Photosynthesis and Nutritive Value in Leaves of Three Warm-Season Grasses before and after Defoliation

Variation in the nutritive value of forages with maturity has been extensively researched (Buxton and MerForage yields are influenced by plant response to defoliation. We tens, 1995; Sollenberger and Cherney, 1995) and shifts examined the photosynthesis and nutritive value of first (first leaves) in proportions of leaf and stem during maturation of and third (third leaves) fully expanded leaves (numbered from the the forage crop often explain variation in nutritive value apex) in three warm-season (C4) grasses. Net photosynthetic rates at uniform temperature and light both before and after a 2-wk exposure (Buxton and Mertens, 1995). Greater variation in yield to full sunlight and the effect of leaf position on nutritive value were than in nutritive value has been associated with environdetermined on vegetative tillers in well-established swards of bermental factors (Buxton and Casler, 1993). The nutritive mudagrass [Cynodon dactylon L. (Per.) cv. Tifton 44], caucasian bluevalue of individual leaves has received less research stem [Bothriochloa caucasica (Trin.) C.E. Hubb.], and Atlantic coastal emphasis but studies of leaf growth and chemistry can panicgrass [Panicum amarum var. amarulum (Hitchcock and Chase) contribute to understanding of forage growth (Volenec P.G. Palmer] growing on a Cecil clay loam (clayey, kaolinitic, thermic and Nelson, 1995; Macadam and Nelson, 2002). Typic Hapludults). Bermudagrass had the greatest level of crude proThe severity of defoliation in a grazing system could tein (CP), followed by panicgrass and bluestem. Fiber was greater in be managed to retain a portion of the older leaves lower the first leaves than in the third leaves for bermudagrass and panicgrass in the pasture canopy if they remain photosynthetically but not for bluestem. Photosynthetic rates of panicgrass and bluestem third leaves estimated 2 wk after defoliation of the surrounding canopy productive. Variation of nutritive value among canopy were less than estimates made before defoliation in the first leaves leaves may also impact the diet of the grazing animal as but were similar to the third leaves before canopy defoliation. The defoliation is managed since the uppermost and younger third leaves of bermudagrass 2 wk after defoliation had lesser photoportions of the canopy would be preferentially removed. synthetic rates per unit chlorophyll than the first or third leaves before The objectives of this study were to test for variation defoliation. Photosynthetic rates were correlated with hemicellulose in the relative responses found in the photosynthetic and across leaf classes and species (r 2 0.93). The photosynthetic decline respiration rates of bermudagrass, caucasian bluestem, observed in third leaves of bermudagrass compared with panicgrass and Atlantic coastal panicgrass first leaves and third and bluestem is evidence of variation in leaf response after defoliation leaves before and after a 2-wk exposure period to ambiamong warm-season grasses. ent sunlight following defoliation of the surrounding canopy. Comparisons of photosynthetic rates among the warm-season (C4) grasses were of less interest than the G canopies contain dynamic populations of relative response to the 2-wk exposure period. Nutritive leaves with growth and senescence taking place value of the first and third leaves was estimated as supsimultaneously. The youngest leaves are often near the porting data by measuring leaf IVDMD, CP, and Van top of the canopy and reach their maximum photosynSoest fiber fractions. thetic ability by full expansion (Jewiss, 1965; Murchie et al., 2002). Grazing often results in the selective removal of young leaf tissue and grazing systems can be MATERIALS AND METHODS managed to vary the residual leaf area. New leaf tissue Field Methods is formed in response to defoliation and some studies Three forage species (bermudagrass, caucasian bluestem, have reported increased photosynthetic rates in the reand Atlantic coastal panicgrass) were established a year before maining older leaves (McNaughton, 1983; Wallace et al., the research on a Cecil clay loam near Raleigh, NC. These 1984; Briske, 1986) but other studies have reported no three treatments were established in a randomized complete increase in net photosynthesis after defoliation (Detling block design with two replicates. The plots measured 25.6 by and Painter, 1983; Ryle and Powell, 1975). Murchie et al. 8.5 m and were sampled over 2 yr. Within these plots, we (2002), in a study with a C3 grass, suggested that leaf selected leaves to represent the leaf classes studied. age was dominant in limiting photosynthesis in the upPlots were burned in late winter and allowed to regrow beper portions of the canopy while acclimation to light fore sampling. Weeds were controlled using 2,4-D (2,4-dichlorophenoxyacetic acid) and Princep [2-chloro-4,6-bis (ethylamino)was dominant in the lower portions of the canopy. s-triazine]. Nitrogen was applied as ammonium nitrate at 90 kg ha 1 four times both years during the summer at 3to 4-wk M.H. Mehaffey, USEPA, Research Triangle Park, NC; D.S. Fisher, intervals beginning in early May (total N 360 kg ha 1 yr ). USDA-ARS, JPCSNRCC, 1420 Experiment Station Road, WatkinsPanicgrass and bluestem were defoliated to a 150-mm stubble ville, GA 30677; and J.C. Burns, USDA-ARS and North Carolina and bermudagrass was defoliated to a 100-mm stubble once State Univ., Raleigh, NC 27695-7620. Received 2 Mar. 2004. Forages. during the summer before heading. *Corresponding author ([email protected]). Data were collected from June through August in both Published in Agron. J. 97:755–759 (2005). doi:10.2134/agronj2004.0049 Abbreviations: ADF, acid detergent fiber; CP crude protein; IVDMD, in vitro dry matter disappearance; NDF, neutral detergent fiber; PAR, © American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA photosynthetically active radiation. 755 Published online April 27, 2005