Marcos J. Oliveira

Comparison of the critical period for weed control in wide- and narrow-row corn

Abstract Field studies were conducted near Blackville, SC, in 2002 and 2003 and near Pendleton, SC, in 2003 to compare the critical period for weed control (CPWC) in narrow- (48 cm) and wide-row (97 cm) corn. A series of treatments of increasing duration of weed interference and length of weed-free period were imposed within each row spacing. Diversity and density of the weed spectrum were greater at Blackville than at Pendleton. Weed interference duration and weed-free period curves were similar between row widths for each of the 3 site-years. Averaged over row width, the CPWC was 36 to 40 d longer at Blackville than at Pendleton. The CPWC began 5 to 9 d after corn emergence (DAE) (one- to two-leaf stage) and ended 45 to 53 DAE (eight- to 10-leaf stage) at Blackville. At Pendleton, the CPWC was only 4 d, beginning 21 DAE (five-leaf stage) and ending 25 DAE (five- to six-leaf stage). Light interception by corn at Blackville at the end of the CPWC averaged 78%, but light interception averaged only 31% at Pendleton at the end of the CPWC, implying that the weed density or weed spectrum may be more of a determinate of the CPWC than canopy formation. Light interception was similar between row widths throughout the growing season, resulting in similar late-season weed biomass between row widths. The CPWC and crop competitiveness with late-emerging weeds was similar between wide- and narrow-row corn when corn light interception did not differ between row widths. Therefore, other strategies, such as increasing the population of narrow-row corn, are likely needed to provide a competitive advantage over wider rows. Nomenclature: Corn, Zea mays L. ‘Asgrow 897 Roundup Ready’.

Sicklepod (Senna obtusifolia) germination and emergence as affected by environmental factors and seeding depth

Abstract Laboratory and greenhouse experiments were conducted to determine the effect of light, temperature, solution pH, solution osmotic potential, and oxygen concentration on sicklepod germination and radicle plus hypocotyl elongation and seeding depth on emergence. Scarified, nondormant sicklepod seeds were used for these experiments. Sicklepod germination was not influenced by red or far-red light nor was light required for germination, which averaged 81% over all light treatments. Sicklepod germinated over a range of constant temperatures from 15 to 50 C, with optimum germination between 15 and 30 C. Germination was optimal near pH 6 for temperatures of 15 and 30 C. Germination and radicle plus hypocotyl length decreased with decreasing solution osmotic potential, and no germination occurred at a solution osmotic potential of −0.75 MPa at 15 C during 7 d incubation. Germination was greater at 20% oxygen than at 2% oxygen. The mean emergence depth for sicklepod was 3.3 and 4.6 cm in a highly disturbed sand and sandy loam soil, respectively. Sicklepod emerged from a 10-cm depth in the sandy loam soil, but no emergence occurred in the sand soil at this depth. Nomenclature: Sicklepod, Senna obtusifolia (L.) Irwin and Barneby CASOB.

Palmer Amaranth and Large Crabgrass Growth with Plasticulture-Grown Bell Pepper

Field experiments were conducted in 2004 and 2005 at Clemson, SC, and in 2004 at Clinton, NC, to quantify Palmer amaranth and large crabgrass growth and interference with plasticulture-grown bell pepper over multiple environments and develop models which can be used on a regional basis to effectively time removal of these weeds. Experiments at both locations consisted of an early and a late spring planting, with the crop and weeds planted alone and in combination. Daily maximum and minimum air temperatures were used to calculate growing degree days (GDD, base 10 C) accumulated following bell pepper transplanting and weed emergence. Linear and nonlinear empirical models were used to describe ht, canopy width, and biomass production as a function of accumulated GDD. Palmer amaranth reduced bell pepper fruit set as early as 6 wk after transplanting (WATP) (648 GDD), whereas large crabgrass did not significantly reduce fruit set until 8 WATP (864 GDD). Using the developed models and assuming Palmer amaranth and large crabgrass emergence on the day of bell pepper transplanting, Palmer amaranth was predicted to be the same ht as bell pepper at 287 GDD (20 cm tall) and large crabgrass the same ht as bell pepper at 580 GDD (34 cm tall). Nomenclature: Large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA, Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA, bell pepper, Capsicum annuum L. ‘Heritage’

