Proposed regression model for calculating optimum crop levels of apple trees

Abstract

The value per ton of Golden Delicious, Granny Smith, Starking and Starkrimson fruit was calculated by assigning values per carton to each of six size groups of fruit harvested from individual trees in experimental orchards during the four seasons 1978/79 to 1982/83. Data for the number of fruits per cm trunk circumference for the current season, kg fruit per cm trunk circumference for the previous season, diameter of fruits 42 days from full bloom (DFFB), hours above 14°C during the first 42 DFFB and average mass (g) per fruit at harvest, were used to calculate the relationship between these variables and the value per ton of fruit. A multiple linear regression equation was fitted to the data to develop a model for calculating optimum crop levels aimed at improving returns per hectare. The model fitted well enough to enable income per hectare for the four seasons to be estimated from data obtained from experimental orchards. Temperature during the first 42 DFFB was shown to be the major variable affecting fruit value. In the case of Golden Delicious it was shown that income would be maximized if crop levels were to be adjusted to calculated optima. In the case of Granny Smith, Starking and Starkrimson however, prices per carton of fruit in the larger size groups were too low to warrant the reduction of crop levels aimed at the production of larger fruit. The model was tested in experimental orchards during five consecutive seasons and was shown to have potential for adjusting thinning guidelines with a view to maximizing income if prices for the different count groups are known and values of input variables are available for a given orchard.

Keywords:

• Apples
• fruit size
• thinning effects