Economics of On-farm Production of Indigenous Fruits - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 247

tasks are carried out during the dry season, weeding takes place during the wet
season and dead branches are removed throughout the year. Fruits ripen
during the wet season. It is assumed that an orchard of IFTs is planted close to
the homestead. Harvesting labour estimates are based on data for harvesting
time of indigenous fruits from trees that farmers have preserved in their fields
and around the homestead.

Modelling benefits of indigenous fruit tree planting

The fruit yield of IFTs is age-dependent and varies among individual trees
(Chidumayo, 1997). For U. kirkiana , yield sets in around years 11-16,
increases up to years 25-30 (Household Monitoring and Farmer Workshop),
and then declines after 45-55 years (C.K. Mwamba, National Institute for
Scientific and Industrial Research, Zambia, 2000, personal communication). It
was assumed that indigenous fruit yield reaches zero at an age of
90-100 years. To model the age-yield relationship, the Hoerl function
(Haworth and Vincent, 1977), u = g e g , has been specified. It describes yield,

u , as a function of age, g ; the coefficients

, , and are estimated via linear

regression after transforming the equation to ln u = ln g . For IFTs

the age-yield function was specified as the relationship between fruit yield and
the productive period of the tree (as a proxy for age) because farmers usually
do not know the age of trees and tend to notice the time when a tree starts
bearing fruits instead.
Three yield functions were estimated using data of farmers' estimates of the
minimum, maximum and modal yield of trees that they had preserved and
managed in their fields. Farmers' data were then complemented by information
from a survey of IFT experts. The database comprised estimates of 38 naturally
grown and farmer-managed U. kirkiana trees. Fruit yield functions for
minimum, modal and maximum yield are given in Table 13.5.

+ ln g +

Table 13.5. Regression results of the parameters of the age-yield function. a

Parameter b

Fruit yield ln Criteria of estimator

Maximum 2.9640*** 1.1270*** 0.0861*** Adj. R 2 = 0.751 F = 48***

(0.311) (0.177) (0.009) SE = 0.73 df = 29

SE

Mode 2.2390*** 1.1550*** 0.0806*** Adj. R 2 = 684 F = 42***

(0.231) (0.150) (0.009) SE = 0.74 df = 36 SE

Minimum 1.4840*** 1.1720*** 0.0735*** Adj. R 2 = 0.661 F = 31***

(0.320) (0.009) (0.182) SE = 0.75 df = 29 SE

a Age of zero years refers to the age at which fruit production begins: this is between 11 and
16 years after germination.
b First row: parameter values.
*: P 0.1, **: P 0.05, ***: P 0.01.
Source: Mithöfer (2005).