Planting Date Effects on Seedling Development of Perennial Warm‐Season Forage Grasses. I. Field Emergence1

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Abstract
Slow establishment of perennial warm‐season forage grasses is a problem in their utilization to improve animal production in summer when cool‐season grasses are less productive. A field experiment was conducted to determine temperature effects on field emergence, estimate base temperature required for emergence in the field, and provide insight toward optimum planting dates for warm‐season perennial forage grasses. Big bluestem (BB) (Andropogon gerardiiVitman), Causasian bluestem (CB) [Bothriochloa caucasica(Trin.) C. E. Hubb.], indiangrass (IG) [Sorghartrum nutans(L.) Nash], switchgrass (SG) (Panicum virgatumL.), and an annual species, crabgrass (CG) [Digitaria sanguinalis(L.) Scop.] were compared in a split‐plot field experiment with five replications. Unchilled seeds of each species were planted at approximately 2‐week intervals in a Mexico silt loam soil (fine, montmorillonitic, mesic Udollic Ochraqualf) near Columbia, MO from early April to late June in 1982 and 1983. Emerging seedlings were recorded frequently. The logistic function was used to fit the emergence data. No emergence data were obtained for CG since natural emergence also occurred. Maximum emergence percentage was obtained following planting on 30 Apr. and 2 June 1982, and 6 and 16 May 1983. Indiangrass had the highest emergence percentages of pure live seed with 38 and 51% in 1982 and 1983, respectively, followed by SG, BB, and CB. Survival percentage of emerged seedlings averaged about 90%, but was reduced to near 70% on late plantings when soil dried out rapidly. Entries with high emergence percentages also had their highest emergence rates (ER) at Et50(time to reach 50% of final emergence). Corrected ER index and ER50(reciprocal of Et50) were closely correlated (r= 0.99, P50is based on actual measurements and has more agronomic significance and biological meaning. Base temperatures required for emergence in 1982 were estimated as 9.4, 9.9, 10.8, and 11.4 °C for IG, BB, SG, and CB, respectively. For 1983 temperatures were 10.0, 10.8, 10.4, and 9.5 °C, respectively. The data suggest optimum planting dates for these species between late April and mid‐May to maximize emergence percentage and reduce risks of high temperature and soil moisture stress.