Beverly R. Durgan

An Emergence Model for Wild Oat (Avena fatua)

Abstract Wild oat is an economically important annual weed throughout small grain producing regions of the United States and Canada. Timely and more accurate control of wild oat may be developed if there is a better understanding of its emergence patterns. The objectives of this research were to evaluate the emergence pattern of wild oat and determine if emergence could be predicted using soil growing degree days (GDD) and/or hydrothermal time (HTT). Research plots were established at Crookston, MN, and Fargo, ND, in 2002 and 2003. On a weekly basis, naturally emerging seedlings were counted and removed from six 0.37-m2 permanent quadrats randomly distributed in a wild oat–infested area. This process was repeated until no additional emergence was observed. Wild oat emergence began between May 1 and May 15 at both locations and in both years and continued for 4 to 6 wk. Base soil temperature and soil water potential associated with wild oat emergence were determined to be 1 C and −0.6 MPa, respectively. Seedling emergence was correlated with GDD and HTT but not calendar days (P = 0.15). A Weibull function was fitted to cumulative wild oat emergence and GDD and HTT. The models for GDD (n = 22, r2 = 0.93, root mean square error [RMSE] = 10.7) and HTT (n = 22, r2 = 0.92, RMSE = 11.2) closely fit observed emergence patterns. The latter model is the first to use HTT to predict wild oat emergence under field conditions. Both models can aid in the future study of wild oat emergence and assist growers and agricultural professionals with planning timely and more accurate wild oat control. Nomenclature: Wild oat, Avena fatua L. AVEFA

Circadian response of annual weeds to glyphosate and glufosinate

Five field experiments were conducted in 1998 and 1999 in Minnesota to examine the influence of time of day efficacy of glyphosate [N-(phosphonomethyl)glycine] and glufosinate [2-amino-4-(hydroxymethyl-phosphinyl)butanoic acid] applications on the control of annual weeds. Each experiment was designed to be a randomized complete block with four replications using plot sizes of 3×9 m. Glyphosate and glufosinate were applied at rates of 0.421 kg ae/ha and 0.292 kg ai/ha, respectively, with and without an additional adjuvant that consisted of 20% nonionic surfactant and 80% ammonium sulfate. All treatments were applied with water at 94 L/ha. Times of day for the application of herbicide were 06:00h, 09:00h, 12:00h, 15:00h, 18:00h, 21:00h, and 24:00h. Efficacy was evaluated 14 d after application by visual ratings. At 14 d, a circadian response to each herbicide was found, with greatest annual weed control observed with an application occurring between 09:00h and 18:00h and significantly less weed control observed with an application at 06:00h, 21:00h, or 24:00h. The addition of an adjuvant to both herbicides increased overall efficacy, but did not overcome the rhythmic time of day effect. Results of the multiple regression analysis showed that after environmental temperature, time of day was the second most important predictor of percent weed kill. Thus, circadian timing of herbicide application significantly influenced weed control with both glyphosate and glufosinate.

Time of Day of Application Effect on Glyphosate and Glufosinate Efficacy

Concurrent with the development of glyphosate- and glufosinate-resistant crops, applied research was conducted to maximize the effectiveness of these two herbicides. The objectives of this study were to examine the influence of time of day of herbicide application, adjuvant, and rate of glyphosate and glufosinate on annual weed control. Time of herbicide application influenced annual weed control of both glyphosate and glufosinate. Greatest annual weed control was observed between 0900 and 1800 h, while less weed control was observed at 0600, 2100, and 2400 h. Additional adjuvant or an increased rate of glyphosate or glufosinate improved efficacy, but did not overcome the time-of-day effect.

Circadian Response of Annual Weeds in a Natural Setting to High and Low Application Rates of Four Herbicides with Different Modes of Action

Four herbicides [glyphosate (GLYT), an amino acid synthesis inhibitor; glufosinate (GLUF), a glutamine synthetase inhibitor; fomesafen (FOME), a protoporphyrinogen oxidase inhibitor; and chlorimuron ethyl (CLIM), an acetolactate synthase inhibitor] were used to examine the influence of time of day of application on the control of a variety of annual broadleaf weeds in field studies conducted in Minnesota (five studies on GLYT and GLUF, three studies on FOME and CLIM). All herbicides were applied with an adjuvant at recommended high and low (half or quarter strength) rates every 3h between 06:00 and 24:00h local time. Visual ratings of percent weed control evaluated at 14d were analyzed by herbicide and application rate for each study and across studies for time-of-day effect by analysis of variance (ANOVA) and single cosinor. A circadian response to each herbicide was found, with greatest weed control observed between 09:00 and 18:00h. Increasing the herbicide application rate did not overcome the time-of-day effect (ANOVA: p≤0.008 for time-of-day effect for each herbicide and application rate). The least-squares fit of a 24h cosine was significant (p≤0.001) for each herbicide and application rate, with double amplitudes of 18–82% (units=% visual control) and estimated peaks (acrophases) near midday between 12:40 and 13:35h. Analysis of residuals obtained from multiple regression that included weed height, herbicide rate, temperature, and relative humidity as independent factors also found a significant time-effect by both ANOVA and cosinor for each herbicide and rate, with acrophases advancing significantly by 3 to 7h for GLYT and GLUF, but not for FOME or CLIM. These results suggest that the four herbicides, while belonging to different families with different modes of action, may reveal different peak times of efficacy when adjusting for environmental factors. Nonetheless, each displays similar circadian patterns when influenced by these factors under natural seasonal field conditions. The within-day rhythmic differences found in weed control are large enough to warrant consideration of the practical financial and environmental importance of the time-of-day that these and other herbicides are applied.

