Weed Population Dynamics

Abstracts:

• Effect of soil disturbance on weed species emergence. 2002. M. W. Myers, W. S. Curran, and D. A. Mortensen, Penn State Univ., University Park; M. J. VanGessel and B. Scott, Univ. of Delaware, Georgetown; B. A. Majek and A. O. Ayeni, Rutgers Univ., Bridgeton, N.J. Proc. 56:5.
• Influence of glyphosate and ALS-type herbicides on weed population dynamics in corn-soybean rotation. 2002. K. Handwerk, W. S. Curran, and D. D. Lingenfelter, Proc. NEWSS 56:98.

Effect of soil disturbance on weed species emergence. 2002. M. W. Myers, W. S. Curran, and D. A. Mortensen, Penn State Univ., University Park; M. J. VanGessel and B. Scott, Univ. of Delaware, Georgetown; B. A. Majek and A. O. Ayeni, Rutgers Univ., Bridgeton, N.J. Proc. 56:5

Integrated pest management (IPM) has become an important and accepted concept in modern agriculture. An important objective of IPM is using herbicides effectively and economically. In order to accomplish this objective, it has become critical to understand weed emergence characteristics such as rate and duration of emergence. Having knowledge of weed emergence characteristics could help in assessing the potential for using single postemergence (POST) herbicide applications. Single POST applications could ultimately decrease the pesticide load on cropping acres and reduce the input costs for growers.

An experiment was conducted in 2001 at four locations in the Northeast to determine the effect of geography and soil disturbance on weed species emergence. The locations included two Pennsylvania sites (Rock Springs and Landisville) and one site in both Delaware and New Jersey. The experimental sites were sown with multiple weed species or left fallow in 2000. Individual plots were either tilled or left undisturbed in Spring 2001. During the course of the season, weed emergence by species, was monitored every 14 days by counting two 0.5 m2 areas per plot. Following each monitoring interval, weeds were removed using 1 lb ai/A paraquat. All four experimental sites were supplemented with irrigation.

A total of 21 weed species were identified over all four locations. The most prevalent weeds were common lambsquarters (Chenopodium album L.), large crabgrass (Digitaria sanguinalis L.), eastern black nightshade (Solanum ptycanthum Dun.), and smooth (Amaranthus hybridus L.) or redroot pigweed (Amaranthus retroflexus L.). In the case of common lambsquarters, spring tillage either had no effect or reduced emergence. At all locations, two distinct periods of emergence occurred with common lambsquarters with the first occurrence at the end of May and the second at the end of June. Large crabgrass also showed a variable response to tillage. At Rock Springs, tillage had no effect on emergence, while at Landisville and New Jersey, large crabgrass emergence was reduced in the tilled plots. Again there were two distinct periods of large crabgrass emergence, although the majority emerged in late May at the initial flush. A smaller second flush emerged in mid-June. Eastern black nightshade also exhibited two distinct periods of emergence and varied only slightly with tillage. Most of the eastern black nightshade emerged at the end of June. Only at Landisville, did tillage alter the emergence pattern with reduced emergence in the tilled plots. Pigweed species were not affected by tillage at Rock Springs or New Jersey. At Landisville, less pigweed emerged in the tilled plots. Pigweed species also had two distinct emergence periods with the first occurring over a two-week period beginning the end of May. The second flush emerged towards the end of June. Although this experiment will be repeated in 2002, first year results showed distinct periods of weed emergence across fours locations. The effect of tillage proved to be more variable and will require more thorough investigation.

Influence of glyphosate and ALS-type herbicides on weed population dynamics in corn-soybean rotation. 2002. K. Handwerk, W. S. Curran, and D. D. Lingenfelter, Proc. NEWSS 56:98.

A corn-soybean-corn (C-S-C) rotation and a soybean-corn-soybean (S-C-S) sequence were established in an experimental farm field with locally existing weed populations. Using six herbicide programs, two glyphosate-based programs, one ALS-based program, two mixed herbicide programs and an untreated check, an experiment was conducted over a four-year period starting in 1998. Glyphosate resistant corn and soybean were planted in all years. Observations and measurements (density counts, biomass) were taken periodically throughout the season to quantify the long-term effect of herbicide programs on weed population shifts.

The primary weed species in the study since 1998 were common lambsquarters (Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), Pennsylvania smartweed (Polygonum pensylvanicum L.), yellow foxtail, (Setaria glauca (L.) Beauv.), and yellow nutsedge (Cyperus esculentus L.). All five of these species were mostly controlled with the herbicide treatment in both corn and soybean. In general, the ALS-based treatment tended to be less effective than the glyphosate-based treatments, especially in soybean; the chlorimuron/thifensulfuron/quizalofop mixture did not effectively control all common lambsquarters, common ragweed, or yellow foxtail. In addition, common ragweed escapes were noted with a single application of glyphosate in corn. The two-pass programs that included soil residual herbicides were effective on all weeds, with the exception of volunteer corn.

Four years after initiation, the five weed species previously mentioned continue to be dominant in both corn and soybean with few exceptions; yellow foxtail is in decline in all treatments, except the soybean ALS-based treatment and the untreated check. Yellow foxtail has increased in the ALS-based treatment in the C-S-C rotation. Yellow nutsedge increased in 1999, but then declined in all treatments in 2000 and 2001. Volunteer glyphosate-resistant corn is an increasing problem in all soybean treatments except the ALS-based program, which includes quizalofop in soybean. Volunteer glyphosate-resistant soybean increased starting in 2000, but only in the two glyphosate-based treatments. In addition, some other weeds were also noted at low frequency over the last four years, including wild buckwheat (Polygonum convolvulus L.) and black medic (Medicago lupulina L.). Finally, common lambsquarters declined especially in the untreated check in 2001, while common ragweed increased. This change was also observed in the single glyphosate treatment in the S-C-S rotation and in the ALS-based treatment in the C-S-C rotation.

In summary, after four years of glyphosate or ALS-based herbicides in corn-soybean or soybean-corn rotation, only minor weed shifts have occurred. The weed seedbank and annual weed management tactics will determine the trajectory of present and future weed problems. If problem weed species exist, monitoring and altering management tactics will be important to preclude the development of future weed problems.