Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 301

However, dependence of both the tolerated period of infestation and the
minimum weed-free period on a wide variety of factors implies that applica-
tion of the critical period concept to field situations requires both extensive
data and careful judgement.
Initial size of crop at establishment
In some crops, possibilities exist for manipulating the size of the crop
at planting time. Rice and many vegetable crops, including tomato, lettuce,
leek, and the cole crops, are often planted in the field as transplants rather
than seeds. Onion sets are essentially transplants, although they are dormant
at planting time. Transplanting into ground that has been freshly tilled or
from which the weeds have been eliminated by herbicides or flaming provides
the crop with a substantial initial size advantage relative to weeds.
The advantages of transplanting for weed management are perhaps so
obvious that they have not been explored much experimentally. Based on
earlier experiments (Weaver & Tan, 1983, 1987), Weaver (1984) and Weaver,
Kropff & Groeneveld (1992) concluded that the minimum weed-free period
for transplanted tomatoes was about 35 days, versus about 70 days for direct-
seeded tomatoes. Also, yield declined more slowly with increased period of
infestation for transplanted tomatoes than for direct-seeded tomatoes. Hill,
De Datta & Real (1990) analyzed several studies of Echinochloa spp.competition
in transplanted and direct-seeded rice, and found that the transplanted rice
was much more resistant to competition. For example, 3 Echinochloa m 2
resulted in 20% yield loss in direct-seeded rice, but 66 plants m 2 were
required to produce the same yield loss in transplanted rice.
Most of the vegetable crops that are commonly transplanted are small-
seeded species that establish slowly and remain noncompetitive for an
extended period when growing from seeds. Early mechanical weeding in the
crop row for these species is essentially impossible, and even hand-weeding
may cause damage to delicate young plants. Consequently, organic growers
usually transplant these crops even though effective direct-seeding equip-
ment is available. If some herbicides are lost, either due to lack of reregistra-
tion or the development of herbicide resistance by major weeds, conventional
growers may increase use of transplanting as well.
Some limited possibilities also exist for the manipulation of seed size.
Kolbe (1980) screened winter barley to obtain two seed size fractions, and
found that plots planted with the large seeds had higher yield and lower weed
cover than plots planted with small seeds. Screening to obtain large seeds is a
possibility in any crop with significant variation in seed size and in which a
market exists for the smaller seed grades that will not be planted. Although