Soil workability days, as determined from an existing soil
moisture model, can be used to specify the actual finish date
constraints for field operations.
Cone penetration resistance (cone index) is strongly a function
of soil moisture content, specific weight and clay ratio (soil
type). A cone index can be predicted from these properties
over a wide range of soil types with a high level of accuracy
and can be used to specify the average soil strength at the
median depth of a segment.
Cone indices were found to be a satisfactory measure of soil
strength for inclusion in the equation predicting both tractor
performance and plough draught.
The characterisation of plough draught by the cone index,
specific weight of soil, moisture content, depth and cut width
of the plough body, speed and tail angle is confirmed.
Dynamic load transfer on the tractor due to implement draught
is included in the analysis of tractor -implement performance.
A simulation of crop yield losses from untimely establishment
for eight crops over a period of 30 years in U.K. was
developed. A satisfactory prediction of crop yield loss
related to the sowing date was found to be a function of the
square of the time duration from the operation sowing date for
both early and late establishment.
The optimum crop sowing date (day number) and peak yield of
each crop were specified for the U.K. as a whole and for
particular sites in Scotland and England.
A discounting cash flow costing procedure was mod fied to
involve the effect of taxation, Government tax allowance and
tax relief and used to calculate the annual cost of a machine
ownership for the machinery selection programme.
The selection of single and multiple combinations of tractors
and implements demonstrates the feasibility of the
suboptimisation sequence for farm operations (tillage and
sowing). The "MSP" machinery selection computer model includes
a tractor implement performance model to select a tractor fleet
matched with proper sizes of ploughs. Each of the technically
feasible combinations is added together to give the total
machinery complement. A specific job during the workdays
available within a specific calendar period; and the costing
routine is used to identify only those tractor -implement
combinations which are economically viable. The final choice
of the appropriate system is dependent on whole farm
constraints of labour team size, level of capital investment.
Timeliness penalties have a major impact on the machinery
selection. For an inadequate machinery system, the timeliness
penalties were very high and could exceed the cost of the
machinery system itself. Once the machinery complement was
sufficient to complete the work on time, the increase in costs
for oversize machines was relatively small.
Labour and fuel costs are calculated within the machinery
selection model for each sequence of field operation. As
tractor size increases, the labour cost of the operation
declines and fuel cost increases. The total tractor operating
costs (including fuel and labour) are surprisingly constant
over a wide range of tractor sizes for a given area.