Agriculture Reference
In-Depth Information
n = k
1
w
(Eqn 6.4)
Hence, planting large mother bulbs at a high planting density results in a
high density of daughter bulbs of small size, whereas planting small mother
bulbs at low density results in a low density of daughter bulbs of large size. Table
6.1 shows the distribution of daughter bulbs that resulted from the extreme
treatments in a French trial. The same size distribution of harvested bulbs can
result from different combinations of mother bulb size and planting density, so
long as they result in equal numbers of daughter bulbs. It can be seen from Table
6.1 that treatments gave a threefold difference in the number of daughter bulbs,
but yield increased by a factor of only 1.3, so mean bulb size was much reduced
from the heavier planting weight.
Although the number of daughter bulbs originating from a mother bulb
depends primarily on its weight, the level of competition experienced by the
mother bulb when it grew has some additional influence. In experiments in
which cv. 'Jersey demi-longue' mother bulbs of equal size were selected from
bulbs produced from densities at planting ranging from 14 to 29/m
2
using
bulbs weighing 10, 15 or 20g, the lower the degree of competition the mother
bulbs were subject to the greater the number of daughter bulbs produced. A
mean of 5.9 daughter bulbs was produced by 10 g bulbs and 8.3 by 20 g bulbs;
but 10 g bulbs themselves produced from 10 g bulbs at 14 plants/m
2
, i.e. under
the lowest competition gave rise to seven daughter bulbs, whereas those raised
from 20 g bulbs at 29 plants/m
2
, i.e. under the highest competition, gave rise to
5.2 daughter bulbs. The corresponding extremes for 20 g bulbs were 10.4 and
7.3 daughter bulbs.
The relation between the yield, Y of shallots and the number of buds (i.e.
daughter bulbs) per ha (N), with k
2
as a fitted constant, is of the form:
Y = k
2
N
1/3
(Eqn 6.5)
Equations 6.4 and 6.5 were derived for spring-planted 'Jersey demi-
longue' shallots grown in western and central France and also apply to
tropical shallots grown in the West Indies. Grey shallots (see Chapter 1) and
autumn-planted shallots do not conform to these equations (Messiaen
et al.
,
1993).
Dates of sowing
In many regions sowing dates are limited by the climate to a short period in the
spring or in the autumn of the year. In many mid-latitude regions - for example,
Israel - sowings can be made throughout the autumn, winter and spring to give a
succession of harvest dates. Autumn sowing is common in low and intermediate
latitude regions with mild winter temperatures which permit some crop growth.
Onions can survive temperatures of -6°C but are killed by temperatures of -8 to
