Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 303

ation within the season is likely to be much less than variation in growth rate.
The competitive balance between the crop and weed depends not just on
the growth rates of individual crop and weed plants, but rather on the ability
of the crop population as a whole to occupy space in the field faster than the
weed community as a whole. Consequently, the optimal planting date will
often depend on both potential weed density and the effectiveness of weed
control measures. If weeds are partially controlled, then yield as a function of
planting date will be maximum for some date that depends on the degree of
control of the several weed species present in the field. Good control will shift
the optimal planting date toward that for the weed-free condition, and poor
control will shift the optimum date toward that for the unmanaged condi-
tion. Thus, the optimal planting date depends on the grower's projection of
the degree of weed control.
Many of the spring-planted row crops grown in the temperate zone,
including maize, sorghum, soybean, and common bean, are subtropical
species with relatively high optimal growth temperatures. Consequently,
early planting is likely to put them at a disadvantage relative to weeds. For
example, Oliver (1979) found that soybean planted in May in Arkansas had
greater yield than July-planted soybean when the fields were kept free of
weeds. However, leaf area index at four weeks was about three-fold greater in
July-planted soybean, and due to day length dependence, growth of Abutilon
theophrasti was slower in July-planted crops.Consequently A.theophrasti caused
little yield loss for the July-planted soybean, but substantial yield loss for
those planted in May. Similarly, Rushing & Oliver (1998) found that fresh
weight of Xanthium stumarium planted with soybean declined with progres-
sively later planting dates; weed suppression by late soybean planting was
particularly great if weed emergence was delayed by two weeks or more. In a
Wisconsin study, Harvey & McNevin (1990) found that control of weedy
Panicum miliaceum and yield of maize and sweet corn improved when crops
were planted in mid May rather than late April.However, late planting of row
crops does not always improve weed control or decrease yield loss from a given
density of weeds (Klingaman & Oliver, 1994).
The situation with spring-planted cereals in the temperate zone is rather
different than that described above for spring row crops.Spring cereals germi-
nate at temperatures as low as 5

C (Cornell University, 1987, p. 144), and are
thus well adapted to cool, wet conditions early in the season. They generally
get maximum yield when planted in late winter in milder climates
(Wibberley, 1989, p. 87), or as early as the ground can be worked in regions
where the soil is frozen much of the winter (Cornell University, 1987, p. 144).
Yield losses become substantial as the season progresses, largely due to

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