Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 302

screening to obtain larger seed would probably increase the cost of the seed,
this might be offset somewhat by increased ease of removing weed seeds
during production of certified seed. Using only large seeds is impractical for
hybrid crops where the seed must be specially grown,and in forages and vege-
tables where no alternative market exists for the seed. Potato seed tubers are
sometimes available in size classes,and the larger grades are likely to be better
suited to growers relying on ecological weed management procedures. Lugo,
Ortiz & González (1997) varied set size and planting density of taro and
showed that the resulting differences in leaf area index had a substantial
effect on weed biomass.
Probablythemosteffectivewaytoobtainthecompetitivebenefitof largeseed
size would be to breed larger-seeded crops. This is probably possible without
yield loss in most crops. Larger seed size would also have benefits for manage-
ment of weeds with cultivation (see Chapter 4) and mulch (see Chapter 5).
Planting date and weed-crop competition
As discussed in Chapter 4, planting date often has a substantial effect
on weed pressure experienced by the crop, because weeds that germinate
before planting are usually destroyed by seedbed preparation. However,
planting date also affects the competitive balance between the crop and the
weeds that emerge after planting.This occurs because the growth rate of both
the crop and the weeds depends on environmental conditions, particularly
temperature and soil moisture, and these typically change through the
season. In weed-free conditions, the optimal planting date for agronomic
crops is often the earliest that will allow successful emergence, since a longer
season allows greater accumulation of resources and hence greater yield
(Gunsolus, 1990). However, seedlings emerging from early plantings may
grow slowly until weather conditions become more benign. Hence, they may
become overgrown with weeds that are better adapted to weather conditions
at that time of year.
Since a field is usually infested with several weed species, each with its own
particular phenology, the problem of choosing a planting date that max-
imizes yield in the presence of weeds is not simple. To a first approximation,
the crop will be most competitive at the planting date for which its growth
rate per unit ground area (g m 2 d 1 ) shortly after emergence is greatest rela-
tive to the weeds that emerge at that same time. This occurs because the early
growth rates of the crop and weeds largely determine which has the greater
stature,leaf area,and root occupation of the soil when they come into compet-
itive contact. Water and nutrient use efficiencies and architecture of both the
crop and weeds may also depend on environmental conditions, but their vari-