Weed Species Shift Over Time -- Part I
Commercial Horticulture Newsletter, November-December 1998
Henry P. Wilson
Professor, Weed Science
Eastern Shore Agricultural Research and Extension Center
Painter, VA 23420
A number of factors influence the species of weeds which are dominant in crop production systems. These factors may include: the species indigenous to the area and those transported into the area, tillage practices, herbicide and herbicide modes of action, weather, the particular crop, crop row spacing and planting dates. We have all observed over the years that the weed species which farmers are currently trying to control often differ from those that they considered important several years ago. We have conducted several studies over the years at the Eastern Shore AREC that document species shifts. During the next several issues of the Vegetable Growers Newsletter, I will discuss results from some of these studies.
Cooperating with Dr. Martin Moscianica, Soil Scientist, at this Center a four-year study was initiated to investigate the effects of tillage, and herbicides on weed proliferation in a wheat/soybean, double crop rotation:
| Tillage treatments were: | conventional (mold board plow) minimum (disc twice only) no-till
| |
| Herbicide treatments: | Fall, 1980 and 1981 -- None Fall, 1982 and 1983 -- No-till plots split with either no herbicide, Gramoxone Extra or Roundup applied prior to wheat emergence |
Following data collection and wheat harvest each year plots were planted back to soybeans using conventional till, minimum till or no-till practices in the same tillage plots planted to wheat. Only data relative to wheat will be presented here.
Results. In the fall of 1980, few weeds were present in the study site. By the spring of 1981 and throughout the remainder of the study, the combination of 2,4-D plus Banvel controlled chickweed and corn chamomile; these weeds were present annually in plots not treated with 2,4-D plus Banvel.
Also, by the spring of 1981 annual bluegrass averaged 1 plant/m 2 in conventional tillage plots and 3 plants/m2 in no-till plots. At that time the two Brome species cheat and soft chess were present in the headlands only. By the fall of 1981, cheat and soft chess were present in the research area. Annual bluegrass and bromegrass species increased in minimum till and no-till in the fall of 1982 (Table 1). It is evident that populations increased as tillage decreased.
Percent of ground covered by annual bluegrass and brome species in the spring of 1983 and in 1984 are presented in Figure 1. It is interesting to note the impact of tillage intensity on the populations of the annual grasses after three and four years in the rotations. Populations of all three grasses increased with decreases in tillage intensity. Applications of Gramoxone Extra and to a lesser extent Roundup controlled annual grasses in no-till. The lower than expected control from Roundup is likely a result of the low rate applied (1 pt/A); it is likely that Roundup Ultra rates of 1.5 to 2 pt/A would have been more effective.
The reduction in yields in no-till wheat was as much as 25% in the fourth year of the study compared to conventional till wheat even where weed and grass control by non-selective herbicides was high. Reasons for this tillage effect on wheat yield are not known.
Conclusions drawn from this four-year study are:
Table 1. Annual bluegrass and brome species present in
wheat planted in the fall of 1982.
Number/m2
| Tillage | Annual Bluegrass | Brome Species |
|---|---|---|
| CT | 1 | 1 |
| MT | 25 | 11 |
| NT | 65 | 27 |
CT = conventional till
MT = minimum till
NT = no-till
Figure 1. Wheat-soybean rotation affects populations of Brome species and annual bluegrass
