| |
return to Farmers Independent Weekly
April 6, 2006
Volunteer spring wheat, emergence timing and implications for management
By Kristi Hildebrand, Lyle Friesen and Rene Van Acker, Department of
Plant Science
The introduction of herbicide tolerance into crops has led to the
development of new herbicide-tolerant volunteers which can be difficult
to control and further limit the effective control methods available to
farmers. The proposed registration of glyphosate-tolerant wheat in
North America (since withdrawn) has highlighted the lack of knowledge
gaps about wheat as a volunteer. The timing of volunteer spring wheat
emergence has not been documented, yet emergence timing affects both
herbicide efficacy and the synchrony of flowering, and therefore gene
flow, between volunteer and cropped wheat. In addition, in western
Canada, volunteer wheat has been shown to persist up to two years
following the harvest of the last wheat crop. However, the mechanism
underlying this persistence is not known, and may be related to either
persistence in the soil or reseeding by plants that escape control.
We conducted a small-plot study over a period of two years (2003 and
2004) to characterize the emergence timing and persistence of eight
lines of Canadian Western Red Spring (CWRS) wheat and to determine
whether emergence timing or persistence was affected by either genotype
(cultivar) or tillage. The CWRS lines in this study were AC Barrie, AC
Domain, AC Intrepid, AC Majestic, CDC Teal, Katepwa, McKenzie, and
Superb. To mimic seed return, 500 seeds m-2 of each cultivar were
spread in the fall on plots at the University of Manitoba research
stations in Winnipeg and Carman. All plots were tilled under in the
fall. In the spring, half of these plots were subjected to spring
tillage, while the other half were left untilled.
Proportional emergence levels (or number of plants emerged relative to
the number of germinable seeds that were spread) were always extremely
low, generally representing approximately 5% of the germinable seed
spread in the previous fall, and never in excess of 13.1%. Although
differences in seed quality were evident in view of the significantly
higher germination percentages recorded for the seed produced in 2002
relative to the seed produced in 2003, seed quality differences did not
account for low proportional emergence since emergence levels were
corrected to reflect only the number of germinable seeds spread.
Site and year significantly affected the proportional level of
volunteer wheat emergence. Soil texture was likely an important factor
in determining the effect of site, since proportional emergence levels
were consistently higher in the sandier soil at Carman than at
Winnipeg, where the soil has a higher clay content. This influence of
soil type and site on persistence could lead to pronounced differences
in persistence throughout the prairies. For example, greater
persistence could be an issue in areas such as western Manitoba that
are dominated by sandier soils (and generally lower precipitation
levels). Because soil moisture seems to be one of the factors that
drive microbial decomposition of seeds, areas with drier climatic
conditions could also experience greater persistence of volunteer
wheat, regardless of soil type. Additionally, farmers in any area
should expect greater persistence under dry conditions.
Results from the two year study indicated that wheat is a relatively
early emerging weed with limited persistence. In all four site years,
emergence began prior to 100 growing degree days (mid to late April),
and peaked prior to 400 growing degree days (mid to late May). The
majority of volunteer wheat seedlings emerged prior to crop seeding.
There was no significant effect of tillage or cultivar on emergence
timing, but there was a significant site-year effect which was
attributed to differences in soil moisture between years. Our findings
showed that the onset of emergence can be delayed when the soils are
dry in the spring.
Our results suggest that in the eastern prairies, where fall and spring
rainfall are reliable and soil moisture levels are relatively high,
volunteer wheat does not form a persistent seedbank. However, as an
early-emerging species, volunteer wheat may have a competitive
advantage over the crop, especially when no control measures are used
prior to seeding. If glyphosate-tolerant wheat is registered,
glyphosate-tolerant volunteers will have the additional advantage of
being largely unaffected by glyphosate-based pre-seeding burnoffs. If
volunteers are allowed to survive and reproduce, they may act as a
pollen source for gene flow to a subsequent wheat crop, or to crops in
neighbouring fields. Based on the results of our study, it seems
unlikely that volunteer wheat persistence is due to a long-lived, or
persistent, seedbank; instead, persistence may be a result of
reseeding. However, the mechanism that underlies volunteer wheat
persistence is perhaps less important than the potential impact of the
persistence.
|
|
