Dormant‐Season Planting and Seed‐Dormancy Impacts on Switchgrass Establishment and Yield

Abstract

Establishment failures linked to seed dormancy are a challenge to wide‐scale use of switchgrass (Panicum virgatum L.) for biomass feedstock and forage production. One prospective strategy for breaking dormancy is dormant‐season planting. The objectives of this study were to evaluate (i) three switchgrass dormant‐season planting dates (1 December, 1 February, and 15 March) vs. a growing‐season (1 May) control; (ii) two seeding rates (6.7 and 10.1 kg pure live seed [PLS] ha−1); and (iii) high‐ and low‐dormancy seed lots. Treatments were assigned in a split‐plot design with three replications at two locations in Tennessee in 2008 and 2009. Neither seeding rate nor seed‐dormancy level affected plant density or yield (P > 0.05). However, a seeding date × year interaction impacted first‐year density at both locations. Although patterns differed by year for the two locations, density of March plantings equaled or exceeded (P < 0.05) those at other dates for both locations and years. These variations in density did not carry over to impact yield in year two. A sigmoidal regression of seedling density vs. yield was significant (P < 0.001) albeit not strong (R2 = 0.13); yield response approached an asymptote above ∼8 plants m−2. Results suggest March planting dates, using standard seeding rate recommendations (6.7 kg PLS ha−1) irrespective of seed‐dormancy rates, may be more reliable than planting in May. Thus, a broader establishment window than traditionally used may be practical. However, results should be validated over a broader range of soils and climatic conditions, especially over a winter severity gradient.

Evaluation of Small Grain Cover Crops to Enhance Switchgrass Establishment

“…Keyser et al (2016) suggested a similar explanation for the stochasticity in switchgrass establishment they observed, reporting moderated weed pressure and greater stand densities during periods of below-average rainfall, but poor stands during substantial drought. At HRREC in 2010, total rainfall for 7 d before and after planting switchgrass in June was 3.6 cm (compared to 7.6 cm for that same period in 2009) and for the 6 wk following planting, only 11.0 cm (compared to 18.2 cm for that same period in 2009).…”

“…Based on combinations of year and location, May plantings provided slightly more consistent second-year yields (greatest in 3 of 6 and least in 1 of 6 instances), with those in June being the least consistent (greatest in 2 and least in 3 of 6 instances), and those in March being intermediate between these two. West and Kincer (2011) and Keyser et al (2016), however, observed no differences in second-year yields associated with planting dates. Likewise, Sanderson et al (1996) found that earlier plantings (early April in their case) produced greater second-year yields than those in early and mid-June.…”

“…However, plantings in March appeared to be more reliable as they more frequently (3 of the 6 year ´ location instances) produced densities equal to or greater than those in May and/or June, and were less likely (1 of 6 instances) to produce densities equal to or lower than observed for both of the other two planting dates. Keyser et al (2016) reported that March plantings produced equal or greater seedling densities compared to those in May. Finally, plantings in May were least likely to produce the densest (or equal to the densest) stands for any year and location combination (1 of 6 instances) and most likely to produce the least dense stands (4 of 6 instances).…”

“…Where there were differences (such as at RECM), switchgrass density with wheat cover crops exceeded that of all other small grain treatments, and all small grains exceeded the density for the fallow control. This outcome is not surprising, given the lack of consistent differences in stand density based on small grains and, where there were density differences (i.e., RECM), all treatments exceeded minimum acceptable thresholds for optimum yield (Schmer et al, 2006;Keyser et al, 2016). Sadeghpour et al (2014b) found no difference in switchgrass tiller density between rye and oat cover crops; they did not examine barley or wheat.…”

“…Similarly, Sanderson et al (1996) reported in situ stratification from early April plantings in southwest Virginia. Keyser et al (2016), working in Tennessee, reported more reliable stand densities from plantings in March vs. those in May. Keyser et al (2016), working in Tennessee, reported more reliable stand densities from plantings in March vs. those in May.…”