Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2005
Publication Date: 7/1/2005
Citation: Tewolde, H., Sistani, K.R., Rowe, D.E., Adeli, A., Tsegaye, T. 2005. Estimating cotton leaf area index nondestructively with a light sensor. Agronomy Journal. 97:1158-1163.
Interpretive Summary: Leaf area index which is a measure of the degree of ground cover by foliage is a key for analysis of crop productivity, water consumption, and management of weeds and other pests. But direct leaf area index measurement is labor-intensive, slow, and destructive. Availability of new instruments in recent years has allowed the measurement of leaf area index inexpensively, quickly, and nondestructively. At least three new instruments became available in the market for measuring leaf area index nondestructively, but the performance of these instruments has not been thoroughly documented. This work evaluated the accuracy and dependability of one of these instruments, AccuPAR, for estimating leaf area index of cotton fertilized with different rates of poultry litter. The research found that this instrument can easily and inexpensively measure leaf area index, but leaf area index measured by this instrument can easily depart from the actual if certain precautions are not followed. The results suggest that taking the measurements when the sun is directly overhead and using a probe that has the same length as the row spacing minimize over- or underestimation of actual leaf area index. Current guidelines in the Owner's Manual of the AccuPAR are inadequate for accurate measurements. Results of this research should, therefore, be useful for scientists and crop consultants that rely on this or similar other instruments for measuring leaf area index.
Technical Abstract: Direct measurement of leaf index (LAI) is labor-intensive, slow, and mostly intrusive. One of three instruments that became commercially available in recent years for measuring LAI indirectly has not sufficiently been evaluated. The objective of this test was to evaluate the accuracy and dependability of the AccuPAR for estimating LAI of cotton (Gossypium hirsutum L.) nondestructively. The test was conducted in 2002 and 2003 on commercial farms at Macon, Cruger, and Coffeeville, Mississippi, USA where poultry litter as an alternative cotton fertilizer and soil amendment was evaluated. Leaf area index was measured nondestructively with the AccuPAR and destructively by taking plant samples three to four times during each growing season at each location. Plant height and chlorophyll index were also measured on the same dates that LAI was measured. Regressing the LAI measured nondestructively by the AccuPAR (LAI2) on LAI measured destructively (LAId) after combining data from all three locations and all dates resulted in the equation LAIa=0.98LAId with r2=0.69. This relationship suggests that the AccuPAR can estimate LAI with acceptable accuracy. However, the meter often overestimated or underestimated LAI when the data for each date were evaluated separately. Most of the overestimation or underestimation seemed to be related to the time and method of measurement, the canopy structure, and, to some extent, to non-foliar plant parts and possibly the degree of greenness of the canopy. These results suggest that the AccuPAR can be a useful means of estimating LAI in row crops such as cotton if care can be taken to minimize overlap of row-to-row shading.