Corn Hybrid Growth Stage Influence on Crop Reflectance Sensing

Abstract

Active-light crop canopy sensing for corn (Zea mays L.) N fertilizer rate decisions typically include measurements of N-suffi cient plants as a reference. When producers use multiple hybrids in one fi eld, the question is raised of whether an N-suffi cient reference is needed for each hybrid. Th e objective of this research was to assess the impact of suffi ciently N-fertilized, similar-maturing corn hybrids on crop-refl ectance measurements. Eleven similar-maturing hybrids were selected from three sites in 2008 and eight hybrids from two sites in 2009. When the corn was about 10 cm tall and on 3 to 5 d intervals canopy refl ectance, leaf chlorophyll, and plant height measurements were obtained. Results were classed into two growth periods based on crop height: 20 to 70 cm and 71 to 120 cm. In three of the four growing periods assessed corn hybrid had no signifi cant eff ect on refl ectance. In 2008, refl ectance for corn 71 to 120 cm tall was aff ected by hybrid; however the eff ect was minor leading to an average N rate recommendation diff erence of 10 kg ha -1 . More pronounced diff erences occurred among hybrids for leaf chlorophyll measurements, resulting in diff erences in N fertilizer recommendations of 55 kg ha -1 . Refl ectance diff erences among similar maturing hybrids would have minimal impact on N fertilizer recommendations. Models were also developed to represent typical upper and lower values for various vegetative indices as a function of corn height. Th ese models can help guard against using questionable data when assessing N-suffi cient corn.

Winter Wheat Genotype Effect on Canopy Reflectance: Implications for Using NDVI for In‐Season Nitrogen Topdressing Recommendations

“…Therefore, when reference strips are used to calibrate sensors, a separate reference strip should be used for each hybrid. Sheridan et al (2012) found that reflectance differences among corn hybrids had a minimal impact on N fertilizer recommendations. On the other hand, Jasper et al (2006) observed that winter wheat inter‐cultivar differences in amber NDVI translates to a misdiagnosis of N uptake by up to 20 kg ha −1 .…”

Model and Sensor‐Based Recommendation Approaches for In‐Season Nitrogen Management in Corn

“…Plant population has the potential to affect crop canopy sensor readings and consequently N recommendation rates. However, previous work showed that reflectance differences among hybrids had minimal impact on fertilizer N recommendations (Sheridan et al, 2012), therefore having a reference strip of the same hybrid may not be critical. With this research we explored whether reference strips of differing plant populations are important in the determination of final in‐season N recommendation.…”

Algorithms for In‐Season Nutrient Management in Cereals

“…It should be noted that ISR index values decrease with increased plant growth, while NDVI increases with plant growth. This inverse relationship has been illustrated (Kitchen et al, 2010;Sheridan et al, 2012).…”

“…Additionally, these N-sufficient reference areas should avoid areas that are unique or historically have had other management problems (e.g., heavy weed infestation, head-lands with soil compaction, terraces, manure history). While different maize hybrids can have an effect on reflectance measurements of N-sufficient plants, the impact is minimal and will not greatly impact algorithm N rate recommendations (Sheridan et al, 2012). However, experience working with producers has demonstrated that other soil and crop management factors may cause variations in reflectance, so producers are encouraged to have a sufficient-N reference area for each field.…”

“…Experience on producers' fields with poor plant stands, even in a small portion of the sufficient-N reference area, will likely result in underestimation of N maize requirements. To guard against this happening, upper ISR values as a function of growth stage for the sufficient-N reference is recommended (Sheridan et al, 2012).…”