Agriculture Reference
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approaches zero. The parameter I allows for the
approximate comparisons of weed competitive-
ness. Estimated I ranged from 0.06 to 2.68,
where I was the percentage yield loss per unit
weed population as density approached zero
( Table 13.2 ).
Weed competition can vary considerably
between years and among potato varieties. For
example, green foxtail competitiveness at low
densities ranged from 0.64 to 2.68% yield loss
per unit weed, depending on the year (Wall and
Friesen, 1990a,b). As a consequence, a very low
weed population density is sufficient to cross an
economic threshold and trigger a treatment.
Green foxtail appears to be more competitive
than the common and troublesome weeds in po-
tato production, such as nightshade spp. and
quackgrass.
The critical period of weed control in potato
represents a large majority of the growing sea-
son. Canadian studies conducted in 1989 and
1990 revealed quackgrass caused a critical
period of weed control that began between 3 and
15 days after emergence and lasted from 23 to as
long as 68 days after emergence (Baziramaken-
ga and Leroux, 1994). The researchers con-
cluded that even a relatively low density of
quackgrass should be controlled for the entire
season to avoid a yield loss. The long critical
period has been observed in potato production
in Brazil (Costa et al ., 2008) and Angola (Mon-
teiro et al ., 2011), where critical periods began
approximately 21 days after treatment and
lasted for longer than 40 days. Although con-
sidered very competitive, potato is still suscep-
tible to yield loss and requires control inputs to
minimize loss.
13.3
Management Tactics
Growers typically identify herbicides as the pri-
mary in-season weed management tactic. Some
growers use a practice called “hilling” prior to
canopy (foliage) row closure, for altering the size
and shape of the potato hill, incorporating fertil-
izer and herbicides, and mechanical disruption
of weeds. Herbicides are often applied early, al-
though applications following hilling are not
uncommon. To prevent phytotoxicity to the po-
tato plant, some herbicides must be applied prior
to potato emergence, or shortly thereafter. Later
in the season, as herbicides dissipate to non-
effective levels, the crop canopy contributes to
weed control by competing for light. Utilizing
good cultural practices, including seedling dis-
ease control and early-season insect control,
Table 13.2. Relative competitiveness of several weeds of potato.
Weed
Variety
Year
Estimated i a
Standard error
Estimated b
Source
Quackgrass
Superior
1989-1990
0.17
0.09
0.77
Baziramakenga
and Leroux, 1994
Quackgrass
Russet
Burbank
1987-1988
0.26
0.02
0.52
VanGessel and
Renner, 1990
Green foxtail
Russet
Burbank
1983
2.68
0.81
0.83
Wall and Friesen,
1990b
1984
1.51
0.36
0.77
1985
0.64
0.17
0.71
Redroot
pigweed
Russet
Burbank
1987-1988
0.54
0.02
0.41
VanGessel and
Renner, 1990
Hairy
nightshade
Russet
Norkotah
2004-2005
0.25
0.02
0.37
Hutchinson et al. ,
2 011
Hairy
nightshade
Russet
Burbank
2004-2005
0.06
0.01
0.19
Hutchinson et al. ,
2 011
Notes : a Yield loss was estimated from figures and charts in various refereed journal articles and fit to the hyperbolic
function with asymptote set to 100% maximum yield loss. Location effects were averaged where appropriate, and weed
densities were converted to per meter of crop row. Estimated i values and standard errors are based on the hyperbolic
yield loss function [ Y = iD /( 1 + iD / A )], where D is the weed density per meter of crop row and i is the percent yield loss
per weed as weed density approaches zero. b Cousens, 1985.
 
 
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