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IV SIMPOSIO INTERNACIONAL DE PASTIZALES San Luis Potosí, S.L.P. México; 22 al 24 de agosto de 2007 Artículos in extenso Área temática: Productividad Table 1. Seasons of NDVI measurements and forage harvest in pastures of the Eastern Research Station, near Haskell, OK. Season/Year † Pasture Spring 2004-2005 1 Spring 2004-2005 2 Summer 2004-2005 1 Summer 2004-2005 2 Summer 2004-2005 3 Summer 2004-2005 4 Fall 2005-2006 1 Fall 2005-2006 2 Fall 2005-2006 3 Fall 2005-2006 4 Spring 2005-2006 1 Spring 2005-2006 2 Spring 2005-2006 3 Spring 2005-2006 4 Summer 2005-2006 1 Summer 2005-2006 2 † Two measurements in spring 2004-2005 and two in the summer of 2005-2006 are missing because scanning of pastures started after two pastures had been already grazed and because of malfunction of the sensor, respectively. Clippings were made with a flail harvester to a stubble height of 10 cm during the period of NDVI readings. Fresh forage was weighed in the field and subsamples of approximately 0.5 kg from each plot were weighed and dried to determine the percent dry matter. Dry matter yields per unit area were calculated. Forage yields reported are the average of five plots (microreliefs) per fertilizer strip. Correlation coefficients (r) along with associated probability levels were obtained for all pairs of yield-NDVI measurements for each season from spring 2004-2005 until the summer 2005-2006 (PROC CORR, SAS Institute, 2003). Number of pairs of observations per season was not equal throughout this research because of measurements in the spring of 2004-2005 started after two of the pastures had been already grazed and in the summer of 2005-2006 ten observations were missing because of malfunction of the sensor. RESULTS AND DISCUSSION The relationship between forage yield and NDVI was inconsistent (Table 2). In four of five seasons NDVI was significantly related with forage yield but from the four significant, the last two had extremely low forage yield levels and had opposite sign relations, negative in the spring, and positive in the summer. From these two seasons it is suggested that the relationship of yield to NDVI under such of low yield levels, seems to be random. Taylor et al. (1998) reported that under low forage yields, correlation coefficients between various radiance light spectrums (including NDVI) with forage yields, decreased dramatically compared with those of high yields. 186
IV SIMPOSIO INTERNACIONAL DE PASTIZALES San Luis Potosí, S.L.P. México; 22 al 24 de agosto de 2007 Artículos in extenso Área temática: Productividad On the other hand, there is a documented weakness of NDVI to relate with vegetative biomass under full canopy coverage and/or with high forage yields (Carlson and Ripley, 1997; Serrano et al., 2000; Aparicio et al., 2000). In agreement with these reports, our results showed a loss of sensitivity of NDVI under high production levels, as observed during the summer of 2004-2005. In this season r was 0.68 (P < 0.001); however, a substantial increase in r was observed during the fall of 2005-2006 (r = 0.76, P < 0.001), when much lower forage yields were recorded. Table 2. Mean yields, NDVI values, coefficients of correlation, probability levels, and number of pairs of observations of five seasons during two years in pastures of the Eastern Research Station, near Haskell, OK. Season/Year Yield (Mg ha -1 ) NDVI r P n Spring 2004-2005 1.19 0.73 0.49 0.214 8 Summer 2004-2005 5.23 0.67 0.68
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