Modelling replicated weed growth data using spatially-varying growth curves

Weed growth in agricultural fields constitutes a major deterrent to the growth of crops, often resulting in low productivity and huge losses for the farmers. Therefore, proper understanding of patterns in weed growth is vital to agricultural research. Recent advances in Geographical Information Systems (GIS) now allow geocoding of agricultural data, which enable more sophisticated spatial analysis. Our current application concerns the development of statistical models for conducting spatial analysis of growth patterns in weeds. Our data comes from an experiment conducted in Waseca, Minnesota, that recorded growth of the weed Setariaspp. We capture the spatial variation in Setaria spp. growth using spatially-varying growth curves. An added challenge is that these designs are spatially replicated, with each plot being a lattice of sub-plots. Therefore, spatial variation may exist at different resolutions – a macro level variation between the plots and micro level variation between the sub-plots nested within each plot. We develop a Bayesian hierarchical framework for this setting. Flexible classes of models result which are fitted using simulation-based methods.

Effects of Common Wheat (Triticum aestivum) Management Alternatives on Weed Seed Production1

Common management alternatives were compared in a factorial arrangement for 2 yr to determine their effects on green foxtail and yellow foxtail seed production in spring wheat in the Northern Great Plains of the United States. Seed production was measured twice, at wheat harvest (in August) and postharvest (after first lethal frost in autumn). Management alternatives were early, middle, and late crop-sowing dates; no-till, chisel, and moldboard plow tillage systems; and broadleaf herbicide only and broadleaf herbicide plus fenoxaprop applications. Fenoxaprop reduced foxtail seed production at wheat harvest but not at postharvest. Early sowing also decreased seed production at wheat harvest but increased postharvest seed production. Tillage system had no consistent effects on foxtail seed production. Postharvest seed production often was greater than or equal to that at wheat harvest regardless of management system. These results indicate that in-crop management alternatives, such as postemergence grass herbicide and early crop sowing, may lower the number of foxtail seeds at harvest substantially, but they must be accompanied by postharvest weed control to reduce overall seed production. Nomenclature: Fenoxaprop; green foxtail, Setaria viridis (L.) Beauv. #3 SETVI; yellow foxtail, Setaria pumila (Poir.) Roem. & Schult. [=Setaria glauca (L.) Beauv.] # SETLU; spring wheat, Triticum aestivum L. ‘Sharpe’. Additional index words: Sowing date, tillage regime. Abbreviations: CP, chisel plow; GDD, growing degree-days; MP, moldboard plow; NT, no-till.

Hard Red Spring Wheat (Triticum aestivum) Tolerance to Postemergence Grass Herbicides1

Eight hard red spring wheat cultivars were tested for tolerance to five postemergence grass herbicides at two locations in Minnesota in 1999 and 2000 at the labeled, one and one-half, or twice the labeled rate. Fenoxaprop plus safener and ICIA 0604 caused the least injury and did not reduce grain yield for most cultivars. Flucarbazone caused intermediate injury and a slight decrease in grain yield for half the cultivars tested. Difenzoquat caused the most injury, regardless of whether the cultivar was genetically sensitive to difenzoquat. Tank-mixing difenzoquat with imazamethabenz reduced injury, even for cultivars that were not genetically sensitive to difenzoquat. Drought stress before application of the postemergence grass herbicides that contain difenzoquat resulted in more potential for crop injury. Excess precipitation combined with high temperatures after application resulted in more potential for crop injury for the other postemergence grass herbicides included in this experiment. Nomenclature: Difenzoquat; fenoxaprop; flucarbazone; ICIA 0604 (proposed common name, tralkoxydim; 2-cyclohexen-[1-one,2-]1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)-(9CI)); imazamethabenz; wheat, Triticum aestivum L. emend. Thell. Additional index words: Crop injury, herbicide tolerance. Abbreviations: HRSW, hard red spring wheat.

