Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/26/2011
Publication Date: 12/12/2011
Citation: Sheridan, A.H., Kitchen, N.R., Sudduth, K.A., Drummond, S.T. 2011. Corn hybrid growth stage influence on crop reflectance sensing. Agronomy Journal. 104:158-164. Interpretive Summary: Nitrogen (N) management in grain crop production has been the subject of considerable research for decades because soils don’t typically provide enough N to achieve optimal crop growth without added fertilizer. However, over-applying N fertilizer results in unused N moving off fields into rivers, lakes, and oceans, causing environmental problems. Crop reflectance sensors can help diagnose crop N need and more precisely determine the amount of N fertilizer to apply. Use of these sensors typically requires farmers to pre-treat a small area within each field with sufficient N fertilizer so that there is no N stress when crop sensing is done. A ratio of sensor readings from this N-sufficient area to sensor readings from the rest of the field is the basis for calculating how much N to apply. Questions have been raised about using this ratio approach when multiple hybrids are planted within a field, which is a typical farming practice. The specific question asked is “Does each hybrid planted within a field require its own N-sufficient reference area?” The objective of this research was to assess how similar-maturing corn hybrids affected crop-reflectance measurements. In three of the four growing periods we assessed, corn hybrid had no effect on reflectance measurements. In the growing period that hybrids were found different, the variation was small and would on average only alter N application by about 5 to 8 percent. The biggest difference between hybrids would result in an N rate change of about 20 percent. We concluded that reflectance differences among similar maturing hybrids would have minimal impact on N fertilizer recommendations. Using this data, we also developed models to represent typical upper and lower values of reflectance as a function of corn height. These models can help guard against using questionable data when measuring N-sufficient corn. The results of this study will help farmers more reliably use reflectance sensor technology for applying the right rate of N fertilizer to corn. When fertilizer rates better match crop needs, N fertilizer loss to the environment will be reduced.
Technical Abstract: Active-light crop canopy sensing for making corn (Zea mays L.) N fertilizer rate decisions typically include measurements of N-sufficient corn as a reference. When producers use multiple hybrids in one field the question is whether an N-sufficient reference is needed for each hybrid. The objective of this research was to assess during mid-vegetative growth stages the impact of sufficiently N-fertilized, similar-maturing corn hybrids on active crop-reflectance measurements. Eleven similar-maturing (114 day maturity plus or minus 1 day) hybrids were selected from three sites in 2008 and eight hybrids from two sites in 2009 out of the University of Missouri Variety Trial Testing Program. Starting when the corn was about 10-cm tall and on 3-5 day intervals through late vegetative growth stages measurements of canopy reflectance, leaf chlorophyll, and plant height were obtained. Results were classed into two growth periods based on crop height: 20-70 cm and 71-120 cm. In three of the four growing periods assessed (two periods for each year) corn hybrid had no significant effect on reflectance measurements. In 2008 for corn 71-120 cm tall reflectance was affected by hybrid, however the effect was minor if the intent was to use reflectance measurements for N fertilizer recommendations. Averaged, the effect of hybrid would result in N rate recommendation differences of 11 kg/ha, with the greatest difference about 40 kg/ha. More pronounced hybrid differences were observed with leaf chlorophyll sensor measurements, which would result in much larger differences in N fertilizer rate recommendations. We conclude that reflectance differences among similar maturing hybrids would have minimal impact on N fertilizer recommendations. Models were also developed to represent typical upper and lower values for several different vegetative indices as a function of corn height. These models can help guard against using questionable data when assessing N-sufficient corn for N fertilizer calculations.