|10-679MS Project Manager: E. M. Barnes|
COTTON YIELD RESPONSE TO RESIDUAL N AND K LEVELS AND CROP REFLECTANCE BASED FERTILIZER NITROGEN MANAGEMENT
M. Wayne Ebelhar and Jac J. Varco, Mississippi State University
Managing soil variability is a complex issue in cotton production, especially in dealing with N fertilization. Technologies including GIS, GPS, soil conductivity, yield monitoring, and remote sensing can be used to map and establish spatial variances within fields as well as provide a basis for spatial management. Crop growth is a result of an integration of all factors influencing it, including the size of the available N pool and water relations. Within-field differences in crop reflectance are an indication of actual field variability in cotton growth less variances caused by human error. Crop reflectance based variable rate N fertilization has the potential to improve profit maximization through improved N-use efficiency, reductions in spatial variability in growth, and reductions in year-to-year carryover of soil nitrate.
Crop rotation can also lead to enhanced growth while unused fertilizer may have carryover effects that lead to environmental issues later. With fertilizer rates above recommended levels or yields below expectation, the potential for unused fertilizer to remain in the system is present. Environmental components such as temperature and rainfall regulate biological activity and biological transformations in the soil system. High rainfall and warm temperatures outside the growing season or even in the growing season provide the means for N losses through denitrification.
This project has two distinct objectives with the first to compare sensor derived vegetative indices for their capability in monitoring plant height and leaf tissue N followed by direct fertilizer application based on the modified vegetative index Canopy Chlorophyll Content Index (CCCI) and the second objective to evaluate the carryover effects of plant nutrients applied in corn for cotton in the following year in the rotation. To address the first objective, initially cotton was grown at the Plant Science Research Center, Mississippi State, MS, to develop techniques and algorithms. Sensor readings for leaf reflectance were taken through the season with a Hydro® (now YARA) N sensor mounted on a three-point tractor hitch and adjusted to a height of 76 inches and run at a speed near 3.6 mph was used to collect crop reflectance across the whole length of each plot. Vegetative indices calculated included Normalized Difference Vegetation Index (NDVI) and the Canopy Chlorophyll Content Index (CCCI). In 2012, the CCCI was compared to a constant rate and a producer applied variable rate based on soil test CEC values. These systems were then used to determine the effects on cotton leaf N values and lint yield. In 2012, the cotton variety ST5288 B2F was planted in a producer's field (approximately 50 acres) south of Natchez, MS. Based on a rigorous data base model relating cotton N response with various vegetative indices at various growth stages, reflectance data was converted to the CCCI index and a fertilizer application map was generated using the model with some grower adjustments. Other treatments included a constant 90 lb. N/acre rate and a farmer represented variable rate based on soil CEC.
To address the second objective, a multi-year field study has been initiated at the Delta Research and Extension Center (DREC) near Stoneville, MS, to evaluate N management for corn. Various N rates (140, 180, 220, and 260 lb. N/acre) and application schedules (combinations of preplant and sidedress applications) from 100% preplant (PP) to 100% sidedress (SD) were evaluated in a randomized complete block design with eight replications through the 2011 growing season and rotated to cotton in 2012 (last year of the study). In 2012, cotton was planted as a rotation crop following the 2011 corn crop. A uniform N rate (lower than needed for optimum production) of 90 lb. N/acre was applied 25 April 2012 across the field and cotton managed as a uniform crop and thus treated the same across the field.
For the sensor-based N study, the farmer applied variable rate treatment based on soil CEC averaged 98 lb. N/acre and CCCI sensor based was 77 lb. N/acre. Leaf N analyses for CCCI sensor based applications were generally lower than either the constant N rate of 90 versus the CEC derived variable rate, but were still above what would be considered adequate. This indicates that even with 13 to 21 lb./acre less fertilizer N applied compared to the other two treatments, there was adequate available N for the cotton crop. Given that leaf N values were elevated above what would be considered adequate even at reduced fertilizer N rates with the sensor derived treatment, the results suggest that the average CCCI sensor based N rate could have been reduced even further. Cotton yields based on harvested transects were 922 lb./acre with 90 lb. N/acre, 911 lb./acre with variable based on soil CEC (98 lb. N/acre average), and 971 lb./acre for variable rate CCCI sensor based (77 lb. N/acre average). These results demonstrate, that sensor driven variable rate fertilizer application using a vegetative index with known sensitivity to chlorophyll and tissue N levels could improve N use efficiency for spatially variable alluvial soils.
In the rotation study, the cotton crop was grown with a single uniform N rate (90 lb. N/acre) during the 2012 growing season and all cultural practices remained constant. Grain yields in 2011 were extremely low compared to the previous years due to lack of rainfall and mistimed irrigation along with an inability to keep up with irrigation in 2011. The cotton crop in 2012 received 90 lb. N/acre prior to planting and no sidedress N during the growing season. Lint cotton yields were similar to those measured in 2011 with all yields above 1,000 lb. lint/acre. There were no individual treatment differences with respect to N management on the corn crop and no interaction between the N rates or application ratios. When averaged across the application management component, cotton yields were significantly higher (50 to 65 lb. lint/acre) where at least 180 lb. N/acre was applied to corn compared to the 140 lb./acre N rate. With the rainfall that was received in December, it was hypothesized that there would be little residual N remaining; however, the lint yields do reflect residual N. After four years, we have seen indications that residual N is remaining following excess N applications to corn in a corn/cotton rotation. In the early years of this evaluation (prior to the most recent objectives), very little response to the higher N rates in corn had been observed. This leads us to conclude that with below normal rainfall, denitification has not occurred at sufficient levels to remove all of the applied N from the system. It was thought that later applications of N (100% Sidedress N), may also have a more pronounced effect on residual N; however, this has not been the effect in 2012 or in any of the previous years. As corn has been brought back into a cotton/corn rotation system, the total N has gone up with as much as twice the N used in corn as in cotton. With high yields, there is much more nutrient removal than with the mono-crop culture.
|Project Year: 2012|
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