Spring- and Fall-Seeded Radish Cover-Crop Effects on Weed Management in Corn

Weeds often limit productivity of organic cropping systems. Radish is a fast-growing, potentially allelopathic cover crop that has the potential to improve weed management in organic systems. To evaluate the effect of radish on density, cover, and biomass of weeds in organically managed corn, 2-yr field experiments were conducted over 4 site years. Four cover-crop planting treatments (fall-only, spring-only, fall + spring, and no cover) were tested in factorial with three cultivation treatments (standard [three to four passes], false seedbed [standard with a false seedbed], and reduced [two passes]). All plots were tilled before planting. Shoot biomass averaged 3,057 kg ha−1 for fall-seeded radish and 385 kg ha−1 for spring-seeded radish. Radish cover crops generally did not improve management of weeds during the corn growing season. However, in the absence of a false seedbed, fall-seeded radish reduced field pennycress density from 9 to < 1 plant m−2 and horseweed density from 6 to 2 plants m−2 in spring in site years where these weeds were present. Fall-seeded radish also reduced cover of summer annual weeds during the fall cover-crop growing season from 4 to 0% in 1 site year, preventing these weeds from setting seed. Radish cover crops did not affect corn grain yield. Nomenclature Field pennycress, Thlaspi arvense L. THLAR; horseweed, Conyza canadensis [L.] Cronq. ERICA; radish, Raphanus sativus L.; corn, Zea mays L. Las malezas a menudo limitan la productividad de los sistemas de cultivos orgánicos. El rábano es un cultivo de cobertura potencialmente alelopático de rápido crecimiento que tiene el potencial de mejorar el manejo de malezas en sistemas orgánicos. Para evaluar el efecto del rábano sobre la densidad, cobertura, y biomasa de malezas en maíz manejado orgánicamente, se realizaron estudios de campo de dos años de duración en 4 sitios-años. Cuatro tratamientos de siembra de cultivos de cobertura (sólo otoño, sólo primavera, otoño + primavera, y sin cobertura) fueron evaluados en forma factorial con tres tratamientos de labranza (estándar [tres a cuatro pases], cama de siembra falsa [estándar con cama de siembra falsa], y reducida [dos pases]). Todas las parcelas fueron labradas antes de la siembra. La biomasa de la parte aérea promedió 3,057 kg ha−1 para el rábano sembrado en el otoño y 385 kg ha−1 para el rábano sembrado en la primavera. Los cultivos de cobertura de rábano generalmente no mejoraron el manejo de malezas durante la temporada de crecimiento del maíz. Sin embargo, en ausencia de la cama de siembra falsa, el rábano sembrado en el otoño redujo la densidad de Thlaspi arvense de 9 a < 1 planta m−2 y la densidad de Conyza canadensis de 6 a 2 plantas m−2 en la primavera, en sitios-años en los que estas malezas estuvieron presentes. El rábano sembrado en el otoño también redujo la cobertura de malezas anuales de verano durante la temporada de crecimiento del cultivo de cobertura de 4 a 0% en 1 sitio-año, previniendo así que estas malezas produjeran semillas. Los cultivos de cobertura de rábano no afectaron el rendimiento de grano del maíz.

Influence of Time of Emergence on the Growth and Development of Wild Oat (Avena fatua)

Abstract Successful control of wild oat in cereal crops requires an accurate prediction of the developmental stages of wild oat plants that emerged during the growing season. The main objective of this research was to evaluate wild oat growth and to predict the phyllochron of wild oat plants that emerge at various times in the Red River Valley region of Minnesota and North Dakota. Field experiments were conducted in 2002 and 2003 in Crookston, MN, and Fargo, ND. Four emergence cohorts were established in 4 successive wk. Research plots were arranged in randomized complete blocks with six replications. From the naturally emerged wild oat population, 10 randomly selected plants per plot were evaluated for plant height, leaves on main stem, tillers per plant, total leaves per plant, days to flag leaf emergence and to heading, biomass per plant, and seeds per plant. Hauns numerical cereal development scale was regressed on days after emergence (DAE), day length (DL), growing degree days (GDD), or photothermal units (PTU). Wild oats that emerged first required more time for flag leaf emergence and heading, were taller, and had more biomass, leaves, tillers, and seed production than wild oat plants that emerged later. Wild oat phyllochron intervals were 5.3 d, 94 GDD, or 1,468 PTU, regardless of emergence timing. These data suggest that wild oat phyllochron is primarily driven by air temperature and is relatively stable during the extended emergence period. Later-emerging wild oat plants, although not as competitive as earlier emerging ones, still have the potential to contribute to the seed bank if left uncontrolled. Nomenclature: Wild oat, Avena fatua L. AVEFA.