NOTES & UNIQUE PHENOMENA Switchgrass Morphological Development Predicted from Day of the Year or Degree Day Models Matt A. Sanderson* and Kenneth J. Moore
ABSTRACT
ronments. T h e data were collected from different environments and with different switchgrass varieties than were used in t h e study of Mitchell e t al. (1997).
We tested recently published linear equations for predicting developmental morphology of warm-season perennial grasses on three cultivars of switchgrass (Panicum virgafum L.). Mean stage count (MSC, based on the Nebraska system) of ‘Cave-in-Rock’and ‘Kanlow’ switchgrass was measured at Ames, IA, and MSC of Cave-in-Rock and ‘Alamo’ switchgrass was measured at Stephenville, TX, during 1995. Measured MSC was compared with MSC estimated from day of the year (DOY) or growing degree day (GDD, base of 10°C) equations. The DOY equation more closely estimated MSC for switchgrass grown at Ames than at Stephenville. The GDD equation did not work well for any cultivar at either location. The equations did not work as well in Texas, probably because of genotypic interactions with daylength and climate for developmental morphology. These results indicate that the DOY equation may be useful with some varieties of switchgrass in the central Great Plains.
MATERIALS AND METHODS The morphological development of Alamo and Cave-inRock switchgrass was measured from March to August 1995 at Stephenville, TX (98”12‘ W, 32’13’ N), and from May to August 1995 for Kanlow and Cave-in-Rock switchgrass at Ames, IA (93’56’ W, 42”33’ N). Switchgrass plots (3 by 6 m) at Stephenville were established in 1992 and at Ames (0.76 by 3.65 m) in May 1990. Soil at Stephenville is a Windthorst fine sandy loam (fine, mixed, thermic Udic Paleustalfs) and at Ames is a Webster silty clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquolls). Nitrogen (as NH4N03)was applied at a rate of 134 kg ha-’ on 11 April at Stephenville and 15 May at Ames. Thirty tillers were marked with colored wire in each of two plots of each cultivar at Stephenville and Ames at the start of the experiment. One tiller was selected at random from each of 30 plants. The developmental stage of the marked tillers was scored according to the Nebraska system (Moore et al., 1991). At Stephenville, measurements began on 22 March and continued until 6 July for Cave-inRock and until 28 August for Alamo. Tillers were scored twice weekly for 6 wk, then weekly for the remainder of the season. At Ames, the sampling dates were 16 May to 31 August. Tiller mortality was about 50% for each cultivar at Stephenville during the sampling period. As marked tillers died, new tillers of approximately the same developmental stage were marked. Total tiller mortality for the year at Ames was 40% for Cavein-Rock and 30% for Kanlow. Dead tillers were not replaced at Ames. A mean stage (MSC, an arithmetic average of stages) was calculated for the 30 tillers in each plot at each date (Moore et al., 1991). To calculate individual stages for the Nebraska scale, an N-value of 4 was used for the leaf (V) stages for both cultivars and an N of 6 (Cave-in-Rock) or 8 (Alamo) for calculating elongation (E) stages. These were the maximum values that occurred in each of the developmental phases. The experimental design at each location was a randomized complete block design, with two blocks. The linear DOY and GDD prediction equations from Mitchell et al. (1997, p. 830, Table 1) were used to estimate developmental stages for each location: MSC = -1.399 + 0.021(DOY); MSC = 0.875 + 0.0017(GDD). Growing degree days were calculated as [(maximum daily temperature + minimum daily temperature)/2] - 10. Positive GDD were summed beginning 1January (R.B. Mitchell, personal communication, 1998). Estimated MSC was regressed against measured MSC to determine the accuracy and precision of the two models. Regressing estimated values on observed values should yield
