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Fluid Journal : Fluid Journal 2005-2007
Winter 2006 Fluid Journal 2 variability and address these factors, and 3) determine the economic feasibility of implementing these technologies in a production environment. Yields Rice/soybean. Average rice yield in 2001 (Table 1) was highly variable within the yield map (Figure 1). Though the yield variability or coefficient of variation (C.V.) was much less in the subsequent soybean crop (Table 1), the apparent yield zones (Figure 2) appear to be consistent with what was seen in the previous rice crop. The yield zone consistency was confirmed by performing a Multi-Year Yield Analysis in which those management zones were defined: high yield, average yield, and low yield. Soil test P results indicated that a P application was warranted over the majority of the field, but the southern portion of the field had a greater probability of obtaining a yield response (Figure 3). Analyses of the yield data collected from the 2003 rice crop indicated a substantial decrease in variability compared to the 2001 crop (Table 1). Figure 4 indicates a definite increase in rice yield in the P-limiting areas of the field as a likely result of variable rate (VTR) P application. Weather differences or other factors may also have been involved. Our studies also showed that P fertility may not have been the only source of yield variability. One hypothesis from our data is that compaction after precision land leveling may have been limiting yields the first two years. Further research has additionally shown a strong correlation between total soil volume that was cut and the difference in yield compared to the fill area. Thus, a second hypothesis that may further define the decrease in field variability from 2001 to 2003 is that organic matter additions (e.g., crop stubble) from the previous cropping year aided in the restoration of the disturbed microbiological ecology that was caused by the land-forming process. Economics A question that is asked often by producers when discussing the implementation of PF is: "Will this technology pay for itself?" A cost analysis was conducted for the field from which these data are reported. When comparing the whole field average rice yield in 2001 to that of 2003, the net increase in grain of 227 lbs/A would amount to a net return of $21.44/A. The cost of applying these PF technologies would be approximately $16.57/A. The Mississippi State University Extension Service (MSU-ES) recommends that when fields have been recently land leveled, that soil samples be randomly collected and composited based on whether the area has been 'cut' or 'filled.' If this method had been used, based on the soil samples that were collected from areas of 'cut' and 'fill', it is highly probable that a blanket application of 30 lbs/A of P2O5 would have been recommended. This would have cost $12.96/A, or $453.60 for the 35-acre field. That is less than the cost of the VRT-P treatment. However, studies by MSU scientists indicate that if P had been uniformly applied at the recommended rate, maximum rice yields would not have been obtained in the area of the field where soil test P was in the very low to low range. That theoretically would have resulted in a lower whole-field yield average. Conclusions Use of PF tools (i.e., DGPS yield monitors, GIS, grid soil sampling, and VTR), coupled with topography maps (i.e., "cut" and "fill" maps): 1) successfully defined management zones, 2) determined yield limiting factors, and 3) addressed one of the key limiting factors: inadequate P fertility. These tools decreased whole-field yield variability and increased total rice production. Although there was an added expense of applying P with VRT, this method was more agronomically appropriate. More precise application of P to areas of need helped to maximize yield and resulted in more consistent production of rice within management zones. Dr. Walker is assistant professor of agronomy at the Delta Research and Extension Center in Stoneville, Mississippi, Dr. Cox is associate professor, Dr. Kingery is professor, and Dr. Oldham is associate extension professor, all in the Department of Plant and Soil Sciences at MSU. Dr. Martin is associate extension professor of agricultural economics and Dr. Street is extension rice specialist at the Delta Research and Extension Center in Stoneville. Credit: Better Crops/Vol. 88 (2004, No. 1). Table 1. Crop Yield average and coefficient of variation (C.V.) over time. Year Crop Average, lbs/A C.V.% 2001 Rice 6,932 38.9 2002 Soybeans 2,662 23.6 2003 Rice 7,159 2.22
Fluid Journal 2002-2004
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