Michael J. Horak

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.

Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean

Abstract Field studies were conducted in 1997 and 1998 at Manhattan and Topeka, KS, to examine the competitive effects of redroot pigweed, Palmer amaranth, and common waterhemp on soybean yield. The experiments were established as a randomized complete block design in a factorial arrangement of three pigweed species, two pigweed planting dates (soybean planting and cotyledon stage), and seven weed densities (0.25, 0.5, 1, 2, 4, and 8 plants m−1 of row, plus a weed-free control). The effect of weed density on soybean yield loss, pigweed biomass, and pigweed seed production were described using a rectangular hyperbola model. Soybean yield loss varied between locations depending on the weed species, density, and time of emergence. Yield loss increased with weed density for each species and location with the first pigweed emergence time. The maximum soybean yield loss occurred at the first planting and 8 plants m−1 of row density, and was 78.7, 56.2, and 38.0% as determined by the model for Palmer amaranth, common waterhemp, and redroot pigweed, respectively. The second planting of pigweed did not significantly reduce soybean yield. The relative ranking of the pigweed species biomass was Palmer amaranth > common waterhemp > redroot pigweed. Maximum seed production for Palmer amaranth, common waterhemp, and redroot pigweed was 32,300, 51,800, and 9,500 seeds m−2. Palmer amaranth produced a larger quantity of seed than did common waterhemp or redroot pigweed at low weed densities (0.25 to 4 plants m−1 of row). But common waterhemp seed production equaled or surpassed Palmer amaranth at high weed densities. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; common waterhemp, Amaranthus rudis Sauer AMATA; redroot pigweed, Amaranthus retroflexus L. AMARE; soybean, Glycine max (L.) Merr.

Soil microbial and nematode communities as affected by glyphosate and tillage practices in a glyphosate-resistant cropping system

Abstract Field experiments were conducted at Ashland Bottoms in northeastern Kansas and at Hays in western Kansas in 2001, 2002, and 2003 to determine the response of soil microbial and nematode communities to different herbicides and tillage practices under a glyphosate-resistant cropping system. Conventional herbicide treatments were a tank mixture of cloransulam plus S-metolachlor plus sulfentrazone for soybean and a commercially available mixture of acetochlor and atrazine for corn. Glyphosate was applied at 1.12 kg ai ha−1 when weeds were 10 or 20 cm tall in both corn and soybean. Soil samples were collected monthly at Ashland Bottoms during the growing period for soil microbial biomass (SMB) carbon determination. In addition, substrate-induced respiration (SIR) and BIOLOG substrate utilization were determined at the end of the growing season each year at Ashland Bottoms, and nematode populations were determined at the beginning and the end of the growing season at both sites. Direct effects of glyphosate rates on soil microbial and nematode communities were also studied in a controlled environment. Values for SMB carbon, SIR, and BIOLOG substrate utilization were not altered by glyphosate. Nematode community response to the glyphosate treatment was similar under both conventional tillage and no-till environments. Total nematode densities were similar with the glyphosate and conventional herbicide treatments. SMB carbon and BIOLOG substrate utilization did not differ between tillage treatments. Nematode densities were greater under conventional tillage than in the no-till system. This study showed that soil health when glyphosate was applied in a glyphosate-resistant cropping system was similar to that of cropping systems that used conventional herbicides. Nomenclature: Glyphosate; corn, Zea mays L. ‘Asgrow RX718RR’, ‘DeKalb 520RRYG’, ‘DeKalb 53-34’; soybean, Glycine max (L.) Merr. ‘Asgrow 3003RR’, ‘Asgrow 3302RR’.

Interspecific hybridization and gene flow of ALS resistance in Amaranthus species

Abstract Several inbred lines of acetolactate synthase (ALS)-inhibiting herbicide-resistant (ALS-R) Palmer amaranth and ALS-susceptible (ALS-S) common waterhemp were developed in the greenhouse. Interspecific hybrids were obtained by allowing several ALS-S common waterhemp females to be pollinated only by ALS-R Palmer amaranth in a growth chamber. Putative hybrid progeny were treated with an ALS-inhibiting herbicide, and the hybrid nature verified using a polymorphism found in the parental ALS gene. Polymerase chain reaction (PCR) was used to amplify a region of the ALS gene in both parental plants and putative hybrids. Restriction enzyme digestion of the ALS-R Palmer amaranth PCR fragment resulted in two smaller fragments, whereas the PCR fragment in the ALS-S common waterhemp was not cut. Restriction digestion of the putative hybrid PCR fragment showed a combination of ALS-R Palmer amaranth double fragments and an ALS-S common waterhemp single fragment. Approximately 4 million flowers were present on 11 common waterhemp females and produced about 44,000 seeds that appeared viable. From the approximately 3,500 putative hybrid seedlings that were screened, 35 were confirmed as hybrids using herbicide resistance as a phenotypic and molecular marker. The data collected here verify that interspecific hybridization does occur between these two species, and even at a low rate, it could contribute to the rapid spread of ALS resistance in these species. Nomenclature: Imazethapyr; common waterhemp, Amaranthus rudis Sauer AMATA; Palmer amaranth, Amaranthus palmeri S. Wats AMAPA.

