1. Development of a Response Index for Corn
Robert Mullen
Ohio State University

2. Response Index
RI first proposed by Johnson and Raun, (inverse of sufficiency index)
RI = Yield of N Rich / Yield of Check (0N)
Direct measure of N response

3. Response Index
Can we reliably predict the response to applied N during the growing season?
Environmentally and economically important
Less N subject to denitrification, leaching, and runoff
Produce more with less inputs
Is the relationship stable at different stages of growth?

4. Response Index
In-season estimates of RI were used to identify RI at harvest for winter wheat

5. Response Index Would RI for corn work as well?
Can RI be identified at early stages of growth (< V8)?

6. Response Index Varvel et al., 1997
Used SPAD meters to identify N response at various stages of growth
V6 – R3
Relationship has been established between GNDVI and SPAD meter reading (Shanahan et al., 2003)

7. Can RI work for corn?
Varvel data shows that pseudo-RI can be determined at various stages of growth
Very little data for some stages of growth
Lack of response at most locations (especially where corn followed soybeans)

8. Can RI be estimated at early stages of growth?
Red NDVI determined significant difference between N rate at V4
N rate
RNDVI
0.209
0.204 60
0.234
0.249
120
0.247
0.239
180
0.263
0.244
Linear
P=0.0149
P=0.0319

9. Can RI be estimated at early stages of growth?
Green and red NDVI determined differences between treatments at V6
N rate
RNDVI
GNDVI
0.531
0.437 60
0.654
0.530
120
0.664
0.546
180
0.678
0.540
Linear
P=0.0001

10. Discussion Practicality of N rich strip
Production scale agriculture without sophisticated sensors
Answer N loss questions and helps make N application decisions

11. Discussion The earlier, the better
Determination of RI early in the season would be better
Lack of high-clearance application equipment
Increase in application window

12. Conclusions?
Continue research to identify relationship between RI-NDVI and RI-Harvest for corn.
“Calibrate” RI at various growth stages with N application rates if yield cannot be predicted accurately (this will be difficult).
Encourage use of N rich strips in production fields to answer simple N application questions.

13. Questions?