Randall S. Currie

Interference of Palmer amaranth in corn

Abstract Palmer amaranth (Amaranthus palmeri) is a major weed in corn (Zea mays) fields in the southern Great Plains of the United States. Field studies were conducted in 1996, 1997, and 1998 near Garden City, KS, to evaluate the effects of Palmer amaranth density and time of emergence on grain yield of irrigated corn and on seed production of Palmer amaranth. Palmer amaranth was established at densities of 0.5, 1, 2, 4, and 8 plants m−1 of corn row both concurrently at corn planting and when corn was at the three- to six-leaf stage. The control plots were weed free. The Palmer amaranth planted with corn emerged with corn, whereas that planted later emerged at the four-, six-, and seven-leaf stages of corn. The Palmer amaranth emerging with corn reduced yield from 11 to 91% as density increased from 0.5 to 8 plants m−1 of row. In contrast, yield loss from Palmer amaranth emerging later than corn was observed only when the emergence occurred at the four- and six-leaf stages. The corn leaf area index (LAI) decreased as Palmer amaranth density increased. Reduction in corn LAI from Palmer amaranth interference was smaller for the second emergence date than for the first emergence date. Seed production per Palmer amaranth plant decreased with greater density, but seed per unit area increased from 140,000 to 514,000 seeds m−2 at densities of 0.5 and 8 plants m−1 of row, respectively, when Palmer amaranth emerged with corn and from 1,800 to 91,000 seeds m−2 at the same densities for later emergence dates. Although Palmer amaranth is highly competitive in corn, this study shows that yield loss is affected more by time of emergence than by density. Nomenclature: Corn, Zea mays L. ‘DK 592SR’; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

Soil water evaporation and crop residues.

Crop residues have value when left in the field and also when removed from the field and sold as a commodity. Reducing soil water evaporation (E) is one of the benefits of leaving crop residues in place. E was measured beneath a corn canopy at the soil suface with nearly full coverage by corn stover or standing wheat stubble. E was also measured from a soil surface that was partially covered with corn stover without crop shading. E was measured with mini-lysimeters that were 300 mm in diameter and 140 mm deep. Surface coverage and amount of dry matter of crop residues influenced E. E was reduced nearly 50% compared with bare soil E when corn stover and wheat stubble nearly covered the surface under a corn canopy during the growing season. Partial surface coverage, from 25% to 75%, with corn stover caused small reductions in E compared with bare soil when there was no crop canopy. Full surface coverage reduced energy limited E 50% to 65% compared with E from bare soil with no shading. No-till management, using crop residues to significantly reduce E, required soil surfaces to be nearly covered. Economic benefits of crop residues for E suppression during the growing season can be as much as

Water use and light interception under Palmer amaranth (Amaranthus palmeri) and corn competition

365 ha-1.

Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-yr multistate study

Abstract A study was conducted near Garden City, KS, under irrigated conditions to determine the effect of full-season Palmer amaranth infestation on corn water use efficiency and light interception in a fully developed corn canopy. Palmer amaranth at densities of 0, 0.5, 1, 2, 4, and 8 plants m−1 was established at corn planting in 1996 and 1997 and at two locations in 1998. Soil water was monitored 240 cm deep in 30-cm increments with a neutron probe each year and at each location every 10 d. Photosynthetic photon flux was measured in 1997 and 1998 by using a circular and a linear quantum sensor for above canopy and in four 50-cm increments for within canopy, respectively. Palmer amaranth reduced corn yield from 11 to 91% as density increased from 0.5 to 8 plants m−1. Water use efficiency of corn declined with increased Palmer amaranth density. Regardless of Palmer amaranth density, soil water extraction was greatest in the top 30 cm of the soil profile. The pattern of corn leaf area distribution was similar across Palmer amaranth densities, with 15, 70 to 75, and 5 to 15% of the total leaf area occurring 1.5 m, 0.5 to 1.5 m, and 0 to 0.5 m above the ground, respectively. In weed-free corn, over 60% of light was intercepted from 0.5 to 1.5 m above the ground. In contrast, in mixed canopies 60 to 80% of light was intercepted 1 m above the ground, where 80% of Palmer amaranth leaf area was concentrated. Under the conditions of this study, water was not a limiting factor. The effect of Palmer amaranth density on total light interception was not significant. However, within each treatment, light interception at different heights differed, emphasizing the importance of evaluating the vertical distribution of light through the canopy to assess the effect of weed height on light competition. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats AMAPA; corn, Zea mays L. ‘DK592 SR’.

