Agriculture Reference
In-Depth Information
alliums. In addition, weeds cause severe problems at harvest for leafy crops like
salad onions and leeks, and interfere with the drying and storage of bulb
onions. Competition from weeds will reduce the mean diameter of onion and
garlic bulbs, putting an increased portion of the crop into low-value, small-size
grades. Weed competition will actually accelerate onion bulb initiation if
photoperiods are approaching those required for bulbing (Shadbolt and Holm,
1956). Therefore, bulbs develop on younger and smaller plants than would
otherwise be so, further reducing the size of the leaf canopy that supports bulb
growth compared with that of a weed-free crop.
The acceleration of bulbing that can be caused by weed competition is
probably due to decreases in the red:far-red ratio (R:FR) incident on leaves
when shaded by weeds (see Fig. 4.33). The likely variation of weed competition
from spot to spot within a field will contribute to increased variation in bulb or
plant size at harvest. The presence of weeds within the leaf canopy of allium
crops will restrict air flow through them, which is likely to increase relative
humidity and prolong periods of leaf wetness, thereby increasing susceptibility
to fungal leaf disease (see Table 5.5). For all these reasons, particularly those
pertaining to quality and weed contamination at harvest, which will not be
tolerated by the supermarket retailers that now sell the majority of vegetables
in developed economies, there is generally a policy of 'zero tolerance' of weeds
in allium vegetable crops (Grundy
et al
., 2003).
Researchers have investigated how long weeds can be left in allium crops
without causing irretrievable losses in yield and, secondly, having controlled the
weeds once, how long crops must be kept weed-free so that any subsequently
emerging weeds do not lower yields. In experiments on bulb onions in the UK,
weeds left until 4 weeks after crop emergence did not reduce yield but, if left
longer than 6 weeks, bulb yield was reduced by 4% for every day their removal
was delayed (Hewson and Roberts, 1973).
Such studies have led to the specification of a 'critical period' during which
the crop must be kept weed-free to ensure there will not be a serious loss of yield
caused by weed competition. Following a review of research on critical periods
in vegetables crops, Grundy
et al.
(2003) concluded that the critical period after
emergence or planting was 4-8 weeks in spring-drilled bulb onions, 5-7 weeks
in spring-transplanted bulb onions and from 4 weeks onward in spring-drilled
salad onions. These critical periods concern effects on yield but not weed
impacts on harvesting or quality. If there is a high density of highly competitive
weeds and temperatures are conducive to rapid growth, the critical period for
drilled bulb onions may commence as soon as 2 weeks after emergence.
Therefore, the timing and duration of the critical period can vary with
weed density, weed competitiveness and weather conditions, in particular
temperature.
Dunan
et al.
(1996) took these factors into account in regression
equations for the effect of duration of time before starting weeding on yields
from commercial drilled bulb onion crops in irrigated fields in Colorado, USA.