Pitted morningglory (Ipomoea lacunosa) germination and emergence as affected by environmental factors and seeding depth

Abstract Laboratory and greenhouse experiments were conducted to determine the effect of light, temperature, solution pH, solution osmotic potential, and oxygen concentration on pitted morningglory germination and radicle plus hypocotyl elongation, and seeding depth on its subsequent emergence. Daily exposure of seed to natural light resulted in lower germination than in darkness, whereas germination was not influenced by brief exposure to red or far-red light. Germination occurred over a wide range of constant temperatures, from 7.5 to 52.5 C, with optimum germination between 20 and 25 C. Germination occurred at solution pH range of 3 to 10 and was optimal from pH 6 to 8. Radicle plus hypocotyl elongation was influenced by the interaction of temperature and solution pH. A combination of acidic medium (pH 6) and high temperature (30 C) resulted in the greatest radicle plus hypocotyl length of 7.6 cm after a 7-d incubation. Germination and radicle plus hypocotyl elongation, averaged over the 15 and 30 C temperature, decreased with increasing moisture stress, with less than 3% normalized germination at −1.0 MPa. Germination was 29, 40, and 51% at 2, 10, and 20% oxygen, respectively, averaged over 15 and 30 C. Germination of seed lying on the soil surface covered with filter paper or without any cover was similar, averaging 64%. Normalized emergence, relative to germination on the soil surface, decreased with increasing burial depth to 4% emergence at 10 cm, with a mean emergence depth of 4.1 cm, averaged across two soil types. Nomenclature: Pitted morningglory, Ipomoea lacunosa L. IPOLA.

Coffee senna (Cassia occidentalis) germination and emergence is affected by environmental factors and seeding depth

Abstract Laboratory and greenhouse experiments were conducted to determine the effect of light, temperature, moisture stress, solution pH, and burial depth on coffee senna germination and emergence. Seeds germinated equally with or without light, and pretreatment with red or far-red light did not affect germination. Optimum temperature for germination was 25 C, and a high germination percentage (> 70%) occurred from 12.5 to 30 C. The low temperature threshold for germination was between 10 and 12.5 C, whereas the upper threshold was near 45 C. Coffee senna germination in response to moisture stress and solution pH differed at 15 and 30 C. At −0.4 MPa, no germination occurred at 15 C, whereas 15% germination occurred at 30 C. Optimum germination was at pH 6, but further increases in pH had a more negative effect on germination at 15 C than at 30 C. Coffee senna germination ranged from 9 to 12% at pH 3, but was 0% at pH 10, which indicates that coffee senna germination was more tolerant of acidic than basic solutions. Depth-mediated emergence inhibition was sigmoidal, with greatest emergence on the soil surface. Emergence from 2- to 10-cm depths reached 95% of the total emergence 1 to 3 d earlier in a sandy loam than in a sand soil. Mean emergence depth was 1.7 cm in the sand and 2.4 cm in the sandy loam soil. Knowledge gained from this research will be instrumental in developing a better understanding of the requirements for coffee senna germination and emergence, allowing further development and improvement of integrated weed management strategies specific to this troublesome weed. Nomenclature: Coffee senna, Cassia occidentalis L. CASOC.

Tillage and Soybean Canopy Effects on Common Cocklebur (Xanthium Strumarium) Emergence

