Agriculture Reference
In-Depth Information
Variability
One of the main problems with leek production is the great plant-to-plant
variation in the crop. In both transplanted and drilled crops the coefficient of
variation in shoot weight at harvest is typically 50-70%. With modern market
outlets requiring uniform grades, e.g. for supermarket pre-packs, variability
results in much work grading the crop after harvest, as well as wastage of plants
that are under- or oversized. In a drilled crop, 25% of the variability at an early
harvest was attributable to differences in the timing of seedling emergence, and
in a transplanted crop 15% was attributable to plant size soon after emergence.
In another experiment on a crop directly sown in late March, the coefficient
of variation of plant weight was 74% in late September and 59% in February. At
these two harvest dates, respectively, 47.6 and 19.7% of the variability could be
accounted for by differences in timing of seedling emergence, seed size, sowing
depth and distance to neighbouring plant or row, all of which were recorded for
individual plants. Of this, only about 7% of variability could be attributed to seed
size or sowing depth differences (Benjamin, 1984). The use of primed seed
reduces the variability associated with differences in the time of seedling
emergence in drilled crops. Variability tends to increase with plant density, in
common with most vegetable crops. In one experiment coefficients of variation
at harvest were 61% at 50 and 44% at 22 plants/m
2
. The increased plant
competition at high densities accentuates any differences in plant size already
established.
These studies have shown that plant-to-plant differences during crop
establishment account for a much lower proportion of variability at harvest
than is the case with other vegetable crops. For example, with lettuce and
carrot, about 50% of variability at harvest is attributable to differences in time
of, or size at, emergence. Therefore, with leek, differences in growth rate after
establishment are more important than for other crops, probably because of
inherent genetic variability within leeks (see Chapter 3). By selecting for
uniformly large seedlings at transplanting, the coefficient of variation was
reduced to 28% at harvest 220 days later, whereas medium and small plants at
transplanting gave coefficients of 39 and 57%, respectively, and the unselected
population 49% (Benjamin, 1984). In this way, genotypes with more uniform
growth rate may be selectable at transplanting, but at the cost of raising many
reject seedlings. Seedling size at transplanting shows a good correlation with
weight and thickness at harvest time (de Clerq and van Bockstaele, 2002).
There can be up to 20% self-fertilization in leek seed crops, and yields from
selfed seed range from 26 to 80% less than from cross-pollinated seed. Thus
selfing is a major source of the variation within a leek seedlot. Some of this
variability will be eliminated when weak seedlings are discarded in trans-
planted crops. However, hybrid leeks, based on male sterility, eliminate the
problem of selfs and give greater uniformity of seedlings and at harvest (de
Clerq and van Bockstaele, 2002).