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of how forage grasses develop morphologically is essential in their management. Systems have been developed for quantifying t h e developmental stages of cool-season (Haun, 1973; Simon and Park, 1983) and warm-season (Moore e t al., 1991; Sanderson, 1992; West, 1990) grasses. A common goal of many of these studies has been to develop quantitative relationships between developmental stage a n d time, whether measured in days o r as thermal time (degree days), as a tool for estimating plant development from readily available information. Sanderson and Wolf (1995) reported a close association between developmental morphology of switchgrass a n d day of year or growing degree days (base = 10OC). Recently, Mitchell e t al. (1997) developed a set of equations f o r predicting t h e developmental stage [as m e a n stage count, MSC, determined by t h e Nebraska system (Moore et al., 1991)] of switchgrass from DOY a n d GDD. T h e equation based on DOY performed NOWLEDGE
better (higher r2 and lower root mean square error) than one based on GDD when tested at Mead, NE, and Manhattan, KS. Mitchell et al. concluded (p. 831) that “...switchgrass management recommendations for adapted cultivars may be based on DOY in t h e central Great Plains.” In this note, we report a test of t h e DOY a n d GDD equations for predicting t h e morphological development of two cultivars of switchgrass in two enviM.A. Sanderson, USDA-ARS Pasture Systems and Watershed Management Res. Lab., Curtin Road, University Park, PA 16802-3702; K.J. Moore, Dep. of Agron., Iowa State Univ., 50011. Received 23 Feb. 1998. *Corresponding author (
[email protected]).
Abbreviations: DOY, day of the year; GDD, growing degree days; MSC, mean stage count; RMSE, root mean square error.
Published in Agron. J. 91:732-734 (1999).
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SANDERSON & MOORE: PREDICTING SWITCHGRASS MORPHOLOGICAL DEVELOPMENT
an intercept of zero and a regression coefficient (slope) of one if the prediction equation is perfect. If the intercept and/ or the slope differ significantly from either condition, the prediction equation is biased. The RMSE is an estimate of prediction error and, along with the coefficient of determination (r’), indicates goodness of fit of estimated with observed values. If the intercept was not significantly different (P > 0.05) from
zero, then the regression was forced through the origin.
RESULTS AND DISCUSSION At Ames, the D O Y equation estimated the development of Cave-in-Rock very well at MSC greater than 2.0 (Fig. 1). The estimated points in this region fell squarely on the one-to-one line. Mean stage count of 2.0 is defined as the onset of stem elongation, whereas MSC values between 1.0 and 2.0 indicate leaf development (Moore et al., 1991). Below MSC 2.0, the equation overestimated MSC, which caused the regression line to deviate from a slope of 1.0. With the G D D equation, however, only those points around MSC of 1.0 were accurately estimated; values above 1.0 were underestimated. Mitchell et al. (1997) also observed that the D O Y equation overestimated MSC (slope = 0.826) at stages less than 2.0. Our data included some values lower than those observed by Mitchell et ai. (1997) and show a greater deviation at lower stage numbers than in the
original study. Sanderson and Wolf (1995) also noted greater variation in the relationship between developmental stage of switchgrass and D O Y at leaf development stages in Texas. The D O Y equation consistently overestimated MSC of Kanlow by about 0.35 mean stage units. As with Cave-in-Rock, the G D D underestimated MSC greater than 1.0, but was accurate at about Stage 1.0. The intercept for the G D D equation of both cultivars was not significantly different from zero, and so the regression was forced through the origin. Regression slopes, however, were significantly less than one. For both cultivars in Iowa, slopes and intercepts for the D O Y equation were significantly different from one and zero, respectively. At Stephenville, the D O Y equation consistently underestimated the MSC of Cave-in-Rock by an average of 0.77 stage units (Fig. 2). The D O Y equation was not able to accurately estimate the MSC of Alamo. The G D D equation performed better than the D O Y equation in estimating the development of Cave-inRock; however, it was still inaccurate. All regressions with the Texas data set had intercepts significantly ( P < 0.05) different from zero and slopes significantly different from 1.0. The root mean square error (RMSE) obtained for our regressions ranged from 0.08 to 0.17 for the D O Y 5
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Stephenville, Texas Cave-in-Rock Switchgrass
Ames, IA Cave-in-Rock Switchgrass 4
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DOY equation
