limitation, questions about on both spatial and temporal variability of yield. Finlay whether management systems are effective at reducing temporal variand Wilkinson (1963), Mead et al. (1986), and Raun ability remain in production agriculture. These questions prompted et al. (1993) used stability analysis to evaluate fertility investigation of a long-term crop rotation study to determine effects treatments or genotypes for stability across time or locaof crop rotation and N fertilization practices in a rainfed environment tion. As noted by Eghball and Varvel (1997), stability on normalized grain yields. Sixteen years of grain yield data from an analysis should not be used for comparisons including experiment with seven cropping systems (three monoculture, two 2dissimilar crops because of the scale dependency of yr rotations, and two 4-yr rotations) and three N fertilizer rates are the analysis. included in the study. Grain yields from 1983 through 1998 for each Eghball and Varvel (1997) used fractal analysis to crop and N fertilizer treatment were normalized and then relative analyze yields from selected treatments in a large longgrain yield within a cropping system and N fertilizer treatment were combined, which resulted in relative yields for each cropping system term study to assess temporal variability. This same and N fertilizer treatment combination in each year. Using the normalprocedure was used by Eghball and Power (1995) to ized yields, overall analyses of the 16 yr of data were conducted to characterize temporal variability for average yield of 10 assess what effects cropping systems and N fertilizer have on yield crops in the United States. In both cases, results indivariability. These analyses demonstrated that crop rotation systems cated that crops were significantly different in terms of are more effective at reducing long-term yield variability than monotemporal variability and that management was having culture systems, even with N fertilizer. As expected, N fertility, oblittle effect on spatial variability because of the domitained from either fertilizer or legumes in monoculture or rotation nance of temporal variability. These results seem to systems, is probably one of the most, if not the most important aspect indicate that management can do little to overcome in reducing yield variability. Analyses of normalized yields also demtemporal variability, unless the environmental factors onstrated that reductions in yield variability could be obtained in many of our cropping systems with proper management. affecting it can be lessened to some extent, such as using irrigation to alleviate water stress. Crop and soil management effects observed over time by researchers, consultants, and farmers seem to contraV both spatial and temporal, has long dict these conclusions. Based on these observations, our been a concern for workers in agriculture. Historiobjective was to use results from a long-term crop rotacally, when fields were first broken out of native grass tion study designed to determine effects of crop rotation or forest areas they were small, and even though they and N fertilization practices in a rainfed environment were managed as a single unit the amount of variability on yield variability by using normalized grain yields they encompassed was relatively small. Development from those systems. of larger equipment and mechanization has resulted in the tendency to combine many of these smaller fields MATERIALS AND METHODS into one large field, which from the standpoint of physical management made these fields easier to manage and The experiment was located on the Agronomy Farm at the allowed farmers to cover more acreage in a much shorter University of Nebraska Agricultural Research and Developtime period. At the same time, this has resulted in much ment Center near Mead, Nebraska on a well-drained Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiularger fields being managed as single units, which generdoll) with an organic matter content of 31 g kg2 in the upper ally encompass more variability. 75 mm. Soil test P and K levels in the upper 75 mm were in Agronomists have long recognized that spatial and the very high categories (according to University of Nebraska temporal variability greatly affects crop production and Soil Testing Laboratory fact sheets) throughout the duration have tried to some extent to reduce those effects. Hybrid of the study. and variety development, fertilizer use, and irrigation Seven cropping systems (three monoculture, two 2-yr rotawhere water is available have been used successfully in tions, and two 4-yr rotations) with three rates of N fertilizer many areas to reduce or minimize some of the effects were included in the study. Monocultures were continuous of variability. These practices have generally resulted corn (Zea mays L.), continuous soybean [Glycine max (L.) in improved average yields over the long term, but little Merr.], and continuous grain sorghum [Sorghum bicolor (L.) Moench]. Two-year rotations in the study were corn–soybean and grain sorghum–soybean, while 4-yr rotations were corn– G.E. Varvel, USDA-ARS and Dep. of Agronomy, Univ. of Nebraska, oat [Avena sativa (L.)] 1 clover (80% yellow sweetclover Lincoln, NE 68583. Joint contribution of USDA-ARS and the Ne[Melilotus officinalis Lam.] 1 20% red clover [Trifolium prabraska Agric. Res. Div., Journal Ser. no. 12880. Received 3 Jan. 2000. tense L.])–grain sorghum–soybean and corn–soybean–grain *Corresponding author ([email protected]). sorghum–oat 1 clover. A mixture of red clover and sweetclover was used to ensure a clover stand, as Melilotus spp. is Published in Agron. J. 92:938–941 (2000).