CHANGES IN RESPONSE INDICES AS A FUNCTION OF TIME IN WINTER WHEAT

Abstract

Nitrogen (N) responsiveness of crops can change with time as it is strongly influenced by in-season environmental conditions. This study was conducted to determine the relationship of N responsiveness using a response index (RI) as a function of time at five locations (Efaw, Stillwater, Lake Carl Blackwell, Perkins and Lahoma, Oklahoma) over a three-year period. Subplots of 4 m² were established at each experimental site that employed a randomized complete block design. Normalized Difference Vegetation Index (NDVI) readings were taken using a Greenseeker (NTech Industries, Inc., Ukiah, CA, USA) handheld sensor at various growth stages. The N responsiveness (RI_NDVI) was determined as the ratio of NDVI readings from a non-N limiting strip and the farmer practice. Then, RI was plotted against days where growing degree days (GDD = (T_min + T_max)/2—4.4°C) were > zero (DGDD > 0). At all sites, RI_NDVI increased with advancing stage of growth. Excluding Perkins 2005 and Stillwater 2006, the relationship between RI_NDVI and DGDD > 0 was positive and highly correlated. When the number of days from planting to sensing where DGDD > 0 was less than 60, it is unlikely that a reliable estimate of RI_NDVI could be obtained since values were all small (close to 1.0), consistent with limited growth at the early stages of growth. Averaged over years and sites for all growth stages, the correlation of RI_NDVI and RI_Harvest was positive and increased up to the Feekes 9 growth stage. Our results further suggested that once RI_NDVI is collected, it should be adjusted using the Equation RI_NDVIadj = RI_NDVI × [1.87/(DGDD > 0 ∗ 0.00997) + 0.5876].

Keywords:

• winter wheat
• normalized difference vegetation index
• response index

Notes

^† no P or K applied.