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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Spring 1993 Table 2. Production characteristics at the Neraska MSEA site. Water Fertilizer Fertilizer Total N Grain Return*** Irrigation applied* N N cost** efficiency yield after fert system (in) (lb/acre) ($/acre) (%) (bu/acre) ($/acre) 1991 Conventional 37 180 $27.50 28 199 $470.50 Surge-Flow 18 110 $20.50 35 196 $469.50 Center-Pivot 13 30 $6.00 58 194 $479.00 1992 Conventional 29 159 $24.30 30 207 $493.20 Surge-Flow 9 65 $13.00 48 200 $487.00 Center-Pivot 8 42 $8.40 48 175 $429.10 * Growing season rainfall roraled 3 and 12 inches in 1991 and 1992, respectively. ** Nitrogen at $0.10/lb, plus $6.50/acre application cost; UAN in starter and for fertigation at $0.20/lb. *** Market value of grain at $2.50/bu minus fertilizer N expenses. N dynamics Three nearly square 33-acre irrigated corn fields with individual irrigation wells were established in 1990 to demonstrate the impact of the above cropping systems on ground water quality. The predominant soil type was a nearly level Hall silt loam. Each of the fields received a preplant application of 150 lb N/acre in 1990. Planted corn received 30 lb N/acre as a starter fertilizer. Flow meters were installed on the wells and fields were furrow irrigated according to traditional producer practices. Grain yields were similar for all three fields (averaging 196 bu/acre) and irrigation application ranged from 36 to 48 inches in 1990. After installation of the irrigation systems in the Spring of 1991 and before planting, soils from each field were sampled to a depth of 4 feet for residual soil N. Yield expectations for the fields were set at 200 bu/acre and used to calculate fertilizer N recommendations according to University of Nebraska procedures. The conventional cropping system is common to the area, which is under a N management program imposed on producers by the Central Platte Natural Resource District (CPNRD). Soil test data, fertilizer applications and nitrate contained in irrigation water for 1991 are shown in Table 1. Three adequately fertilized test strips (six rows wide receiving 150 lb/acre as sidedress N) were established in each field for comparison purposes to evaluate crop N status in the surge-flow and sprinkler irrigated fields. Chlorophyll meters (SPAD 502 manufactured by Minolta Corp.) were used to routinely monitor crop N status of fields on a weekly basis by comparing meter readings from the bulk field with those from adequately fertilized test strips. An N sufficiency index of 95% was established as the threshold level to trigger fertigation (20 to 30 lb N/acre as UAN). Comparison data from the sprinkler irrigated field never indicated crop N stress, therefore no fertilizer N was applied via fertigation in 1991. Similarly, chlorophyll meter data collected from the surge-flow irrigated field never indicated an N stress. Provisions and procedures were not available to fertigate the surge-flow irrigated field in 1991. These two improved N and water management strategies were able to maintain productivity in 1991 without statistically reducing yields (Table 2). Slight apparent yield reductions for the surge-flow and sprinkler irrigated fields are attributed to minor equipment problems resulting in delayed application of the first irrigation in late June when climactic conditions were unusually hot and dry. Grain yields in 1992 were slightly lower (approximately 3%) under surge- flow irrigation than under conventional practices (Table 2). Yields determined at 12-row intervals across both fields indicated that the reduction occurred in the area of the surge-flow irrigated field where laser grading removed up to 8 cm of topsoil in the fall of 1990. These areas were visible in aerial photographs taken at silking, but only one of the three producer test strips that received extra fertilizer N could be detected in the photo. Nevertheless, average yield from the three test strips in the surge- flow irrigated field was 206 bu/acre, compared to 209 bu/acre for adjacent strips that only received starter fertilizer and fertigation applied N (Table 1). Yield reductions of 15% under the sprinkler irrigation strategy in 1992 are attributed to 1) lower levels of residual soil N in the spring compared to the surge-flow irrigated field and 2) a lower rate of starter fertilizer application than in 1991 (Table 1). Inadequate N availability during the rapid growth stages (four weeks prior to silking) could also contribute to the lower yield under sprinkler irrigation in 1992. However, chlorophyll meter data indicated that the average crop N sufficiency level was never lower than 95%, compared to the adequately fertilized reference strips, except just prior to the single fertigation application. Yield reductions under the sprinkler system in 1992 suggest an apparent failure of the chlorophyll meter strategy to schedule fertigation, but aerial photographs showed the problem was with locating the adequately fertilized test strips. Unfortunately, one of the old 1991 test strips confounded interpretation of the 1992 chlorophyll meter data, which resulted in the erroneous decision to limit fertigation. Yields and grain protein contents (averaged 8.3%) were similar for the three fields in 1991, resulting in grain N removal of approximately 124 lb N/ acre. Grain N removal averaged 123, 117 and 90 lb N/acre in 1992 for the conventional, surge-flow and sprinkler irrigated fields, respectively. Comparing grain N removal with the sum of N availability for each irrigation
Fluid Journal 1996-1998