Agriculture Reference
In-Depth Information
Beyond simply mean bulb weight and diameter, as predicted by Eqns 6.1
and 6.2, equations have been derived to predict the distribution of bulb yield
over different weights and diameters (de Visser and van den Berg, 1998). Thus:
F(w) = 1/(1 + e ((m-d)/b) )
(Eqn 6.3)
F(w) = the proportion of the total bulb weight with a diameter less than or
equal to d
d = bulb diameter, mm
m = the diameter of a bulb at the mean weight (i.e. where F(w) = 0.5)
b = a constant that varies with the diameter of a bulb at the mean weight
(b = 0.0991m)
m is related to the cube root of the mean bulb weight and can be found
using the equation: m = 13.2 w m 1/3 , where m is in mm and w m
is the mean
bulb weight, g.
Equation 6.3 was developed in The Netherlands using data from plots with
plant densities ranging from 52 to 185 plants/m 2 of the fairly spherical
'Rijnsburger' type onions. Predicted size distributions from Eqn 6.3 along with
Eqn 6.1, using the values for A and B for 1970 given by Frappell (1973), are
compared with those observed for cv. 'Pukekohe Long Keeper' in New Zealand
in Fig. 6.8. At densities between 33 and 130 plants/m 2 , i.e. in the range typical
for bulb onions, the predictions are good despite the difference in location and
cultivar. Thus, if a target mean yield can be achieved, the weight and number
of bulbs in each grade can be predicted.
Trials comparing various row widths with the same overall plant density
have consistently shown increases in yields as row widths decrease. For example,
in moving from rectangularities (inter-row distance/mean within-row distance
between plants) of 8 to 1, increases in bulb yield of 10-20% have been reported.
Inter-row spacings of 45-60 cm, necessary for inter-row cultivation before
herbicidal weed control was well established, do not give maximum yields of bulb
onions. Herbicides have enabled growers to adopt the 'bed' system of vegetable
growing and to use plant spacing arrangements that enable crops to fully exploit
light and soil resources and give high yields.
Planting density and shallot size
Similar principles apply to the control of bulb size in shallots (Cohat, 1986;
Messiaen et al. , 1993). The size of bulbs harvested depends on the number of
daughter bulbs formed per unit area, and this can be varied by two factors: the
number of mother bulbs planted per unit area and/or their size. The growing
points giving rise to daughter bulbs are already present in a mother bulb when
it matures, and cannot be modified by storage conditions, planting date or
density. The number of daughter bulbs produced per mother bulb planted
depends primarily on the weight of the mother bulb. For bulbs raised under
similar conditions, Eqn 6.4 gives the relation between bulb weight, w and the
number, n of buds within where k 1 is a constant:
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