How Does Plant Population Density Affect the Yield, Quality, and Canopy of Native Bluestem (Andropogon spp.) Forage?

The density at which a crop is grown is known to affect its growth and quality, but little is known about how plant density affects the growth of native perennial forage grasses. The objectives were to investigate the effects of plant population density on the forage yield, forage quality, and plant canopy structure of two native bluestem species. Big bluestem (Andropogon gerardii Vitman) and sand bluestem (A. hallii Hack.) were established at six plant densities (1.2, 1.8, 2.7, 3.6, 5.4, and 10.8 plants m) in a split plot experiment with whole plots in four randomized blocks. Plant density was the main plot and bluestem species was the split plot. Data were analyzed with a mixed model analysis of variance with blocks within year as random effects and year as a repeated measure. The optimum plant density for forage production was between 3.6 and 5.4 plants m. However, the optimum for crude protein (CP) concentration occurred at 1.2 and 10.8 plants m and followed a quadratic response as plant density increased. The greatest leaf area index (LAI; leaf area per square meter) was at 10.8 plants m. At 10.8 plants m, the average yield loss from maximum was about 8.5% dry matter (DM). Also, the CP concentration was 8.5% greater for plants grown at 10.8 plants m than for those grown at 3.6 to 5.4 plants m. A plant density of 10.8 plants m would produce high quality forage with only slight reductions in DM yield as compared with plants grown at 3.6 to 5.4 plants m. AGREAT DEAL is known about the effects of plant population density on the yield of horticultural and field crops (Boquet, 1990; Brown et al., 1970; Cuomo et al., 1998; Cusicanqui and Lauer, 1999; Lauer, 1995; Lege et al., 1993; Robinson and Nel, 1989; Wade and Douglas, 1990; Wade et al., 1988; Weerasinghe and Fordham, 1994; Widders and Price, 1989; Wilson and Dixon, 1988). However, plant density effects on native forage species is less well defined (Bolger and Meyer, 1983; Cooksley and Goward, 1988; Graybill et al., 1991; Jefferson and Kielly, 1998; Pinter et al., 1994; Sanderson andReed, 2000; Springer et al., 2003). For field crops such as corn (Zea mays L.), sorghum (Sorghum spp.), and soybean [Glycine max (L.) Merr.], grain yield can be maximized by adjusting the seeding rates to match the moisture conditions of the environment, that is, densely populated stands utilize moisture and nutrients more quickly than sparsely populated stands (Jones and Johnson, 1991; Sanderson et al., 1996). Plant morphology can affect plant density. Skalova and Krahulec (1992) found that tiller numbers of Festuca rubra L. decreased as plant density increased. Similarly, Hiernaux et al. (1994) found that the main purpose of tillering was to compensate for low plant density that resulted from drought or intense grazing. Information on the effects of plant density on forage quality and feed value is limited to tropical forage corn or forage sorghums (Cuomo et al., 1998; Cusicanqui and Lauer, 1999; Pinter et al., 1994; Sanderson et al., 1996). These studies found little or no response of forage quality or feed value to plant population density. Understanding plant growth and development and forage quality of native warm-season grasses as they relate to the population density at which these grasses are grown will allow producers to improve the management and utilization of these forages. Our objectives were to investigate the effects of different plant population densities of two native bluestem (Andropogon spp.) species on forage yield, forage quality, and plant canopy structure. MATERIALS AND METHODS This study was conducted at the USDA-ARS, Southern Plains Range Research Station, Woodward, OK (368259 N, 998249W, elevation 586 m) on an Eda loamy fine sand (mixed, thermic Lamellic Ustipsamment). In May 2000, two native bluestem species, big bluestem cv. Kaw and sand bluestem cv. Chet were transplanted at six plant population densities in a split plot experiment with whole plots in four randomized blocks. Plant density was the main plot and bluestem species was the split plot. Variable plot sizes were used to obtain the desired plant population densities. The treatments consisted of six population densities representing 1.2, 1.8, 2.7, 3.6, 5.4, and 10.8 plants m. The actual plot dimensions, plant spacing within plot, number of plants per plot, harvested area, and number of plants harvested per plot are given in Table 1. During the establishment year, plots were maintained weedfree by hoeing, and dead plants were replaced to maintain the correct population densities. In subsequent years (2001–2003), plots were burned in March and atrazine [2-chloro-4-ethylamino-6-isopropylamino-s-triazine] was applied 7 to 14 d later for weed control at 1.68 kg a.i. ha. Nitrogen was applied in the form of urea (46–0–0) at 70 kg N ha in April each year of the study. Plant canopy height was measured before harvest each year by placing a meter stick near the center of each plot through the forage to the soil surface and reading the meter stick directly. Ten culms were randomly chosen from each plot and the number of leaves was counted for each culm and the leaf area for each culm was determined by passing its leaves through a LI-COR LI-3100C area meter (LI-COR Biosciences, Lincoln, NE). After forage harvest, the number of culms per plant was counted for five plants per plot. Culm density was calculated by multiplying the average number of culms per plant times the plant density per square meter. Leaf density was calculated USDA-ARS, Southern Plains Range Research Station, 2000 18th St., Woodward, OK 73801. All programs and services of the USDA offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap. Received 13 Dec. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 47:77–82 (2007). Forage & Grazinglands doi:10.2135/cropsci2005.12.0464 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CP, crude protein; DM, dry matter; IVDMD, in vitro dry matter digestibility; LAI, leaf area index; LTA, long-term average. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 77 Published online January 22, 2007