Differential Kochia (Kochia scoparia) Populations Response to Glyphosate

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage. Nomenclature: Glyphosate; Amaranthus spp. #3 AMASS; Ipomoea spp. # IPOSS; corn, Zea mays L. ‘Roundup Ready®’ # SETFA. Additional index words: Herbicide-resistant crops, weed interference. Abbreviation: POST, postemergence.

Field‐evolved resistance to four modes of action of herbicides in a single kochia (Kochia scoparia L. Schrad.) population

Abstract Kochia is a troublesome weed throughout the western United States. Although glyphosate effectively controls kochia, poor control was observed in several no-till fields in Kansas. The objectives of this research were to evaluate kochia populations response to glyphosate and examine the mechanism that causes differential response to glyphosate. Glyphosate was applied at 0, 54, 109, 218, 435, 870, 1305, 1740, 3480, and 5220 g ae ha−1 on 10 kochia populations. In general, kochia populations differed in their response to glyphosate. At 21 d after treatment, injury from glyphosate applied at 870 g ha−1 range from 4 to 91%. In addition, glyphosate rate required to cause 50% visible injury (GR50) ranged from 470 to 2149 g ha−1. Differences in glyphosate absorption and translocation and kochia mineral content were not sufficient to explain differential kochia response to glyphosate. Nomenclature: Glyphosate; kochia, Kochia scoparia (L.) Schrad.

Corn Yield Response to Deficit Irrigation

BACKGROUND Evolution of multiple herbicide resistance in weeds is a serious threat to weed management in crop production. Kochia is an economically important broadleaf weed in the U.S. Great Plains. This study aimed to confirm resistance to four sites of action of herbicides in a single kochia (Kochia scoparia L. Schrad.) population from a crop field near Garden City (GC), Kansas, and further determine the underlying mechanisms of resistance. RESULTS One-fourth of the GC plants survived the labeled rate or higher of atrazine [photosystem II (PSII) inhibitor], and the surviving plants had the Ser-264 to Gly mutation in the psbA gene, the target site of atrazine. Results showed that 90% of GC plants survived the labeled rate of dicamba, a synthetic auxin. At least 87% of the plants survived up to 72 g a.i. ha(-1) of chlorsulfuron [acetolactate synthase (ALS) inhibitor], and analysis of the ALS gene revealed the presence of Pro-197 to Thr and/or Trp-574 to Lue mutation(s). Most GC plants also survived the labeled rate of glyphosate [5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor), and the resistant plants had 5-9 EPSPS gene copies (relative to the ALS gene). CONCLUSION We confirm the first case of evolution of resistance to four herbicide sites of action (PSII, ALS and EPSPS inhibitors and synthetic auxins) in a single kochia population, and target-site-based mechanisms confer resistance to atrazine, glyphosate and chlorsulfuron.

Differential Response of Grain Sorghum Hybrids to Foliar-Applied Mesotrione

Because dwindling water supplies are limiting crop production, a field study was conducted during 2005-2009 in southwest Kansas to determine the yield response of corn to irrigation and evapotranspiration (ETc), and to document plant growth parameters and soil water use. Corn was grown in a five-year rotation of corn-corn-wheat-grain sorghum-sunflower. Results from the corn after sunflower and corn after corn are presented here. Six irrigation treatments were produced by applying 25 mm of irrigation every 5 to 17 days. Year-to-year grain yields averaged over irrigation and crop sequence appeared to be correlated with leaf area index, which possibly reflected the severity of hail events that occurred in four of the five years of the study. However, dry matter accumulation per plant did not vary across irrigation treatments. Surface residue coverage from the previous years crop was 38% for sunflower and 61% for corn. ETc and productivity, also known as water use efficiency (WUE), decreased significantly as irrigation decreased. The deficit irrigation treatments used more of the previous non-growing season precipitation than the fully irrigated treatment due to greater soil water storage capacity in the drier soil profile. Furthermore, these treatments extracted more soil water during the growing season as irrigation decreased. Linear models of ETc predicted grain and dry matter yields with R2 values of 0.67 and 0.59, respectively. The relationship of relative grain yield and ETc was also linear and more pronounced, with an R2 value of 0.82. In contrast, the relationship of relative yield and irrigation followed a curvilinear model. During the five-year study, variability in yields increased as irrigation decreased, illustrating a greater income risk with less irrigation. Yield response to irrigation, especially over multiple years, is essential information to build economic studies of cropping alternatives, deficit irrigation management, and income risk. These relationships need to be developed regionally to characterize the effects of environmental factors, especially precipitation.