Abstract Field experiments were conducted in Pendleton, SC, in 2004 and 2005, to determine the influence of tillage with or without soybean on common cocklebur emergence. Treatments included no-till/no soybean (NTNS), no-till plus soybean (NTS), tillage/no soybean (TNS), and tillage plus soybean (TS). Emergence was monitored from an artificial seed bank in 2004 and a natural seed bank in 2005. Overall, common cocklebur emerged from early May through late October and presented multiple emergence. In no-till plots with or without soybean, initial emergence was delayed 7 d in both years. In TNS plots, major emergence (daily emergence > mean emergence plus standard deviation) of common cocklebur occurred from early May to late July. In NTNS plots, major emergence occurred from late May through late August. No-till reduced total common cocklebur emergence by 59 to 69% compared with tillage. At the V5 to V6 soybean growth stage, the daily soil thermal fluctuation at 2.5 cm soil depth diminished from approximately 15 to 5 C and reduced common cocklebur emergence by 84 to 91% for the rest of the growing season. Common cocklebur emergence was higher when the mean soil temperature was > 15 C, and the daily thermal fluctuation was > 7.5 C. This study suggests that strategies that promote early crop canopy development and minimum tillage should reduce common cocklebur emergence. Nomenclature: Common cocklebur; Xanthium strumarium L. XANST; soybean; Glycine max (L.) Merr

A Model For Predicting Common Cocklebur (Xanthium Strumarium) Emergence in Soybean

Abstract The objective of this research was to develop a model to predict common cocklebur seedling emergence in spring tillage and no-spring-tillage systems in the presence and absence of a soybean canopy. A Weibull function was used to accumulate heat units (i.e., growing degree days) at a 2.5 cm soil depth on days when mean soil temperature, soil water potential, and soil thermal fluctuation were above established thresholds. The base temperature, soil water potential, and soil thermal fluctuation thresholds used for model development were 17 C, −100 kPa, and 7.5 C, respectively. A single function adequately described common cocklebur seedling emergence in the presence and absence of drill-seeded soybean from data combined over an artificial (2004) and natural seedbank (2005) (R2  =  0.986). Model parameterization differed between the artificial and natural seedbank in the absence of spring tillage, but emergence was adequately described, regardless of soybean presence. Separate parameter estimates for the artificial and natural seedbanks were needed to adequately describe emergence in the system without spring tillage (R2  =  0.975 to 0.984). The ability of the model to account for reduced emergence when soil moisture is limited or when daily thermal fluctuation requirements are not met could assist practitioners with assessments associated with field scouting for weeds as well as other management decisions. Nomenclature: Common cocklebur, Xanthium strumarium L. XANST, soybean, Glycine max (L.) Merr

Role of Light Quality and Temperature on Pitted Morningglory (Ipomoea Lacunosa) Germination with After-ripening

Abstract Pitted morningglory seed were collected in the fall of 2003 from Blackville, SC, and 2004 from Pendleton, SC, to assess the effect of after-ripening and burial on light and temperature requirements for germination. Pitted morningglory germination was evaluated over a 12-mo period after maturation. Germination was neither stimulated by red light or inhibited by far-red light, nor was it reversible by red or far-red light. Light was not essential for germination of buried seed. Direct exposure to sunlight prevented germination of recently mature seed, but not once seed had sufficiently after-ripened. Pitted morningglory was capable of germination in darkness over a wide range of constant and fluctuating temperatures immediately after maturation. Germination in response to temperature varied with time of year after maturation, with the population from Pendleton having increased germination in May. Thermal fluctuations increased germination of both populations at suboptimal temperatures. Thermal amplitude regulation of germination varied over time and appeared to play a more important role in germination of after-ripened seed than recently mature ones. The ecological significance of changes in germination requirements with after-ripening is discussed. Nomenclature: Pitted morningglory, Ipomoea lacunosa L. IPOLA.

Changes in Senna obstuifolia Germination Requirements over 12 Months under Field Conditions

Senna obtusifolia seeds were collected in Fall 2003 and immediately field sown to assess dormancy alleviation and effect of after-ripening over a 12-month period on light and temperature requirements for germination. Seeds did not exhibit physical dormancy at maturation and readily germinated over a broad range of light and thermal conditions. Dormancy gradually increased during Winter months, resulting in only a small fraction of the population capable of germination by early Spring. Dormancy break did not occur at a specific time of the year nor did dormancy alleviation increase over the 12-month period following maturation. Conditions during Spring and Summer coincided with thermal requirements for germination of the nondormant fraction of the population. Senna obtusifolia seeds were nonresponsive to red and far-red lights, and seeds did not acquire a light requirement following burial for 6 months.