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Planting Date Effects on Seedling Development of Perennial Warm-Season Forage Grasses. II. Seedling Growth¹

Perennial warm-season forage grasses can improve animal performance in summer when cool-season grasses become dormant and low in forage quality. Rapid establishment of acceptable stands is a factor limiting their use. Field experiments were conducted to determine optimum planting dates of perennial warm-season forage grasses, determine effects of planting date on seedling growth and partitioning of dry matter between roots and shoots, and compare strategies of seedling growth among perennial warm-season grasses and an annual weed. Seeds of big bluestem (BB) (Andropogon gerardii Vitman), Caucasian bluestem (CB) [Bothriochloa caucasica (Trin.) C. E. Hubb.), indiangrass (IG) [Sorghastrum nutans (L.) Nash], switchgrass (SG) (Panicum virgatum L.), and an annual species, crabgrass (CG) [Digitaria sanguinalis (L.) Scop.] were planted at approximately 2-week intervals from early April to late June in 1982 and 1983. A split-plot design was used with five replications. Soil was a Mexico silt loam (fine, montmorillonitic, mesic Udollic Ochraqualf). Weeds were removed by hand shortly after their emergence in 1982, while weed emergence was prevented by methyl bromide (bromomethane) treatment in 1983. All species within a given planting data were harvested when BB reached the fifth collared-leaf stage. Crabgrass developed more leaves, had greater leaf area, and accumulated more dry matter than did perennials showing that it would exhibit strong competition to perennials during establishment. Among perennials, SG and IG developed more leaf area and accumulated more dry matter than did CB and BB. However, CB developed more leaves and tillers than did other perennials, but leaves were small. Both SG and CB had higher leaf area ratios and lower root/shoot ratios than did IG and BB. Big bluestem required fewer days to reach the fifth collared-leaf stage on later plantings than on early plantings, but in 1982 total dry matter accumulation at the fifth leaf stage was similar among planting dates. In 1983, however, dry matter accumulation and leaf area of BB were highest when seeds were planted on 6 and 16 May. In both years the root/shoot ratio of BB was highest for early plantings and decreased as planting was delayed. Other species showed similar trends. Dry matter production averaged over planting dates for CB, IG, and SG, respectively, was 1.2, 2.5, and 2.1 times greater than BB in 1982, and 1.6, 2.6, and 3.4 times greater than BB in 1983. Perennial warm-season forage grasses are adapted for planting between late April and mid-May, a period when low temperatures occur to partially chill seeds and adequate soil moisture is available to allow development of the permanent root system prior to drought conditions.

Key Words: Root/shoot ratio • Leaf area ratio • Big bluestem • Andropogon gerardii Vitman • Caucasian bluestem • Bothriochloa causica (Trin.) C. E. Hubb • Indiangrass • Sorghastrum nutans (L.) Nash • Switchgrass • Panicum virgatum L. • Crabgrass • Digitaria sanguinalis (L.) Scop

² Former graduate assistant (now associate research fellow, Dep. of Forage Crops, Taiwan Livestock Res. Inst., Hsinhua, Tainan, Taiwan, ROC) and professor, Dep. of Agronomy, Univ. of Missouri, Columbia, MO. 65211.

Received for publication July 18, 1984.