Granular applicators equipped with variable-rate technology (VRT) have gained popularity in recent years as
a result of increased interest in variable-rate application. The purpose of this investigation was to characterize distribution
patterns at varying rates for different granular applicators. Uniform-rate (UR) tests were conducted to assess the accuracy
of variable-rate application from four granular applicators: two spinner-disc spreaders (A and B), and two pneumatic
applicators (C and D). Pattern results indicated a consistent triangular pattern for spinner spreader B and consistent patterns
for the pneumatic applicators (C and D). However, applicator D produced pattern variations at the center and right side.
Simulated overlap analysis generated CVs <20% for applicators B and C. Applicator A performed well at the two lower rates
(CVs <19%) but not at the highest rate (CV = 27%). Pattern unevenness for applicator D produced CVs between 25% and
34%. The spinner-disc spreaders over-applied, while the pneumatic applicators under-applied at the margins, suggesting an
adjustment to the effective swath spacing or spinner-disc speed is needed to improve application accuracy. Further, overlap
plots indicated pattern variability even when acceptable CVs were attained for applicators B and C. Therefore, it is
recommended that CVs accompany simulated overlap pattern plots to ensure proper calibration of VRT equipment. Swath
spacing analysis indicated that three of the four applicator spacings could be changed from the recommended value to improve
application uniformity. Pattern comparisons showed that pattern shifts occurred for applicator A (P = 0.0092) with increasing
application rate but not for applicators B, C, and D. These results demonstrate potential application errors with VRT and
the need for proper calibration to maintain acceptable performance. Further, this investigation demonstrates the need for
a VRT equipment testing standard.
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