Impact of Palmer Amaranth (Amaranthus palmeri) on Corn (Zea mays) Grain Yield and Yield and Quality of Forage1

Abstract The selection of herbicide-resistant weeds in grain sorghum production has prompted researchers to explore alternative herbicides to prevent, delay, and manage herbicide-resistant weed biotypes. Greenhouse and field experiments were conducted to evaluate the differential response of sorghum hybrids to POST application of mesotrione. In a greenhouse experiment, 85 sorghum hybrids were treated with 0, 52, 105, 210, and 315 g ai/ha mesotrione when plants were at the three- to four-leaf collar stage. Sorghum response ranged from susceptible to tolerant sorghum hybrids. ‘Pioneer 84G62’, ‘Pioneer 85G01’, and ‘Triumph TR 438’ were the three most susceptible, whereas ‘Dekalb DKS35-70’, ‘Frontier F222E’, and ‘Asgrow Seneca’ were the three most tolerant hybrids. One week after treatment (WAT), the mesotrione rate causing 50% visible injury ranged from 121 to 184 and 64 to 91 g/ha in the most tolerant and susceptible hybrids, respectively. Mesotrione dose–response studies were conducted under field conditions on four sorghum hybrids. One WAT, injury symptoms were greater (up to 23%) in Pioneer 85G01 than in Asgrow Seneca (< 14%). However, all plants appeared normal by the end of the growing season. In addition, sorghum yields were not reduced by mesotrione treatments as verified by correlation coefficient analysis. Nomenclature: Mesotrione; sorghum, Sorghum bicolor (L.) Moench. SORBI

Impact of Land Management Practices on Carabids (Coleoptera: Carabidae) and Other Arthropods on the Western High Plains of North America

Studies were conducted at one location in 1997 (W97) and in two locations in 1998 (E98 and W98) near Garden City, KS, to evaluate the impact of Palmer amaranth at densities of 0, 0.5, 1, 2, 4, and 8 plants/m of row on forage yield and quality of irrigated corn and to determine if harvesting Palmer amaranth–infested corn for forage rather than for grain would reduce losses. Weed-free corn, Palmer amaranth alone, and corn–Palmer amaranth harvested together were evaluated for forage yield and quality. Forage quality was determined by evaluating in vitro dry matter digestibility (IVDMD), acid detergent fiber (ADF), neutral detergent fiber (NDF), and crude protein (CP). Corn grain and forage yield declined with increasing Palmer amaranth density. However, decline in forage yield ranged from 1 to 44% of the weed-free yield at Palmer amaranth densities of 0.5 and 8 plants/m of row, whereas decline in grain yield ranged from 11 to 74% of the weed-free yield at the same densities. Digestibility of weed-free corn forage, expressed as IVDMD, was significantly higher than that of Palmer amaranth alone at W97 and W98. The CP of weed-free corn forage was similar to that of Palmer amaranth forage at W98, but it was lower at the other two locations. In contrast, CP did not differ between corn–Palmer amaranth harvested together and weed-free corn. The IVDMD of weed-free corn forage was greater than that of corn–Palmer amaranth mixture at W98, but no differences were observed at the other two locations. The relative feeding values of weed-free corn and corn–Palmer amaranth mixture were similar, and greater than that of Palmer amaranth in monoculture. These results indicate that Palmer amaranth interference in corn may not affect forage quality but can cause a decline in yield. This decline in yield is less when harvesting corn and Palmer amaranth together for forage rather than for harvesting corn grain alone. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. #3 AMAPA; corn, Zea mays L. Additional index words: Forage quality, weed density, yield loss. Abbreviations: ADF, acid detergent fiber; CP, crude protein; DM, dry matter; IVDMD, in vitro dry matter digestibility; NDF, neutral detergent fiber; RFV, relative feed value.