Reducing agroecosystem vulnerability to weed invasion - Invasive Plants: Ecological and Agricultural Aspects - page 176

Combining optimal factors
Arable land is repeatedly disturbed [16] and maintained at very early stages of
succession [14]. In the past, researchers have been successful at isolating
important factors that benefit weed management specifically and crop produc-
tion generally. Factors such as competitive cultivar selection, careful seed
placement, high seeding rates, strategic fertilizer placement and diverse crop
rotations can all lead to significant weed management improvements. For
example, in canola, competitive cultivars [57], timely weed removal [83, 84]
and higher than normal plant densities [57, 85] are individual factors that have
been reported to improve weed management. However, even simple plant
communities are far too intricate to expect that single manipulated factors will
yield consistent positive returns [86]. As Tilman [87] indicated: “…we know
all too little about how ecosystems work”.
Restricting weed populations may be more successful if several factors are
combined at optimal levels for a multi-pronged restraint of weed invasions.
There are few reported cases of such studies in the literature. The following
examples from recently conducted research illustrate how the combination of
several optimal agronomic factors can provide effective weed management.
A study involving relatively competitive (InVigor 2153) and non-competi-
tive (Exceed) canola cultivars, three stages of herbicide application (2-, 4-,
and 6-leaf canola) and three canola seeding rates (100, 150, and 200 seeds
m -2 ) was conducted over three years at two Alberta locations [28]. The best
factor combination (InVigor 2153, 2-leaf herbicide application, 200 seeds
m -2 ) yielded 41% higher than the worst combination (Exceed, 6-leaf herbi-
cide application, 100 seeds m -2 ). More importantly, the best yield combina-
tion also provided greater weed management and lower weed biomass vari-
ability. In another recent study involving a barley-pea-barley-pea rotation in
a direct seeded production system, combining early seeding, higher crop seed
rates, and spring-applied fertilizer provided the most competitive cropping
system over the four years of the study [88]. In-crop herbicides applied at
50% versus 100% recommended rates sometimes resulted in greater weed
biomass and lower crop yields with recommended crop seed rates but few dif-
ferences between herbicides rates were noted at relatively high crop seed
rates. In addition, after 4 years, the amount of weed seed in the soil seed bank
was similar at 50% compared to 100% herbicide rates when high crop seed
rates were utilized.
In an ongoing study (unpublished observations) we are investigating culti-
var, seeding rate, herbicide rate and crop rotation effects on barley productivi-
ty and weed management. Individually, these factors had considerable effects
on wild oat, but when combined, the effects were dramatic (Tab. 1). For exam-
ple, the higher seeding rate decreased wild oat biomass and seed numbers
approximately four-fold while the tall barley cultivar decreased wild oat bio-
mass and seed numbers approximately ten-fold compared to the short cultivar.
Combining higher seeding rates with the taller barley cultivar decreased wild