Isozyme Variability and Breeding Systems in Populations of Yellow Nutsedge (Cyperus esculentus)

The importance of seeds in the reproduction and maintenance of yellow nutsedge (Cyperus esculentus L. #3 CYPES) populations was evaluated. Isozyme analysis using starch gel electrophoresis was performed on 20 individuals of each of 10 widely separate populations in California. Genetic variation among individuals served as an indicator of the relative importance of asexual and sexual reproduction in each population. Eight enzyme systems were assayed from which 12 loci were resolvable, with four of those loci exhibiting variability. A maximum of four isozyme genotypes appeared in any population; only nine of 81 total potential genotypes were identified. Five populations were isozymically uniform, apparently composed of a single genotype. The remaining five populations were genotypically variable; these frequently deviated strongly from the genotype frequencies expected of a sexually reproducing population. These results indicate that sexually produced seeds are unimportant in the maintenance of yellow nutsedge populations in agricultural environments. Although viable seed may be produced, tubers appear to be the primary mode of reproduction in this species. Additional index words. Clonal reproduction, seed reproduction, electrophoresis, genetic variation, isozyme analysis,

Pollen morphological differences in Amaranthus species and interspecific hybrids.

Abstract This study examined pollen morphological variation among Amaranthus species and interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and several putative hybrids resulting from interspecific mating between common waterhemp and Palmer amaranth were grown in a greenhouse. Mature pollen was collected, viewed, and photographed with a scanning electron microscope (SEM). The pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture arrangement. Differences were observed between the monoecious and dioecious Amaranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious species, Palmer amaranth, whose pollen was similar to that of the monoecious species spiny amaranth. However, pollen grain diameters did not differ between the monoecious and dioecious plants. Significant differences also were noted between the pollen from the putative common waterhemp × Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal parent but had an aperture number that was intermediate between parents. This indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; grain amaranth, Amaranthus cruentus L. AMACR; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; Powell amaranth, Amaranthus powellii S. Wats. AMAPO; prostrate pigweed, Amaranthus blitoides S. Wats. AMABL; redroot pigweed, Amaranthus retroflexus L. AMARE; sandhills amaranth, Amaranthus arenicola I. M. Johnst. AMAAR; smooth pigweed, Amaranthus hybridus L. AMACH; spiny amaranth, Amaranthus spinosus L. AMASP; tall waterhemp, Amaranthus tuberculatus (Moq.) J. D. Sauer AMATU; tumble pigweed, Amaranthus albus L. AMAAL.

Relative time of redroot pigweed emergence affects dry matter partitioning

Abstract The partitioning coefficient is defined as the proportion of new dry matter partitioned among different plant parts. Partitioning coefficients can be used to model plant dry matter accumulation. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of density and relative time of emergence of redroot pigweed on dry matter partitioning to stem, leaves, and reproductive parts throughout the season. Redroot pigweed was grown with sorghum and in monoculture at densities of 2, 4, and 12 plants m−1 of row each year at each location. Dry matter partitioning during vegetative growth was not influenced by plant density. However, partition coefficients during the reproductive growth stage changed as a linear function of the time of pigweed emergence relative to the sorghum leaf stage. The later the emergence time relative to sorghum leaf stage, the higher the partitioning coefficient values for leaf (PCleaf) and stem (PCstem) and the lower the partitioning coefficient values for reproductive parts (PCrp). The observed differences in partitioning coefficients due to relative emergence time are valuable information to those interested in simulating growth of competing plant species, especially with reference to their seed production. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) ECHCG; common lambsquarters, Chenopodium album L. CHEAL; redroot pigweed, Amaranthus retroflexus L. AMARE; sorghum, Sorghum bicolor (L.) Moench.

Sex Pheromone of the Noctuid Moth, Tyta luctuosa (Lepidoptera: Noctuidae), a Candidate Biological Control Agent of Field Bindweed

Abstract Laboratory studies were conducted to identify the sex pheromone of Tyta luctuosa (Denis and Schiffermuller), a Eurasian noctuid moth that has been imported and released to aid in control of field bindweed, Convolvulus arvensis L. Using gas chromatography-mass spectrometry, together with electroantennogram and windtunnel bioassays of male moths, two compounds, (Z)-9-tetradecenal and (Z)-11-hexadecenal, were identified as the major pheromone components. Whole-gland extracts contained these components, as well as two other major compounds, (Z)-9-tetradecanol and (Z)-11-hexadecanol. However, the two alcohols were not detected in airborne emissions of calling females. Ratios of Z9–14:ALD to Z11–16:ALD were markedly different for whole-gland extracts and airborne emissions (1:3 and 2:1, respectively). Also, although the total amount of the two compounds varied nearly eight-fold among individuals (22–167 ng, mean 75 ± 56 ng [SD]) in gland extracts, the Z9–14:ALD/Z11–16:ALD ratio was relatively constant (0.3 ± 0.15). On average, calling females released 94 ng of Z9–14:ALD and 45 ng Z11–16:ALD per hour, with a mean ratio of 2.2. In wind tunnel tests, 69% of males exhibited complete upwind flights and touched the stimulus source in response to a synthetic pheromone blend that mimicked the female-produced Z9–14:ALD/Z11–16:ALD airborne concentration and ratio, as compared with 82% and 50% in response to calling females and pheromone gland extracts, respectively. Tyta luctuosa adults are vagile, and both adults and larvae are cryptic in the field-factors that make recoveries of released insects unlikely. Therefore, our data will contribute to the development of a pheromone-based monitoring tool to help assess colonization and establishment of this potentially useful weed biological control agent.