Cover crops can reportedly improve soil fertility, suppress weed growth and pest pressure, and contribute to cotton (Gossypium hirsutum L.) yield improvements. To systematically evaluate cover crop effects on cotton yield and weed suppression, we conducted a random‐effects meta‐analysis investigating 10 moderating variables in 104 articles, yielding 1117 independent studies over 48 yr. Globally, cover crops increased cottonseed and lint yield by 6 and 5%, respectively, while decreasing weed biomass by 20%. Overall, leguminous cover crops increased seed cotton yield by 17 to 43% (Vicia spp. and Pisum spp., respectively). Monocots, nonlegume dicots, and legumes were effective at suppressing weed growth (21, 52, and 10%, respectively). Incorporation of cover crops by tillage (rather than chemical burn down) resulted in lint (14%) and seed (42%) yield increases. Cover crops increased cotton yields markedly on loamy soils, whereas there were lesser increases for other textures (p > 0.05). Greater efficacy of cover crops at controlling weeds was proportionate to soil silt content, which was inversely related to sand content. In addition, cover crops were effective at controlling multiple types of weed species; however, weed species with shorter maximum heights were most suppressed by cover crops. Overall, cover crops had a positive effect on cotton yield and weed suppression. Their effectiveness, however, can vary depending on soil texture, management strategy, and cover crop or weed genera. These results are useful in developing recommendations for suppressing weed growth and improving yield via cover crop integration in cotton cropping systems per geographic region and soil texture.

253 RESEARCHS witchgrass is a C4 perennial grass native to North American prairie lands (Newell and Eberhart, 1961). Like most grasses, switchgrass is produced in a sward, which is a densely planted expanse of grass, rather than in rows or as single plants. In switchgrass, acceptable stand density in a sward is ~0.3 m between plants (Launchbaugh and Owensby, 1970;Schmer et al., 2006;Vogel, 1987;Vogel and Mitchell, 2008;Weaver, 1968). For breeding purposes, switchgrass evaluations are usually performed on single plants at a lower planting density, allowing a breeder to walk through the nursery and evaluate each individual plant. The discrepancy in evaluation and selection conditions vs. production conditions may lead to less effective or ineffective selection because of differences in plant performance under varying degrees of competition. Studies in a large number of grass species have examined some of the issues arising from these disparate evaluation and production conditions. Differences between spaced and sward evaluations have been noted in many grass species that are produced under sward conditions. Waldron et al. (2008) found that in an evaluation of 22 tall fescue (Festuca arundinacea Schreb.) half-sib families, values of ABSTRACT Switchgrass (Panicum virgatum L.) is an important emerging biofuel crop. In breeding nurseries, plants are typically widely spaced; however, production is in densely planted swards. This disconnect may hinder cultivar improvement. This study measured the efficiency of lowdensity, spaced-plant selection on improving biomass and ethanol yield in a high-density, simulated sward. Fifty-six full-sib families were clonally replicated in two adjacent nurseries in Knoxville, TN. The spaced-plant nursery consisted of single plants on 1-m centers. The simulated-sward nursery was created by planting four by seven plants on 0.33-m centers with 1-m alleys. In 2013 and 2014, biomass yield, ethanol yield, and morphological traits were evaluated. Trait means, correlations, and efficiency of indirect selection (E) were calculated. Significant interaction (p < 0.05) between year and nursery was observed for all traits except ethanol yield. The identified high-yielding ideotype differed between biomass and ethanol yield and between spaced-plant and simulated-sward nurseries. Selection under spaced-plant conditions for simulated-sward performance was efficient for ethanol yield (E = 0.96) but highly inefficient for biomass yield (E = −0.31). Several morphological traits evaluated under spaced-plant conditions were identified as efficient indirect selectors for simulated-sward biomass or ethanol yield. Results suggest selections for sward biomass yield may be more appropriate under sward-like conditions, but that spaced-plant nurseries are efficient for selection of ethanol yield performance under sward-like conditions and for indirect selection of sward yield traits using morphological traits.

Switchgrass (Panicum virgatum L.) takes 3 yr to reach maximum biomass yields, delaying selection and cultivar improvement. This study evaluated the efficiency of early selection in improving third‐year biomass and ethanol yields in lowland switchgrass. Fifty‐six full‐sib families were planted in Knoxville and Crossville, TN, with 1‐m spacing in a randomized complete block design. In 2012 to 2014, plants were evaluated for biomass yield, ethanol yield, and morphological traits at 8 wk after emergence and in years one through three. Trait means, efficiency of indirect selection, and genetic gain were calculated. Biomass yield differed significantly by year (92 g in yr 1, 1069 g in yr 2, and 1425 g in yr 3) while ethanol yield did not (x¯=63mgnormalg‐1). Juvenile selection was ineffective. Biomass selection was most efficient using first‐year fall height with genetic gains per year 2.4 times those of direct selection on third‐year biomass. First‐year biomass and second‐year biomass, fall height, spring height, and tiller diameter also exhibited high efficiency (1.3 to 1.9). For ethanol yield, negative efficiency indicated selection for low first‐year height and low second‐year spring height, and tiller diameter would result in 1 to 1.3 times the genetic gain expected from direct selection on third‐year ethanol yield. Results indicate that early selection and indirect selection using morphological traits may speed yield improvement due to greater selection efficiency and higher genetic gains.

Niger (Guizotia abyssinica, L.) is a desirable oilseed crop for birdseed, especially for finches (Spinus spp.) because of its high ratio of unsaturated to saturated fatty acids and relatively high oil content. In 2012, phenotypic traits, seed oil and fatty acid content measurements were made on 14 plant introductions (PIs) from the United States Department of Agriculture germplasm collection. The PIs originated in Ethiopia (ten), India (three) and USA (one). The phenotypic traits analysed included seed/plant, branches/plant, capitula/plant1, average seed/capitulum and plant height. After initial assessments of the 14 PIs, three were selected for use as parents to produce two one-way and two two-way F1 crosses for the purpose of evaluating self-incompatibility (SI). Parent and F1 progeny seeds were planted in a greenhouse and transplanted to a field site at the East Tennessee Research and Education Center (2012, 2013 and 2014). Comparisons from 2012 showed seed oil of the 14 PIs ranging from 32.9 to 37.9% (PI 508076 (Ethiopia) and PI 509436 (India), respectively). Major fatty acids included stearic, palmitic, oleic and linoleic; with linoleic acid in highest amount. PI508079 (Ethiopia) had the best combination of seed yield, seed oil and linoleic acid content. Over 2013 and 2014, SI ranged from 91.1 to 100.0%. W6 18860 (USA) had the most self-compatibility, and the F1 plants generated from crosses between W6 18860 and other plants tended to be self-compatible when the former was used as a pollen recipient. The results obtained from this study should be useful for niger breeding and production purposes.